Fluid Manipulations Using a Piezo Electric Transformer for Sensing and Spray Generation
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Western Michigan UniversityScholarWorks at WMU
Dissertations Graduate College
8-2017
Fluid Manipulations Using a Piezo ElectricTransformer for Sensing and Spray GenerationApplicationsZeinab RamshaniWestern Michigan University, zeinab.ramshani@gmail.com
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Recommended CitationRamshani, Zeinab, "Fluid Manipulations Using a Piezo Electric Transformer for Sensing and Spray Generation Applications" (2017).Dissertations. 3162.https://scholarworks.wmich.edu/dissertations/3162
FLUID MANIPULATIONS USING A PIEZO ELECTRIC TRANSFORMER FOR
SENSING AND SPRAY GENERATION APPLICATIONS
by
Zeinab Ramshani
A dissertation submitted to the Graduate College
in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
Electrical and Computer Engineering
Western Michigan University
August 2017
Doctoral Committee:
Massood Z. Atashbar, Ph.D., Chair
David B. Go, Ph.D.
Bradley J. Bazuin, Ph.D.
Paul D. Fleming, Ph.D.
FLUID MANIPULATIONS USING A PIEZO ELECTRIC TRANSFORMER FOR
SENSING AND SPRAY GENERATION APPLICATIONS
Zeinab Ramshani, Ph.D.
Western Michigan University, 2017
A steady effort has been made on the development of fluid manipulation techniques for
sensing and actuating systems. Conventional techniques of fluid manipulation for sensing and
spray generation purposes, often need trained staff or high input power and they are
complicated, time consuming and expensive. Therefore, it is necessary to develop new systems
which can overcome these drawbacks.
In this dissertation, the author has developed piezoelectric-based systems for fluid
manipulation. The focus of this dissertation involves novel approaches of enhancing the
sensing and actuating systems using the piezoelectric devices. This includes designing a
system, which theoretically can overcome the limitations associated with conventional fluid
manipulation systems, fabricating the device and examining the functionality of the system to
prove the claim.
The purpose of the first project of this research work was to design and fabricate a
piezoelectric based system to enhance the sensitivity of the system towards the sensing of toxic
materials in liquid media. The functionality of the designed sensing system was investigated
towards several toxic heavy metals including lead, cadmium, nickel and mercury. The SH-
SAW sensor was fabricated on a 64° YX-LiNbO3 piezoelectric substrate using
photolithography techniques and placed in the sensor groove of an acrylic based material flow
cell. Then, varying concentrations of target analytes were injected into the flow cell using a
programmable syringe pump. A network analyzer was used to measure the phase response (S21)
of the SH-SAW sensor towards the test analytes. System control, data acquisition and post
processing of the network analyzer measurements was performed using a LabView™ based
application. The significance of this research was based on the contribution that this sensing
system could enhance the detection of the toxic heavy metal ions to pico molar concentration
levels while conventional methods often work in the micro molar concentration levels.
Further, a piezoelectric based system which can be utilized for spray generation from a
desired liquid was designed and fabricated. A linear 128 Y-cut lithium niobate (LiNbO3)
crystal was used and a 3D printed stand was designed to pin the piezoelectric transformer on
the second resonance standing wave nodes. The piezoelectric transformer (PT) was actuated
by a signal generator connected to a radio frequency (RF) amplifier. To generate the spray,
various aqueous solutions prepared using deionized (DI) water was filled in an adjacent
reservoir and a paper bridge was placed from the reservoir and in contact with the surface of
the PT. The generated piezoelectric driven spray resulted in a broad area, uniform, continues
spray appropriate for coating applications. The spray generation system also made it possible
to generate the spray out of the liquid by applying around 15 Vamp, AC input voltage amplitude,
while traditional techniques typically require around 100 Vamp for spray formation.
Finally, the PT driven spray generation technique was used for membrane coating
applications. Different polymers such as poly(allylamine hydrochloride) (PAH)/ poly(styrene
sulfonate) (PSS) and poly(diallyl-dimethylammonium chloride) (PDADMAC) were
sequentially sprayed on to a polycarbonate track-etched (PCTE) membrane. The polymer
coated membrane was tested towards water permeability and ion rejection ratio to investigate
the functionality of this novel spray generation system for membrane coating purposes.
ii
ACKNOWLEDGEMENTS
In the beginning, I would like to express my sincere appreciation to Prof. Massood
Zandi Atashbar for all his help, advice and guidance trough 5 years of my research program.
I am very grateful of Prof. David B. Go who gave me an opportunity to pursue my
research work in University of Notre Dame as well as Dr. Paul Rumbach and Dr. Michael
Johnson, who passed their knowledge to me. Without their support, the second phase of this
dissertation would not have been possible.
My thanks also go to the other committee members Dr. Bradly Bazuin and Dr. Paul D.
Fleming for their time and comments.
I would like to show my appreciation to Dr. Dean Johnson who helped me through my
teaching experience in Western Michigan University.
I would like to express my great thanks to my fellow lab members Dr. Ali Eshkeiti, Dr.
Sai Guruva Reddy Avuthu, Dr. Amer Chlaihawi, Sepehr Emamian and Dinesh Maddipatla for
their support and friendship and specially Dr. Binu Narakathu who was not only a perfect co-
worker in the lab but also a dedicated teacher for me.
Finally, I would like to dedicate this dissertation to my family. To my dad who may not
always agree with me but when the time comes, supports me the most. To my mom and her
unconditional love. To my brothers who have taken care of all my responsibilities at home,
while I was pursuing my dreams overseas.
Zeinab Ramshani
Copyright by
Zeinab Ramshani
2017
iii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ........................................................................................... ii
LIST OF TABLES ....................................................................................................... vii
LIST OF FIGURES ................................................................................................... viii
CHAPTER
I. INTRODUCTION .................................................................................................. 1
1.1 Motivation ........................................................................................................ 1
1.2 Author’s Contributions ..................................................................................... 2
1.3 Dissertation Organization ................................................................................. 4
1.4 References ........................................................................................................ 6
II. LITERATURE REVIEW ....................................................................................... 9
2.1 Introduction ...................................................................................................... 9
2.2 10Fluid Manipulation Techniques for Sensing Applications ......................... 10
2.2.1 Toxic Heavy Metal Deficiencies ............................................................... 10
2.2.2 Sensors ....................................................................................................... 11
2.3 Fluid Manipulation Techniques for Spray Generation ................................... 17
2.3.1 Pneumatic Sprays ...................................................................................... 17
2.3.2 Acoustic Based Sprays .............................................................................. 17
2.3.3 Electrically-Driven Sprays ........................................................................ 20
2.4 Summary ........................................................................................................ 23
2.5 References ...................................................................................................... 24
iv
Table of Contents-Continued
CHAPTER
III. INVESTIGATION OF A PIEZOELECTRIC BASED SENSOR FOR TOXIC
HEAVY METAL DETECTION .......................................................................... 38
3.1 Introduction .................................................................................................... 38
3.2 Theory ............................................................................................................ 39
3.3 Experimental .................................................................................................. 42
3.3.1 Chemicals, Materials and Sample Preparation .......................................... 42
3.3.2 Synthesis of the Chemical Sensing Layer ................................................. 42
3.3.3 Sensor Fabrication ..................................................................................... 43
3.3.4 Flow Cell Fabrication ................................................................................ 45
3.3.5 Experiment Setup ...................................................................................... 46
3.4 Results ............................................................................................................ 46
3.5 Summary ........................................................................................................ 52
3.6 References ...................................................................................................... 53
IV. PIEZOELECTRIC TRANSFORMER BASED SPRAY GENERATIONSYSTEM ............................................................................................................... 59
4.1 Introduction .................................................................................................... 59
4.2 Experiments and Results ................................................................................ 60
4.2.1 Piezoelectric Transformer and Stand Design ............................................ 60
4.2.2 Spray Generation ....................................................................................... 61
4.2.3. Spray Coating Profilometry ....................................................................... 63
4.2.4 Volumetric Flow Estimation ..................................................................... 64
4.2.5 Linear Flow Speed Visualization ................................................................ 65
v
Table of Contents-Continued
CHAPTER
4.2.6 Bulk Motion of Droplets Directly on the PT Device .................................. 67
4.2.4 Measurement of Output Voltage ............................................................... 67
4.2.5 Effect of Liquid Conductivity on Spray Current ....................................... 70
4.2.7 Analysis of the Load Resistance and the Input Current .............................. 72
4.2.6 Effect of Liquid Conductivity and Surface Tension on the Onset
Voltage ................................................................................................................. 73
4.2.7 Chemical Solution Preparation .................................................................. 75
4.4 Summary ........................................................................................................ 75
4.5 References ...................................................................................................... 76
V. PIEZOELECTRIC TRANSFORMER BASED SPRAY GENERATION
FOR THIN FILM MEMBRANE COATING ...................................................... 79
5.1 Introduction .................................................................................................... 79
5.1.1 Spray Coating Techniques ........................................................................... 79
5.1.2. Membrane Coating Materials ..................................................................... 80
5.2 Experimental .................................................................................................. 81
5.2.1 Spray Generation Setup ............................................................................. 81
5.2.2 Material Preparation .................................................................................. 82
5.2.3 Moving Stage (Lazy Susan) ...................................................................... 82
5.2.4 Water Permeability and Salt Rejection Tests ............................................ 83
5.3. Results ............................................................................................................ 84
5.3.1. Bilayer coating of PAH and PSS ................................................................ 84
5.3.2. PDADMAC Spray Coating ....................................................................... 89
vi
Table of Contents-Continued
CHAPTER
5.4 Summary ........................................................................................................ 90
5.5 References ..................................................................................................... 90
VI. CONCLUSION..................................................................................................... 93
6.1 Conclusion ...................................................................................................... 93
APPENDICES
A. List of publications............. ............................................................................ 97
A.1 Inventions ........................................................................................ 97
A.2 Journal Papers .................................................................................. 97
A.3 Conference Papers ........................................................................... 98
B. Matlab code for scale used to graph the spray volumetric flow rate
used in Chapter 4 ......................................................................................... 100
vii
LIST OF TABLES
5.1 Water permeability and ion rejection results for PAH/PSS PT spray coating . 86
5.2 Water permeability and ion rejection results for PDADMAC PT spray coating .............................................................................................................. 89
viii
LIST OF FIGURES
Figure 2.1: Schematic of a planar IDE design SAW device ............................................. 15
Figure 2.2: Curved IDE designed SAW device ................................................................ 15
Figure 2.3: Flexible SAW device fabricated on a nanocrystal film ................................. 16
Figure 2.4: (a) Overcoming the surface tension to break the droplet in SAW devices.
(b) SAW device configurations. ....................................................................................... 18
Figure 2.5: Schematic of a typical electrospray system. High voltage applied between
the metal nozzle electrode and substrate generates the
spray from the precursor liquid delivered by the syringe. ................................................. 21
Figure 2.6: Paper spray setup used for mass spectrometry. .............................................. 23
Figure 2.7: Taylor cone formation in paper spray. ........................................................... 23
Figure 3.1: Schematic of a delay line SAW device with input and output IDEs .............. 40
Figure 3.2: Structure of QDPPZ ....................................................................................... 43
Figure 3.3: (a) Schematic of the SH-SAW device and (b) fabricated SH-SAW
sensor. ........................................................................................................................ 44
Figure 3.4: Flow cell with the SH-SAW sensor in the sensor groove (Inset: PDMS
microfluidic flow channel. ................................................................................................ 45
Figure 3.5: Experimental setup. ........................................................................................ 46
Figure 3.6: Magnitude (insertion loss) and phase of the frequency response for SH-
SAW sensor. ..................................................................................................................... 47
Figure 3.7 (a) SH-SAW sensor phase response (S21) towards varying concentrations of
PbNO3 and (b) Percentage change in frequency shift of SH-SAW sensor response...... .. 48
Figure 3.8: SH-SAW sensor phase response (S21) towards varying concentrations of
CdNO3 ........................................................................................................................ 50
Figure 3.9: SH-SAW sensor phase response (S21) towards varying concentrations of
Ni(NO3)2 ........................................................................................................................ 50
ix
List of Figures-Continued
Figure 3.10: SH-SAW sensor phase response (S21) towards varying concentrations of Hg(NO3)2 ................................................................................................................................................................................ 51
Figure 4.1: 3D printed PT stand to pin the device in L/4 of the tips. Conductive tape was used to apply the input voltage to the razor blade in contact with top and bottom electrodes. ........................................................................................................................ 61
Figure 4.2: (a) PT-driven electrospray configuration with solution delivery from a paper wick. (b) Geometric configuration of the PT with resulting standing displacement and stress wave. (c) Photograph of PT-generated electrospray of 50 mM NaCl solution. ................................................................................................................... 62
Figure 4.3: 3D profilometry of a glass slide coated by 1 µm beads. The roughness was determined to be 0.05 μm over an area of 15 mm × 5 mm. (b) SEM image of as
deposited particle ........................................................................................................... 64
Figure 4.4: Generated spray volume for 5 mM NaCl solution in DI water versus time. The slope of the curve fit indicates a constant volumetric flow rate of 20 µl/min over 5 min of applied constant 19 Vamp AC input voltage. The dash lines represent the standard deviation in the spray volume from three repeated experiments ...................................... 65
Figure 4.5: Front tracking of the red dye during spray generation ................................... 66
Figure 4.6: Distance of the front of red dye through the paper wick as a function of time with applied constant 20 Vamp AC input voltage at 59.91 KHz. The dotted line is a linear curve fit indicating a constant speed. ................................................................................ 66
Figure 4.7: Time evolution of droplet motion under a 13 Vamp, 59.9 kHz input to the PT. The droplet nearest the corner eventually translated to the edge of the PT, wetting it, but no spray was formed. The more central droplet only exhibited vibrational motion, but no bulk translation ...................................................................................................... 67
Figure 4.8: (a) PT output voltage measurement configuration. (b) PT output voltage as a function of the applied input voltage at a frequency of 60 kHz. .................................... 69
Figure 4. 9: PT driven generated spray current measurement configuration. ................... 70
Figure 4.10: (a) Logarithmic plot of output spray current for different (a) concentrations and (b) conductivity of HCl and NaCl in DI water with an applied voltage of 18 Vamp. The dash line in (b) is a linear curve fit with a coefficient of determination equal to r2 =0.76. ............................................................................................................... 71
x
List of Figures-Continued
Figure 4.11: Input current for different concentrations of HCl and NaCl in DI water for
an input voltage of 18 Vamp and slightly different frequency around 60 KHz dependent
on the liquid. ..................................................................................................................... 72
Figure 4.12: (a) PT equivalent circuit. (b) Simulated input and output current (I in , I out)
as a function of load resistance (RL) ................................................................................. 73
Figure 4.13: a) Logarithmic plot of the onset voltage for various concentration of NaCl
in DI water. The dash line is the linear curve fit with a slope equal to -0.32. b) Onset
voltage as a function of surface tension for various concentration of glycerol in DI
water ........................................................................................................................ 74
Figure 5.1: Sequential PT driven Spray coating of membrane using PAH and PSS by
placing the membranes on a Lazy Susan .......................................................................... 83
Figure 5.2: SEM images of PCTE membranes sprayed with different numbers of
bilayers. Different numbers of PAH/PSS bilayers were sprayed onto a 50 nm PCTE
membrane. The sprayed membranes were taped onto a SEM grid, sputtered with
iridium (2 nm), and visualized in SEM ............................................................................. 85
Figure 5.3: Logarithmic plot of the normalized water permeability vs. number
of bilayers......................................................................................................................... 87
Figure 5.4: Salt rejection vs. number of bilayers. A different number of PAH/PSS
bilayers were sprayed alternatingly with 10 s duration on a 50 nm PCTE membrane.
The salt rejection test was carried out in a dead-end filter cell filled with either 10 mM
KCl or 10 mM MgCl2. ....................................................................................................... 87
1
CHAPTER I
INTRODUCTION
1.1 Motivation
A steady effort has been made on the development of fluid manipulation techniques for
sensing and actuating systems. Conventional fluid manipulation mechanisms by external
pressure using pumps and air pressure are simple, but not precise1. Ultrasonic standing waves
produced by piezoceramic and sound reflector, have been used for fluid manipulations in a
variety of applications as a more accurate solution2. Nilsson et al. 3 have used this method in
separation of plasma and erythrocytes in blood stream based on particle gathering in the nodes
of the propagating ultrasonic standing wave. In this dissertation, the author focuses on two
aspects of the fluid manipulation applications: sensing and spray generation systems.
Among research developments in sensing systems, heavy metal detection systems have
been receiving an increasing attention due to its adverse effects in aquatic and atmospheric
ecosystems, even at the micro- or nano-molar concentration levels4,5. In the case of aqueous
systems, this eco- and phyto toxicity on vital microorganisms is influenced by environmental
factors6. Traditional detection methods for toxic heavy metals, mostly found in their ionic form
in nature, include spectrometric sensing systems or biological / chemical sensors.7,8,9,10,11,12,13.
Spectrometric sensing systems work based on the interaction between applied electromagnetic
radiation and the heavy metal ions, while the chemical and biological sensors rely on the change
on a specific ligand’s functionality in the presence of the heavy metal ions. However, these
techniques often need trained staff and they are complicated as well as time consuming14.
Therefore, it is necessary to develop sensing systems that are easy-to-use, cost effective, highly
sensitive and employ rapid detection techniques.
2
Various mechanisms can be used to manipulate and atomize fluids, from simple
approaches like a pneumatic atomizer, in which there is no control on the droplet size, to more
precise ones such as an acoustic based atomizer. Surface acoustic wave devices are proven to
work well for atomizing the desired aqueous sample in chemical analysis approaches and mass
spectrometry. However, they are often complicated to fabricate and need around 100 Vamp input
voltage15. To get an electrically driven spray, approximately 1-5 KVamp has to be applied to the
liquid to form the droplet plume in electrospray systems16 . Among all the drawbacks in
conventional spray generation systems, the high amount of required applied voltage can be a
significant reason for illustrating the need for more research in fluid manipulation.
Piezoelectric devices produce mechanical strain under applied electrical field which
results in propagating mechanical vibrations17. Acoustic waves have been used to manipulate
the liquid medium in sensing technologies, due to its benefits such as small sample volume,
high sensitivity and bio compatibility 18 , 19 , 20 . The high sensitivity arises from the large
electromechanical coupling factor of the piezoelectric crystal21. Moreover, the aforementioned
features make piezoelectric devices an apt choice in actuating systems such as aerosolization.
The mechanical vibration along with the large polarization produced on the surface of the
piezoelectric crystal can be used to overcome the liquid film surface tension and break it into
the droplets.
1.2 Author’s Contributions
In the first project, described in chapter3, the author designed and fabricated a surface
acoustic wave device, contributed in synthesis of proper sensing layer and conducted some
experiments in order to investigate the functionality of the system towards lead, mercury,
cadmium and nickel ions in pico molar concentration in water. From this part of research,
author published a decent journal paper along with 5 conference papers.
3
In the second project, described in chapter 4, author designed a novel piezoelectric
transformer based spray generation system. Author was responsible for design and fabrication
of the proper piezoelectric transformer and stand fabrication for fluid manipulation and spray
generation from a liquid reservoir. The author then performed some experiment to measure the
various spray parameters such as current and voltage and to study the effect of liquid properties
on the generated spray characteristics. From this research work, author published an
intellectual property, a patent and a decent journal paper.
In the third project, described in chapter 5, author found a practical use for the PT driven
spray system. The capability of the system was examined towards the membrane coating. The
author conducted different experiments to collect adequate data which was proving the
reliability of this approach. Form this part of the research, author was published an intellectual
property, a patent and a paper.
Overall, the author’s research work has resulted in two intellectual properties
disclosures, two patent applications, three high quality peer reviewed journal and six
conference publications mentioned in Appendix A. The results of the projects have been
published in prestigious journals such as Sensors and Actuators: B Chemical; Applied Physics
Letter and Analytical Methods. The research outcomes have been shared with other scholars in
this field during the author’s presentation and proceedings in international conferences such as
IEEE Sensors Conference (2013, 2015, 2016), Electro/Information Technology (EIT) (2013),
International Meeting for Chemical Sensors (IMCS) (2014), Frequency Control Symposium &
the European Frequency and Time Forum (FCS) (2015) and Electrostatic Society of America
(ESA) (2016). The author also was awarded the All-University Graduate Research and
Creative Scholar Award for 2016-17 by Western Michigan University.
4
1.3 Dissertation Organization
This dissertation presents details of the research projects that was performed, including
developing piezoelectric based systems for fluid manipulation and investigating the capability
of the designed piezoelectric based systems for sensing applications and spray generation.
In Chapter 2, the literature review is provided. This chapter presents a brief introduction
to conventional fluid manipulation systems used for sensing and spray generation. The basic
definition of sensor followed by previous sensing systems that have been successfully used for
toxic heavy metal detection in liquid media are discussed. Traditional techniques of spray
generation are also reviewed in this chapter. The drawbacks of these systems which led the
author to develop a piezoelectric based fluid manipulation system in this dissertation, are also
included in the discussions.
In Chapter 3, a project on developing a piezoelectric sensing system is presented. An
efficient piezoelectric transformer was fabricated for the detection of heavy metal compounds.
An acrylic flow cell, which consists of inlet and outlet ports for the microfluidic chamber as
well as polydimethylsiloxane (PDMS) based microfluidic channels was used to provide a user
friendly, re-usable sensing system. A chemical sensing layer was utilized to selectively bind to
heavy metal ions. The capability of the system for detecting varying concentrations of heavy
metal ions, such as lead (Pb), cadmium (Cd), mercury (Hg) and nickel (Ni) in liquid
environments, through phase shifts in the frequency response of the piezoelectric sensor, was
studied. This work demonstrated the capability of the developed system to detect heavy metal
compounds at pico molar concentration levels.
In Chapter 4, a project for developing a novel spray generation system using a
piezoelectric transformer is presented. The author demonstrated the use of a piezoelectric
transducer to induce the generation of a broad area electrospray. A piezoelectric transducer was
designed and top and bottom electrodes was patterned to generate adequate output voltage from
5
an applied input voltage. A proper PT stand was designed and 3D printed to access the
piezocrystal second resonant frequency. A continues broad area spray was formed by providing
a persistent liquid film using a paper bridge connected to a liquid sample reservoir. The effect
of various liquid parameters on the generated spray was studied
In Chapter 5, the piezoelectric transformer based spray production system was used for
membrane coating. The capability of the system for spray formation out of some high demand
coating materials was investigated. Polymer coating of a track etched membrane was chosen
due to the recent high demand. Solutions of these polymers were sprayed using the PT_ driven
spray system. Coated membranes were tested for the water permeability and ion rejected ratio.
The outcomes will be presented to indicate the abilities and limitations of the system for this
purpose.
Finally, in Chapter 6, a conclusion of the research work and suggestions for the future
works are presented.
6
1.4 References
1 Teh, Shia-Yen, Robert Lin, Lung-Hsin Hung, and Abraham P. Lee. "Droplet
microfluidics." Lab on a Chip 8, no. 2 (2008): 198-220.
2 Bruus, Henrik, Jurg Dual, Jeremy Hawkes, Martyn Hill, Thomas Laurell, Johan
Nilsson, Stefan Radel, Satwindar Sadhal, and Martin Wiklund. "Forthcoming Lab on a Chip
tutorial series on acoustofluidics: Acoustofluidics—exploiting ultrasonic standing wave forces
and acoustic streaming in microfluidic systems for cell and particle manipulation." Lab on a
Chip 11, no. 21 (2011): 3579-3580.
3 Nilsson, Andreas, Filip Petersson, Hans W. Persson, Henrik Jönsson, and Thomas
Laurell. "Autologous blood recovery and wash in microfluidic channel arrays utilizing
ultrasonic standing waves." In Micro Total Analysis Systems 2002, pp. 625-626. Springer
Netherlands, 2002.
4 Rooney, Corinne P., Fang-Jie Zhao, and Steve P. McGrath. "Phytotoxicity of nickel
in a range of European soils: Influence of soil properties, Ni solubility and
speciation." Environmental pollution 145, no. 2 (2007): 596-605.
5 Eshkeiti, A., Binu Baby Narakathu, A. S. G. Reddy, A. Moorthi, M. Z. Atashbar, E.
Rebrosova, M. Rebros, and Margaret Joyce. "Detection of heavy metal compounds using a
novel inkjet printed surface enhanced Raman spectroscopy (SERS) substrate." Sensors and
Actuators B: Chemical 171 (2012): 705-711.
6 Lock, Koen, and Colin R. Janssen. "Ecotoxicity of nickel to Eisenia fetida,
Enchytraeus albidus and Folsomia candida." Chemosphere 46, no. 2 (2002): 197-200.
7 Abebe, Fasil A., Carla Sue Eribal, Guda Ramakrishna, and Ekkehard Sinn. "A ‘turn-
on’ fluorescent sensor for the selective detection of cobalt and nickel ions in aqueous
media." Tetrahedron Letters 52, no. 43 (2011): 5554-5558.
7
8 Hu, Bo, Lin-Lin Hu, Ming-Li Chen, and Jian-Hua Wang. "A FRET ratiometric
fluorescence sensing system for mercury detection and intracellular colorimetric imaging in
live Hela cells." Biosensors and Bioelectronics 49 (2013): 499-505.
9 Trieu, Khang, Emily C. Heider, Scott C. Brooks, Fernando Barbosa, and Andres D.
Campiglia. "Gold nanorods for surface plasmon resonance detection of mercury (II) in flow
injection analysis." Talanta 128 (2014): 196-202.
10 Quang, Duong Tuan, and Jong Seung Kim. "Fluoro-and chromogenic
chemodosimeters for heavy metal ion detection in solution and biospecimens." Chemical
reviews 110, no. 10 (2010): 6280-6301.
11 Knecht, Marc R., and Manish Sethi. "Bio-inspired colorimetric detection of Hg2+ and
Pb2+ heavy metal ions using Au nanoparticles." Analytical and bioanalytical chemistry 394,
no. 1 (2009): 33-46.
12 Gammoudi, Ibtissem, Hakim Tarbague, Ali Othmane, Daniel Moynet, Dominique
Rebière, Rafik Kalfat, and Corinne Dejous. "Love-wave bacteria-based sensor for the detection
of heavy metal toxicity in liquid medium." Biosensors and Bioelectronics 26, no. 4 (2010):
1723-1726.
13 Kozlova, Tatiana, Chris M. Wood, and James C. McGeer. "The effect of water
chemistry on the acute toxicity of nickel to the cladoceran Daphnia pulex and the development
of a biotic ligand model." Aquatic Toxicology 91, no. 3 (2009): 221-228.
14 Hossain, SM Zakir, and John D. Brennan. "β-Galactosidase-based colorimetric paper
sensor for determination of heavy metals." Analytical chemistry 83, no. 22 (2011): 8772-8778.
15 Rajapaksa, Anushi, Aisha Qi, Leslie Y. Yeo, Ross Coppel, and James R. Friend.
"Enabling practical surface acoustic wave nebulizer drug delivery via amplitude
modulation." Lab on a Chip 14, no. 11 (2014): 1858-1865.
8
16 Törnkvist, Anna, Stefan Nilsson, Ardeshir Amirkhani, Lena M. Nyholm, and Leif
Nyholm. "Interference of the electrospray voltage on chromatographic separations using
porous graphitic carbon columns." Journal of mass spectrometry 39, no. 2 (2004): 216-222.
17 Park, Gyuhae, Hoon Sohn, Charles R. Farrar, and Daniel J. Inman. "Overview of
piezoelectric impedance-based health monitoring and path forward." The Shock and Vibration
Digest 38, no. 2 (2003): 91.
18 Luo, J. K., Y. Q. Fu, Y. Li, X. Y. Du, A. J. Flewitt, A. J. Walton, and W. I. Milne.
"Moving-part-free microfluidic systems for lab-on-a-chip." Journal of Micromechanics and
Microengineering 19, no. 5 (2009): 054001.
19 Renaudin, Alan, Vincent Chabot, Etienne Grondin, Vincent Aimez, and Paul G.
Charette. "Integrated active mixing and biosensing using surface acoustic waves (SAW) and
surface plasmon resonance (SPR) on a common substrate." Lab on a Chip 10, no. 1 (2010):
111-115.
20 Wang, Zhuochen, and Jiang Zhe. "Recent advances in particle and droplet
manipulation for lab-on-a-chip devices based on surface acoustic waves." Lab on a Chip 11,
no. 7 (2011): 1280-1285.
21 Kondoh, Jun, Takao Muramatsu, Tetsuo Nakanishi, Yoshikazu Matsui, and Showko
Shiokawa. "Development of practical surface acoustic wave liquid sensing system and its
application for measurement of Japanese tea." Sensors and Actuators B: Chemical 92, no. 1
(2003): 191-198.
9
CHAPTER II
LITERATURE REVIEW
2.1 Introduction
Fluid manipulation at the microfluidic scale plays a significant role in developing lab-
on-a-chip functional technologies. These miniaturized structures, need small volumes of
reagents and result in a fast and accurate response due to their enhanced sensitivity that is
critical for sensing systems1. In this chapter, the author has reviewed conventional methods
used for fluid manipulation in sensing or spray generation systems. After a brief explanation
of sensor basics and the elevating need for improvements in toxic heavy metal sensing systems,
due to their environmental drawbacks, the author introduces some traditional methods of toxic
heavy metal sensing systems (spectrometric, chemical/biological and acoustic sensors). The
reasons behind the author’s choice of the sensing method further in this dissertation, is also
discussed.
The well proved techniques of spray generations are also reviewed in this chapter.
Despite all the methods described (pneumatic, acoustic and electrical spray), the author
explains the necessity for further improvement in spray formation technology for specific
applications.
10
2.2 Fluid Manipulation Techniques for Sensing Applications
This section provides a brief historical background about conventional fluid
manipulation techniques for sensing applications. Sensors mostly produce a mechanical or
electrical signal in response to the change in their physical properties. Sensors have been widely
used in various applications since their first discovery in the early 19th century2. Among the
different usage of sensors, the author is focusing on the sensing systems that target toxic heavy
metal compounds in aqueous media.
2.2.1 Toxic Heavy Metal Deficiencies
Heavy metals are the elements with atomic weight between 63 and 209 g/mol in the
periodic table of elements3. The toxic effect of heavy metals has been a growing concern in the
biomedical and environmental industries, and hence a major focus of research in the
development of different sensing systems. Although, heavy metals are typically formulated as
non-toxic compounds, it has been shown that they can cause serious health issues even at micro
molar concentration levels4.
The rising demand for heavy metals in applications such as plating, mining, solar cells,
pharmaceutical and chemical industries intensifies their role as environmental pollutants5,6.
Toxic heavy metals can be released in the environment and pollute the area for a long time
during their mining process 7 . Some toxic heavy metals are also used to mine the other
materials8. These toxic ions accumulate in human body since they are not biodegradable like
organic contaminants9. In plants, toxic heavy metals can affect the root growth as well as water
entry and exit and cause some deficiencies in their physiological functions 10 . The extra
aggregation of the toxic heavy metals in vegetables from contaminated water can aafect food
safety11. The detection of these heavy metal compounds is thus of utmost importance.
11
2.2.2 Sensors
An electronic sensor is defined as a device which responds to a stimulant by producing
an electrical signal12. The sensor converts a physical property to a readable signal which makes
it possible to record the data for future analysis. The output of the sensor can be used to trace
any changes in amplitude, phase or frequency of the voltage or the current of the electrical
signal. Sensors can be classified as active or passive. An active sensor requires additional
energy source other than the physical property which being sensed, however a passive sensor
does not need any external source of energy for sensing activities13.
For example, there are passive and active sensors designed for satellites14 . Active
sensors, such as radar, use the electromagnetic radiation as the external source and compares
the sent and received signals to measure the distances to objects. On the other hand, a satellite’s
passive sensors may utilize reflected sunlight or thermal radiation.
2.2.2.1 Spectrometric Sensors
Spectrometric sensing systems are categorized as optical sensors, where the sensor is
designed to use a light source to detect the changes in the measurand. These sensing systems
use electromagnetic radiation and record changes in the intensity as a function of the frequency
or wavelength15.
After the initial launch of the atomic absorption spectrometer in the 19th century, an
enhanced cold vapor atomic absorption spectroscopy (CVAAS) was developed as a technique
for heavy metal ion detection. Hatch and Ott transferred mercury ions to the spectrometer
optical measurement by neutralizing the ions with stannous chloride to generate the vapor form
of mercury16. A pure, dry gas such as air or argon is used to carry the mercury vapor. The light
12
absorption of mercury is directly proportional with the ion concentration. However, any other
ions in the inert gas, which can reduce the florescence emission, can affect the results.
A faster spectrometric approach for heavy metal detection, when compared to the
atomic adsorption technique, is inductively coupled plasma mass spectroscopy (ICP-MS).
Argon atoms are ionized by being placed inside an electromagnetic field generated by an RF
oscillation circuit producing a plasma discharge. This can ionize an aerosolized heavy metal
sample, which is introduced to the discharge and connected to the mass spectrometer for heavy
metal ion detection17. Micro molar level concentration of heavy metal has also been detected
using inductively coupled plasma atomic emission spectrometry (ICP-AES). Excited atoms
and ions produced by inductive coupled plasma emit electromagnetic radiation at wavelengths
that are related to the specific material18.
Even though these methods have reliable results, they are relatively expensive, time
consuming and need trained personnel.
2.2.2.2 Chemical and Biological Sensors
A chemo/bio sensor records a readable signal in response to a controlled binding event
by combining a chemical/biological sensing element with a transducer19,20. Prasad et al. 21
demonstrated that specific antigen antibody binding can be used to detect low concentrations
of environmental and physiological hazardous agents for neural studies. Metal ions can change
the enzyme function, suppressing them. The immobility of enzyme oxides or proteins have
been used to detect micro molar concentration levels of the toxic heavy metal compounds in
liquid media22,23. The need to develop alternative techniques of enzyme based sensors seems
to be necessary due to enzyme price, time consuming reactions and the required complex
enzyme reactions since the pollutants could not be recognized with a simple enzyme
reaction24,25. For this reason, Yamasaki et al.26 implemented a microbial based sensing system,
13
for copper detection in water. The presence of copper was found to suppress the bacterial
growth and a detection limit was reported to be in the milimolar concentration level. However,
it is worth mentioning that the use of microbes in this detector can be hazardous to the user.
2.2.2.3 Acoustic Sensors
In acoustic sensors, the change in the environment can be detected by the investigation
of an acoustic wave, which propagates throughout the media. These types of sensors are
typically based on piezoelectricity, which involves electrical to mechanical vibration
conversion and vice versa. Piezoelectric sensors have been used in various sensing applications
for measuring the changes in the propagating acoustic wave characteristics, such as magnitude
or phase shift in the frequency response27. An example of an acoustic sensor is a quartz crystal
microbalance (QCM) sensor, consisting of a quartz crystal and metal electrodes, that shows a
shift in resonant frequency with any change in the applied mass 28 . QCMs have been
successfully tested for heavy metal ion detection in liquid media at milimolar concentration
levels29.
A surface acoustic wave (SAW) device contains patterned electrodes on the top
piezoelectric substrate. Even though different SAW device configurations have been used,
since its discovery by Lord Rayleigh in 188530, the use of interdigital electrodes (IDEs) or
interdigital transducers (IDTs) and reflectors by White and Voltmer in 196531 and Staples et
al. in 197432, respectively have proven to minimize the power dissipation and hence obtain
optimized SAW generation and detection. Several advantageous features such as small size,
high resonant frequency, low power consumption and compatibility with CMOS technology,
makes them an appropriate choice when compared to conventional sensing systems33, 34.
SAW devices have been typically used in the Rayleigh, Lamb, shear horizontal (SH-
SAW) and Love propagation modes35. In the Rayleigh propagation mode, particle displacement
14
has two components: one is parallel to the SAW propagation direction and the other is normal
to the substrate, thus forming an elliptical trajectory for the particle. Most of the SAW energy
is confined to the surface of the substrate and is often diminished to zero at a substrate depth
of 4 to 5 times the wavelength36. Due to the normal component of the particle displacement
and interference with other media, above the substrate surface, this mode is not suitable for a
liquid environment.
The Lamb wave37 is similar to the Rayleigh wave and are typically generated using
very thin substrates that are only a few wavelengths thick.38 Like the Rayleigh waves, Lamb
waves have both normal and shear components and can be used for gas sensing, but they are
also well-suited for liquid sensing. Due to the low phase velocity of Lamb waves, there is no
energy penetration inside liquid media placed on the SAW surface, and hence no acoustic
streaming.
On the other hand, in the SH-SAW propagation mode, the particle displacement
happens only in parallel with the substrate surface, which prevents the vibration vector from
transferring into any secondary media and hence prohibits SAW energy attenuation when the
substrate comes into contact with a liquid39. Knodoh et al. proved SH-SAW liquid sensing
system’s capability by detecting various types of Japanese tea. The liquid properties were
evaluated by monitoring the frequency shift and amplitude change of the SH-SAW sensor
output40. SH-SAW sensor, has also been used for investigating oil contamination of ground
water by detecting hydrocarbons in micro molar concentration levels41. The use of the SH-
SAW device is thus a promising solution for heavy metal compound detection in liquid
environments.
Often, a thin-film called a guiding layer is applied to the surface of the device to activate
a Love wave mode42, which also inhibits radiation of acoustic pressure into the liquid making
Love wave operation the most common for sensing in liquids.43
15
Devices that can generate SH-SAWs, with much less bulk leakage, began to appear in
the 1990s, and often use a Love thin-film wave guide to further prevent leakage into a liquid
on the surface.44,45,46,47
The SAW device is operated at a designed resonant (or center) frequency (f) imposed
by the IDE design, wherein a typical IDE the electrode spacing is an integer multiple or 1/2,
1/3, or 1/4 of the wavelength = 2π/k, with k being the wavenumber and wavelengths typically
between 0.01 mm to 1.0 mm.
A variety of IDE designs have been implemented to affect both the resonance and the
propagation of the SAW. The most basic is a planar design, where the IDE consists of simple
linear electrode ‘fingers’ and the interdigitated spacing is consistent for each finger pair, as
shown in Figure 2.1.
Figure 2.1: Schematic of a planar IDE design SAW device48.
Figure 2.2: Curved IDE designed SAW device49.
16
However, other clever IDE designs are possible, such as curved or focused IDE (Figure
2.2) configurations to produce focused SAWs 50 or variable-spacing ‘chirped’ IDEs that
produce multiple resonant frequencies.51
A single IDE design is generally used to generate planar traveling waves that propagate
along the surface of the device, called a traveling wave, but sets of two or more opposing IDEs
can also be used to set up constructive interference and produce standing surface waves. To
inhibit reflections and scattering off the edges of the SAW substrate and minimize wave
reflection interference, acoustic absorbents such as gels (e.g. alpha gel) are often applied to the
edges of the piezoelectric substrate.52
Ultimately, the IDE design depends heavily on the nature of the SAW device and the
application in question. The progress in SAW device fabrication methods have focused on
developing novel flexible substrates instead of conventional, rigid single-crystal piezoelectric
substrates. Jin et al. developed flexible SAW devices which operate in Rayleigh and Lamb
wave propagation mode by depositing ZnO nanocrystals on bendable Kapton polyimide
films.53
Figure 2.3: Flexible SAW device fabricated on a nanocrystal film54.
17
2.3 Fluid Manipulation Techniques for Spray Generation
Spray technologies for the atomization of fluids play an important role across a vast
range of technologies ranging from drug delivery 55 and chemical analysis 56 to combustion
systems57 and material coatings58. In the biomedical field, aerosolized drug delivery for cystic
fibrosis 59, pneumonia60, influenza61, respiratory tract infections62 and pulmonary aspergillus63
have been under clinical trials. The results illustrate the controllable and concentrated drug
delivery in targeted regions which positively affects the dosing and duration of the therapeutic
procedures. Various mechanisms can be used to atomize fluids including pneumatic 64 ,
acoustic65 and electrically-driven sprays66.
2.3.1 Pneumatic Sprays
Pneumatic sprays work based on the interaction of two fluid phases, gas and liquid.
Compressed and high velocity gas is pressurized into a liquid jet to produce the spray. These
types of atomizers are able to produce the spray out of viscous materials at high throughput,
which makes them a subject of interest in painting, air brushes and fuel combustion systems67.
This atomizer structure can be designed to be simple and cost effective. However, there
is no precise control over the resultant aerosol size and hence the thickness and uniformity of
the covered area68. The characteristics of the spray are drastically affected by the size, geometry
and location of the liquid nozzle69,64,70, 71.
2.3.2 Acoustic Based Sprays
After Wood and Loomis made the first acoustic based spray system in 1927, Sollner
explained that the mechanical vibration of the surface causes the cavitation beneath the surface
of the liquid, which results in mist formation72. Lang showed that when an ultrasonic transducer
18
bridges between liquid and air, generated acoustic waves produce capillary waves that are able
to atomize the liquid73 . Since then, ultrasonic atomizers have been used for a variety of
applications such as plastics, coatings or in more advanced spray production system designs,
for organic film deposition74,75.
Piezoelectric crystals convert the electrical signal to mechanical vibrations; hence they
are apt choices for this approach. Surface acoustic wave (SAW) devices, explained in part
2.2.2.3, are well-known spray generation techniques. The delivered electrical signal to the
SAW device converts to particle displacement and vibrations due to the piezoelectric features
of the substrate. A sufficient amount of the input electrical signal can initiate particle
displacement in the axis normal to the surface; big enough to leak the energy into the droplet
or liquid film placed on the substrate, as shown in Figure 2.4(a).
Figure 2.4: (a) Overcoming the surface tension to break the droplet in SAW devices. (b) SAW
device configurations76.
a)
b)
19
The pressure difference and induced capillary wave can destabilize the liquid surface
and overcome the surface tension at a specific threshold to break the liquid into atomized
droplets. In a typical SAW device, this threshold can be achieved by delivering around 100 V
amplitude sinusoidal input voltage77.
SAW based spray generation has been used and available since 1990. 78 , 79 , 80 The
physical mechanism behind the spray generation and the designing optimization for SAW
atomizer have been well documented in the literatures.81,82,83
Friend, Yeo and collaborators published several papers about the use of microscale
aerosolized droplets generated by SAW devices in pharmaceutical84 and chemical85 industries.
It was also demonstrated that the resonant frequency of the SAW device and the geometrical
configuration of the interdigital electrodes (IDEs), shown in Figure 2.4(b), can affect the size
of the droplets, the uniformity of the mist and the onset voltage for atomization that is
achievable by SAW device design and fabrication86,.
The resonant frequency of the SAW device is a function of the width of the metalized
IDEs patterned on the piezoelectric substrate, calculated by87
𝑓 ∝1
4𝑤(2.1)
where w is the width of the IDE. The radius (R) of the generated aerosol using the SAW
atomizer, on the other hand, this is88
𝑓 ∝1
𝑅(2.2)
Therefore, a fabricated SAW device with high center frequencies (in the MHz range)
can produce relatively small and mono-dispersed atomized droplets. For atomization
applications, the shape of the IDEs can play a major role on the minimum required input
voltage, known as onset voltage. This is because patterning specific IDE configurations will
enable the acoustic power to be focused on a desired point of the substrate89. However, SAW
20
device fabrication means patterning micron size metalized fingers on the piezoelectric substrate
in a clean room, which can be complicated, time consuming and expensive.
2.3.3 Electrically-Driven Sprays
Electrically-driven sprays, where a high voltage is applied to a liquid flow or film, have
a long and distinguished history90,91. To this day, electrosprays are widely used for mass
spectrometry systems92 and emerging applications include micro-propulsion93, spray coating94,
materials synthesis95 and printing96.
In electrosprays, positive and negative ions get separated by the electrical field. In the
late 16th century, William Gilbert discovered the effect of the electrical field on water droplets,
and in the mid-20th century, Sir Geoffrey Ingram Taylor modeled the shape of the cone made
by applying the electrical field to the droplets97. A high voltage between 1 to 20 kV is applied
between a nozzle electrode and a flat grounded electrode (Figure 2.5). The liquid is provided
continually to the nozzle electrode and a substrate is placed on the grounded electrode for thin
film formation. When the input voltage, applied to the liquid in the nozzle electrode, crosses
the threshold, the liquid is drawn into a cone. The liquid reaching the nozzle electrode tip forms
a Taylor cone, which emits a liquid jet through its apex due to charge accumulation at the apex
of the cone which overcomes the surface tension of the liquid.
In fact, Taylor cones are the result of the electrified interface between the conductive
liquid and air, when the charge reaches a critical level. Initially, a 100 µm size thin liquid jet
and eventually a series of small and highly charged liquid droplets will be formed. The droplet
diameter is directly proportional to liquid mass density and indirectly proportional with liquid
conductivity and surface tension98. One limitation of most electrospray systems is the nature
of the droplet plume, which typically arises from the tip of a liquid cone (Taylor cone99 or cone-
21
jet100) and spreads conically to cover an area101. Thus, the projected area of the droplet plume
is circular, which is not ideal for applications such as uniform coating.
Figure 2.5: Schematic of a typical electrospray system102. High voltage applied between the metal
nozzle electrode and substrate generates the spray from the precursor liquid delivered by the syringe.
The size of this plume can be affected by the spray and solution conditions 103 .
Alternative strategies, such as using alternating current (AC) fields, can also be used to generate
linear jets of droplets rather than plumes104. Yet, developing ways to generate a uniform, broad
area droplet plume could be important for applications such as spray coating.
As another type of electrically-driven spray formation, paper spray is an ambient
ionization method that is well known as a reliable, time efficient, easy way for mass
spectrometry with no sample preparation105. Similar to electrospray ionization, it appears to
produce charged droplets via Taylor cones. Typically, 3-5 kV input voltage is required and
applied to a piece of paper which contains the desired liquid, via copper clips to make the spray
out of the liquid as shown in Figure 2.6106. Delivering a sufficient electrical field will produce
charged droplets by liquid breakage due to Columbic forces according to107
𝐹 =𝑉
𝑟 (2.3)
22
where V is the applied voltage and r is the radius of the cut paper knowing that the
minimum necessary field strength for field ionization which is around 107 Vcm-1 107.
For this purpose, various types of papers, ranging from chromatographic paper to fiber
glass papers, have been cut to have a sharp point with a tip angle between 60° to 150° and used
for paper spray108. Research has shown that fiber glass paper shows poor performance when
compared to chromatographic papers109. The sharp tip of the paper can provide a higher electric
field as well as help in microfluidic transportation110 . High resolution images and phase
Doppler analysis have demonstrated the mechanism behind paper spray ionization by Espy et
al.107. High speed photographic images suggested this phenomenon occurs in two separate
modes depending on the solvent flow rate. Taylor cones are formed and numbered in the
(Figure 2.7). The first mode happens for high solvent flow rates by applying 3.5 to 4.5 kV and
0.1 to 0.2 µA, where Taylor cones result in a wide range of droplet size formations. After a
sufficient amount of solvent is depleted, mode two will occur. In this mode, the solvent flowrate
is smaller and the current is higher when compared with the first mode; also, corona discharge
generates smaller and monodispersed droplets. The short spitting mode in paper spray makes
it a proper method for mass spectrometry but not practical in procedures that needs a longer
endurance such as coating applications.
23
Figure 2.6: Paper spray setup used for mass spectrometry 111.
Figure 2.7: Taylor cone formation in paper spray107. Cones are shown with numbers.
2.4 Summary
This chapter provided a brief background and introduction to fluid manipulation in
sensing and spray generation systems. The conventional systems for sensing toxic heavy metal
ions in liquid media were introduced. The conventional sensing systems are expensive, time
consuming and need complicated procedures with trained staff which suggests introducing a
new procedure that overcomes these drawbacks. Spray generation technologies were also
24
summarized in this chapter. Based on the high applied voltage, small area coverage and non-
continuity of the spray generated by the conventional systems, this field of study still needs an
innovative idea that overcomes these deficiencies. The author will discuss how piezoelectric
devices can be a proper choice to design new systems for sensing and spray generation
techniques that conquer the obstacles of the traditional approaches.
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38
CHAPTER III
INVESTIGATION OF A PIEZOELECTRIC BASED SENSOR FOR TOXIC
HEAVY METAL DETECTION
3.1 Introduction
Some of the most common heavy metal pollutants are lead (Pb) and cadmium (Cd)1.
Pb has been categorized as a neurotoxic element due to its severe impact on the nervous system,
causing neurotoxicity and nephrotoxicity in the gastrointestinal and renal systems2,3,4,5. Cd,
which has been increasingly used in industrial product lines, is known to cause kidney failure,
hypercalciuria and osteoporosis6,7,8. Mercury (Hg) is the second most toxic heavy metal in the
planet. Hg is a neurotoxicant, which targets the central nervous system as well as liver and
heart muscles in the human body9. Nickel (Ni) has been seeing a rising demand in the surface
coating and jewelry industries. Ni is a carcinogenic contaminant, which causes pulmonary
fibrosis, asthma, lung and nasal cancer in case of long term exposures10. All these elements are
often released from natural and manmade sources and have the capability to accumulate in the
vital organs and cause fatalities11,12.
Heavy metal detection methods including spectrometric techniques such as cold vapor
atomic absorption spectroscopy, inductively coupled plasma mass spectroscopy,
electrochemical impedance spectroscopy, UV-visible absorbance spectroscopy, x-ray, laser
and colorimetric analysis as well as biosensing detection techniques based on enzyme
activation or bacteria have been reported13,14,15,16,17,18,19,20. However, these techniques are often
expensive, time consuming, laborious, hazardous and require complex labeling methods.
39
Therefore, it is necessary to develop sensing systems with easy-to-use, cost effective, highly
sensitive and rapid detection techniques.
In this work, the author has designed and fabricated an efficient piezoelectric device,
consisting of gold (Au) interdigital electrodes (IDEs) on a 64° YX-LiNbO3 based piezoelectric
substrate known as a shear horizontal surface acoustic wave (SH-SAW) sensor, for the
detection of heavy metal compounds. The main focus of the author was to design a system
which has a small limit of the detection to be able to show a recordable signal for low
concentrations of the toxic heavy metals in liquid media. The range of the detected heavy
metals by this system was desired to be as wide as possible. Proper chemical sensing layer,
Phenol or naphtho[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione (QDPPZ) are used to
selectively bind to the heavy metal ions. An acrylic based flow cell was used to hold the sensor
and PDMS microfluidic channel. The capability of the system for detecting different
concentrations of heavy metal compounds, such as lead nitrate (PbNO3), cadmium nitrate
(CdNO3), mercury (II) nitrate, (Hg(NO3)2) and nickel (II) nitrate (Ni(NO3)2) in liquid
environments, through phase shifts in the frequency based response (S21) of the SH-SAW
sensor are investigated.
3.2 Theory
For sensing applications, the SAW devices typically work at higher frequencies (~100-
500 MHz) whereas most microfluidic devices operate at lower frequencies (~10-100 MHz).
The most common piezoelectric substrates are single crystal lithium niobate (LiNbO3), lithium
tantalate (LiTaO3), and quartz (SiO2) with common cuts including 128 y-x cut for LiNbO3 and
ST and AT cuts for quartz substrates.
The piezoelectric based SH-SAW devices, generally consists of two sets of metalized
input and output IDEs mounted on a piezoelectric substrate. The surface acoustic wave is
40
generated by an impulse or sinusoidal input electrical signal applied at the input IDEs. It then
propagates on the piezoelectric substrate through the delay line, which is the distance between
the input and output IDEs and is converted back to an electrical signal, as shown in Figure 3.11.
Figure 3.1: Schematic of a delay line SAW device with input and output IDEs.
The SH-SAW device output signal can finally be obtained from the output IDEs. The
SAW resonant frequency can be mathematically calculated by21
𝑓0 =𝑣𝑠
𝜆 (3.1)
𝜆 = 4×𝑤 (3.2)
where, f0 is the resonant frequency of the SAW, vs is the SAW propagation velocity on
the piezoelectric substrate, λ is the SAW wavelength and w is the width of the IDEs. The
resonant frequency of the SH-SAW sensor is dependent on the velocity of the acoustic wave
passing through the piezoelectric substrate. The velocity of the wave is known to vary due to
mass, stiffness, conductivity, dielectric coefficient, temperature and pressure changes22. The
absorbance of the different concentrations of toxic heavy metals by the sensitive layers, which
are coated on top of the piezoelectric substrate, causes a mass loading effect to take place. mass
loading also changes the SAW propagation velocity and thus the resultant center frequency,
which can be used to quantify the concentration levels23.
41
The principle behind SAW-based sensing devices is that the target chemicals that
adsorb on a functionalized surface on the SAW substrate fundamentally change the frequency
of the SAW, and measurement of the frequency shift (∆f) can be correlated to the concentration
of the target species. Based on this principle, SAW sensors were initially developed more than
three decades ago24,25, and are currently employed as fundamental components in a variety of
microsensing systems. 26 The main advantage associated with SAW sensors is their high
sensitivity and capability for the trace detection of a wide variety of chemical materials
including bio/chemicals27,28, organic and inorganic vapors29,30, and explosives.31
Chemical SAW sensing systems utilize a sensing layer on the SAW substrate that is
designed to react or bind with the analytical target. The sensing layer interacts with the target
to cause detectable changes in the features of the acoustic wave, such as the velocity or
amplitude, which are then manifested as a frequency shift or insertion loss.
According to the literature, increasing the resonant frequency can enhance the sensor
performance by increasing the amount of the frequency shift response to the change in media
and subsequent sensitivity. However, at frequencies in the GHz range, the noise factor will
have more effect on the results. The sensitivity of the sensor is defined as32
𝑆 =∆𝑓
𝑓0⁄
𝑢(3.3)
where the Δf is the frequency shift of a sensor with resonant frequency equal to fo in
response to the unit measurand of u.
42
3.3 Experimental
3.3.1 Chemicals, Materials and Sample Preparation
PbNO3, CdNO3, Hg(NO3)2 and Ni(NO3)2 in crystalline form, were purchased from the
Sigma Aldrich Chemical Company. Various solutions of deionized water and PbNO3, CdNO3,
Hg(NO3)2 and Ni(NO3)2 in concentrations of 1 pM ,100 pM, 1 nM, 100 nM, 1 μM, 50 μM,
100 μM and 750 μM were prepared. PbNO3, CdNO3, Hg(NO3)2 and Ni(NO3)2 are all soluble
in the DI water and the process will not be affected by NO3- anions33. All test analytes were
stored at −20 ºC in 10 ml aliquots before use. Tubing (Inner diameter - 0.01"; Outer diameter
– 0.0625") and tube connection accessories for sample transfer were purchased from Upchurch
Scientific.
3.3.2 Synthesis of the Chemical Sensing Layer
The sensitive layer for this experiment, which has been synthesized for sensing
applications, was acquired from the Chemistry Department at Western Michigan University.
The chemical sensing layer QDPPZ was synthesized using a two-step process34,35,36,37. In the
first step, concentrated sulfuric acid (H2SO4, 20 mL) and concentrated nitric acid (HNO3,
10 mL) were added dropwise to a mixture of 1,10-phenanthroline (1.00 g, 5.56 mmol) in the
presence of potassium bromide (KBr) (5.95 g, 50 mmol) at 0°C. The solution was refluxed for
2 hours then cooled to room temperature, yielding a black, oily product. The contents of the
flask were diluted with 400 ml deionized water and neutralized with sodium bicarbonate
(NaHCO3), yielding a clear yellow solution. The product was extracted with methylene
chloride and dried over anhydrous magnesium sulfate (MgSO4). The solvents were removed
using a rotary evaporator, resulting in a yellow solid. The product was purified by
43
recrystallization from methanol. The average yield of the product, 1,10-phenanthroline-5,6-
dione was (1.11 g, 5.31 mmol) which was calculated to be 95%. The resulting material was
achieved to be formulated as 1H NMR (400 MHz, CdCl3, 25 oC) δ: 9.12-9.10 (t, 2H, J = 2.95
Hz), 8.51-8.48 (d, 2H, J = 1.83 Hz), 7.60-7.55 (m, 2H, J = 4.71 Hz).
In the second step, 1,10-phenanthroline-5,6-dione (0.50 g, 2.38 mmol) was refluxed in
ethanol for 15 min. 9,10-diaminoanthroquinone (0.981 g, 2.38 mmol) was then added, resulting
in a purple solution, and the solution was refluxed for 4 hours. The dark purple product was
collected using vacuum filtration, washed with methanol and concentrated in vacuum. The
reaction yield was 80%. Resulting material was achieved to be formulated as 1H NMR (400
MHz, CdCl3, 25 oC): δ 9.83 (d, 1H), 9.64 (d, 1H), 9.28 (d, 2H), 8.64 (dd, 2H), 8.27 (q, 2H),
7.82 (m, 4H). The final structure is shown in Figure 3..
Figure 3.2: Structure of QDPPZ.
3.3.3 Sensor Fabrication
Figure 3.3(a) illustrates the schematic of the SH-SAW device. The SH-SAW sensor
was fabricated on a 64° YX-LiNbO3 piezoelectric substrate using photolithography techniques.
Eight pairs of input and output IDEs, 0.1 μm thick Au, were patterned by metal sputtering
44
technique on the piezoelectric substrate. The electrode aperture is 760 µm, while electrode
width and gap are 10 μm thereby resulting in a 40 µm acoustic wave length (λ). The SH-SAW
sensor also consists of 120 reflectors fabricated on the outer side of the input and output IDEs
with similar dimensions and 20 reflectors in between the IDEs to reduce the scattering losses
of surface acoustic waves38. Since the SAW propagation velocity on the 64° YX-LiNbO3
piezoelectric substrate is 4474 m/s and the designed wavelength is 40 µm, the resultant
resonant frequency is calculated to be 111.8 MHz39. The photograph of the fabricated SH-SAW
sensor, with overall device dimensions of 11 × 12 mm2 is shown in Figure 3.3(b). Two sets of
input IDE, output IDE, reflectors and electrical pads were patterned on one substrate for cost
efficiency purposes. Author just used one set for this experiment.
Figure 3.3:(a) Schematic of the SH-SAW device and (b) fabricated SH-SAW sensor.
Input IDEs Output IDEs
Reflectors ReflectorsReflectors
45
3.3.4 Flow Cell Fabrication
The flow cell for this experiment was provided by the Sensor Technology Laboratory
(STL) in the Electrical and Computer Engineering Department at Western Michigan
University. The flow cell, with overall device dimension of 70 × 50 × 52 mm (w/l/h), was
designed in AutoCAD™ and CNC machined using acrylic material (Figure 3.4). A
microfluidic flow channel, with dimensions of 710 × 6800 × 710 µm (w/l/h) and a total
channel volume of approximately 3.4 µl, was also fabricated with PDMS. The flow of the test
analyte through the PDMS microfluidic channel was obtained by integrating two sets of inlet
and outlet ports in the flow cell.
Figure 3.4: Flow cell with the SH-SAW sensor in the sensor groove (Inset: PDMS microfluidic
flow channel.
The effective closing of the flow cell, which results in the tight sealing of the PDMS
microfluidic flow channel around the sensing area of the SH-SAW sensor, was made possible
by the use of an axially magnetized set of neodymium magnets (Diameter - 0.25"; Thickness -
0.375"; Magnetic strength – 13,200 Gauss) purchased from K&J Magnets, Inc.
SH-SAW Sensor in Sensor Groove
Gold Tipped Electrical Spring
Probes
Neodymium Magnets for proper
flow cell closing PDMS Microfluidic Channel
(Volume ~ 3 µl)
46
3.3.5 Experiment Setup
The experiment setup is shown in Figure 3.5.The SH-SAW sensor was placed in the
sensor groove of the flow cell. Calibration for the wires and probes was done before the
measurements. Before use and at the end of each experiment, the sensor was cleaned with
acetone, and then blow dried with pressurized air. The measurements were performed at
constant room temperature (25 °C), using a heater occupied with thermocouple, since any
changes in the temperature would affect the SAW velocity and attenuation40.
Initially, a reference signal for deionized (DI) water was obtained. Then varying
concentrations of analytes were injected into the flow cell using a KD Scientific (KDS210P)
programmable syringe pump, at a flow rate of 50 μL/min. An Agilent 4395A network analyzer
was used to measure the frequency response (S21) of the SH-SAW sensor towards the test
analytes. System control, data acquisition and post processing of the network analyzer
measurements was performed using a LabView™ based application.
Figure 3.5: Experimental setup.
3.4 Results
To investigate the practicality of the system, results were first obtained towards
different concentrations of PbNO3. Figure 3.6 shows the frequency response (S21) of the SH-
SAW measured using network analyzer which contains both magnitude or insertion loss and
Agilent 4395ANetwork Analyzer
Flow Cell with SH-SAW Sensor
KD Scientific Programmable Syringe
Pump (KDS210P)
LabViewTM
application on PC
Withdrawal
Infusion Connected
Via SMA Cable
Via GPIB
Connected
47
phase. The author decided to record the phase aspect of the frequency response since it results
in an easier to detect shift.
Figure 3.6: Magnitude (insertion loss) and phase of the frequency response for SH-SAW sensor.
Figure 3.7 shows the frequency response (S21) of the SH-SAW towards different
concentrations of PbNO3. It was observed that the resonant frequency of the reference signal
established by DI water shifted from 108.564 MHz to 108.551 MHz, 108.543 MHz,
108.529 MHz, 108.508 MHz, 108.503 MHz, 108.499 MHz, 108.494 MHz and 108.479 MHz
for the 1 pM, 100 pM, 1 nM, 100 nM, 1 μM, 50 μM, 100 μM and 750 μM concentrations of
PbNO3 solution, respectively (Figure 3.7 (a)). This results in frequency shifts of 13 kHz,
21 kHz, 35 kHz, 56 kHz, 61 kHz, 65 kHz, 70 kHz and 85 kHz along with percentage changes
of 0.011 %, 0.019 %, 0.032 %, 0.051 %, 0.056 %, 0.059 %, 0.064 % and 0.078 % for the 1 pM,
100 pM, 1 nM, 100 nM, 1 μM, 50 μM, 100 μM and 750 μM concentrations of PbNO3 solution,
respectively when compared with DI water (Figure 3.7 (b)). The experiment was repeated
48
seven times using two different fabricated sensors and standard deviations of 3.5 kHz, 1.7 kHz,
2.1 kHz, 6.8 kHz, 3.5 kHz, 4.6 kHz, 2.1 kHz and 5.6 kHz from the average value of 15 kHz,
22 kHz, 33 kHz, 48 kHz, 57 kHz, 64 kHz, 69 kHz and 80 kHz were achieved for the 1 pM,
100 pM, 1 nM, 100 nM, 1 μM, 50 μM, 100 μM and 750 μM concentrations of PbNO3 solution,
respectively .
Figure 3.7: (a) SH-SAW sensor frequency response (S21) based on phase variation towards
varying concentrations of PbNO3 and (b) Changes in frequency shift of SH-SAW sensor response.
Then, varying concentrations of CdNO3 were injected onto the SH-SAW sensor. It was
observed that the resonant frequency of the reference signal established by DI water shifted
from 109.290 MHz to 109.100 MHz, 109.076 MHz, 108.989 MHz, 108.979 MHz,
(
a)
b)
49
108.963 MHz, 108.948 MHz, 108.944 MHz, 108.935 MHz and 108.933 MHz for the 1 pM,
100 pM, 1 nM, 100 nM, 1 μM, 50 μM 100 μM, 250 μM and 750 μM concentrations of CdNO3
solution, respectively. The frequency response (S21) of the SH-SAW sensor demonstrated
frequency shifts of 190 kHz, 214 kHz, 301 kHz, 311 kHz, 327 kHz, 342 kHz, 346 kHz,
355 kHz and 357 kHz along with percentage changes of 0.173 %, 0.195 %, 0.275 %, 0.284 %,
0.299 %, 0.312 %, 0.316 %, 0.324 % and 0.326 % for the 1 pM, 100 pM, 1 nM, 100 nM, 1 μM,
50 μM 100 μM, 250 μM and 750 μM concentrations of CdNO3 solution, respectively when
compared with DI water (Figure 3.8). The experiment was repeated eight times using two
different fabricated sensors and standard deviations of 4.2 kHz, 1.1 kHz, 1.1313 kHz, 1.6 kHz,
1.7 kHz, 2.5 kHz, 1.6 kHz, 1.5 kHz and 1.3 kHz were obtained from the average value of
109 kHz, 129 kHz, 175 kHz, 192 kHz, 203 kHz, 217 kHz, 221 kHz, 235 kHz and 271 kHz for
the 1 pM, 100 pM, 1 nM, 100 nM, 1 μM, 50 μM 100 μM, 250 μM and 750 μM concentrations
of CdNO3 solution, respectively.
Also, Figure 3.9 shows the SH-SAW sensor resonant frequency shift for the varying
concentrations of Ni(NO3)2 solution, when compared with DI water. Frequency shifts of
273.3±60 kHz, 441.7±50 kHz, 566.7±49 kHz, 673.3±43 kHz and 806.7±57 kHz was observed
for the 1 pM, 100 pM, 1 nM, 100 nM and 1 μM concentrations of Ni(NO3)2 solution,
respectively.
50
Figure 3.8: SH-SAW sensor frequency shift towards varying concentrations of CdNO3 .
Figure 3.9: SH-SAW sensor frequency shift towards varying concentrations of Ni(NO3)2.
The resonant frequency shifts can be attributed to the change in SAW propagation
velocity, in the delay line, caused by the varying concentrations of the test analytes. The
development of a selective and sensitive chemical sensing layer relied on the deposition of the
organic ligand, QDPPZ, which is a 1,10-phenanthroline derivative that consists of nitrogen and
oxygen groups that will coordinate to attach the heavy metal ions. Initial work has
demonstrated that the quinolone unit was essential in inducing selectivity towards the heavy
metal ions, which was not observed if 1,10-phenanthroline alone was used. The binding of
51
QDPPZ to the heavy metal ions causes an increase in mass between the electrodes, thus
affecting the surface acoustic wave function. As the ions flow through the sample channels,
QDPPZ molecules coordinate with the metal ions, forming a complex and increasing the mass
of the material between the electrodes.
However, QDPPZ was not an apt choice for mercury ion detection. Performing the
same experiment using mercury nitrate did not show any frequency shift in our SAW sensing
system. This suggested that unlike lead, cadmium and nickel, QDPPZ is not an adsorbent for
mercury ions. In the second attempt for mercury nitrate detection using our system, the author
used Phenol as the sensing layer on the SAW sensor, based on the approved binding event
between phenolic group and metals such as copper41, and the ability of the system towards the
mercury nitrate detection was tested by introducing different concentration of Hg(NO3)2
solution.
Figure 3.10 shows the SH-SAW sensor resonant frequency shift for the varying
concentrations of Hg(NO3)2 solution, when compared with DI water. Frequency shifts of
184.8±23 kHz, 378.3±62 kHz, 458.3±80 kHz, 600.0±96 kHz and 748.3±116 kHz were
observed for the 1 pM, 100 pM, 1 nM, 100 nM and 1 μM concentrations of Hg(NO3)2 solution,
respectively.
Figure 3.10: SH-SAW sensor frequency shift towards varying concentrations of Hg(NO3)2.
52
These sets of frequency responses also displayed detection levels as low as picomolar
concentrations and the ability of this piezoelectric sensor to distinguish among a wide range
(micro, nano and pico level) of sample concentrations. The results satisfied the main goal of
this chapter which was introducing a sensing system with a small limit of detection.
The results obtained demonstrated that phenol and QDPPZ can be employed as
sensitive layers for Hg(NO3)2 and PbNO3, CdNO3 and Ni(NO3)2, respectively. It is worth
noting that the approved toxicity level of Pb and Cd according to the United States Food and
Drug Administration (USFDA) is 1.25 mM and 207 µM, respectively 42 .The United States
Environmental Protection Agency (EPA) reported maximum allowable level for Hg(NO3)2 and
Ni(NO3)2 in drinking water to be 10 nM and 0.3 µM, respectively43, 44. Considering these
toxicity limits, this system can follow any trace of these toxic heavy metals before the toxicity
reaches to the dangerous level to avoid any health issues.
3.5 Summary
In this chapter, the author provided a brief introduction about the conventional methods
of toxic heavy metal detection in aqueous solution as well as the needs of improving these
methods. The author then described theoretical explanations of the piezoelectric sensing based
system. This was followed by a detailed report of the performed experiments including the
chemicals, materials and sample preparation; synthesis of the chemical sensing layer; sensor
fabrication; flow cell fabrication and experiment setup. Final results of the experiments are also
represented.
To summarize, a SH-SAW sensor was successfully fabricated on a 64° YX-LiNbO3
piezoelectric substrate. An efficient flow cell was also designed and fabricated using acrylic
material. The flow cell consisted of inlet and outlet ports for the microfluidic chamber and
PDMS based microfluidic channels. The feasibility of using the SH-SAW sensor for detecting
53
heavy metal compounds was demonstrated through the quantitative detection of PbNO3,
CdNO3, Ni(NO3)2 and Hg(NO3)2. A proper chemical adsorbent was used as the sensing layer.
The measured resonant frequency response shift of the SH-SAW sensor demonstrated
a 13 kHz, 190 kHz, 273 kHz and 184 kHz frequency shift for the 1 pM concentrations of
PbNO3, CdNO3, Ni(NO3)2 and Hg(NO3)2 respectively, when compared with the DI water. The
results show the capability of the SH-SAW sensor to detect pico molar concentrations of toxic
heavy metals in water, which is several orders of magnitude lower than the maximum allowable
level for toxicity in drinking water.
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26 Inoue, Yasunobu, Yoshihiro Kato, and Kazunori Sato. "Surface acoustic wave
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27 Berger, Manuel, Alexander Welle, Eric Gottwald, Michael Rapp, and Kerstin Länge.
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28 Sankaranarayanan, Subramanian KRS, Reetu Singh, and Venkat R. Bhethanabotla.
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58
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59
CHAPTER IV
PIEZOELECTRIC TRANSFORMER BASED SPRAY
GENERATION SYSTEM
4.1 Introduction
The drawbacks of the conventional spray generation systems, were discussed in detail
in the literature review (Section 2.2), including the lack of thickness and uniformity control in
pneumatic sprays, the time and money consuming fabrication process of surface acoustic
devices in acoustic spray systems, and applied high applied voltage and the plum shaped spray
area in electro sprays. These challenges indicate the necessity of further research and system
design in this area of this technology. In this chapter, the author has been investigating the
ability of piezoelectric crystals, in a simple and reliable design, significantly improve spray
generation .
In this study, the author utilized a piezoelectric transformer for inducing the generation
of a broad area electrospray. A piezoelectric transformer (PT) is a piezoelectric device that
directly amplifies a low input alternating current (AC) voltage (~10 V) to values that are several
orders of magnitude larger (~102-103 V) through the electromechanical coupling effect of the
piezoelectric substrate.
The physics underlying the generation of this spray are complex, and the exact
mechanism is unclear. It is well known that the PT generates both mechanical displacement
(vibration) and high surface voltages (with accompanying electric field) at the end of the crystal
where it is in contact with a paper wick. It has been previously demonstrated that ultrasonic
60
piezoelectric actuation can be used to vibrationally break a liquid film and atomize liquid on
the surface1. Furthermore, on 128°YX LiNbO3 crystals, both surface acoustic Rayleigh and
Lamb waves have been shown to atomize fluid on the surface2,3, although at radio frequencies
(~MHz) much higher than those used here.
In this chapter, the author introduces a novel approach of broad spray formation using
a piezoelectric transformer (PT). A 100 mm × 15 mm × 0.5 mm (l/w/t) 128°YX LiNbO3
piezoelectric crystal PT device was used at the second resonant frequency. Using a rectangular
micro fiber glass paper, liquid film was delivered to the crystal surface. Applying the AC
voltage at the resonant frequency led to the liquid film break down and spray formation along
the entire width of the PT device. A comprehensive study was performed by the author to
investigate the mechanism and the effect of parameters such as liquid properties or power
delivery on the spray formation.
4.2 Experiments and Results
4.2.1 Piezoelectric Transformer and Stand Design
A 100 mm × 15 mm 5 mm (l/w/t) cut of a piezo electric crystal was used as the
piezoelectric (PT) device. Although different geometric configurations can be used4, the author
utilizes one that is based on a linear 128 Y-cut lithium niobate (LiNbO3) crystal. Bottom and
top electrodes were formed by painting silver electrodes half the length of the PT on both sides.
A proper stand was designed using AutoCAD software and 3D printed to hold the PT in a
location that support the second resonance standing wave nodes (Figure 4.1).
61
Figure 4.1: 3D printed PT stand to pin the device in L/4 of the tips. Conductive tape was used to
apply the input voltage to the razor blade in contact with top and bottom electrodes.
The PT was actuated by a signal generator (Agilent Model 33220A) connected to an
RF amplifier (Powertrone Model 500A). Sinusoidal waveforms were used for the studies. An
oscilloscope (Tektronix DPO 2024B) was used to measure the applied voltage and current via
a resistor-capacitor-inductor (RLC) notch filter set at the driving frequency. A PT-driven
electrospray was generated by applying an AC input voltage, at the second harmonic resonant
frequency of the 128°YX LiNbO3 crystal, to two electrodes on the top and bottom of the
piezoelectric surface. The resonant frequency is a strong function of the surrounding
capacitance of the system, and for these studies, the second resonance was typically ~60 kHz.
4.2.2 Spray Generation
To generate the spray, various deionized (DI) water based aqueous solutions were filled
in an adjacent reservoir and a paper wick was placed in the reservoir and in contact with the
surface of the PT, as shown in Figure 4.2(a). The paper bridge serves to continuously wick the
solution to the PT surface , allowing the generation of a continuous electrospray, similar to
methods used for surface acoustic wave atomization5. Various types of paper were examined
62
as the wick, the paper_ PT contact and spray consistency were each observed. Ultimately,
fiberglass paper (Ahlstrom Company) produced the most consistent spray.
As shown in Figure 4.2(b), inputting an AC voltage at one end of the crystal generates
a standing mechanical wave, and due to electromechanical coupling, a large polarization is
generated along the surface of the crystal. By mounting the crystal, so that the nodal resonance
of the standing wave is pinned, substantial surface voltages are reached at the crystal tip.
Figure 4.2: (a) PT-driven electrospray configuration with solution delivery from a paper wick.
(b) Geometric configuration of the PT with resulting standing displacement and stress wave. (c)
Photograph of PT-generated electrospray of 50 mM NaCl solution.
Notably, it has been shown that these surface potentials are sufficient to breakdown gas
and form a plasma6,7,8. Here, the author utilizes this surface voltage to generate a continuous
electrospray by supplying an aqueous solution to the crystal surface.
Figure 4.2(c) shows an image of a PT-generated electrospray of a sodium chloride
solution (50 mM, NaCl) using an input AC voltage amplitude of 15 Vamp at 59.78 kHz. The
author found that the spray forms directly from the paper wick, and only when the wick is in
good contact with the PT. Unlike conventional electrospray, there is no secondary or ‘counter’
electrode used here, nor is the liquid actively pumped through a capillary by an external source
such as a syringe pump. Instead, liquid flow is driven through a combination of capillary action,
63
to saturate the paper, and an electrokinetic flow as mass is removed by the spray. Thus, the
spray is inherently self-limited by the amount of electrokinetic flow that can be generated.
Unlike conventional electrospray, the droplets do not appear to be formed from Taylor cones;
rather the behavior is more like a spitting mode9. Also, of note is that, because there is no
counter electrode and the field is inherently AC, the droplets leaving the paper are not directed
by an electric field as in conventional DC electrospray. Instead, they essentially follow ballistic
projectile trajectories under the influence of gravity, falling to the surface beneath the PT. The
spray is generated uniformly along the width of the LiNbO3 crystal, which is 15 mm for these
experiments, such that the spray covers a much wider area than the circular area covered by a
typical capillary electrospray. This could be especially useful for spray applications to cover a
wide area.
4.2.3. Spray Coating Profilometry
To demonstrate this spray generation system’s potential for wide area coating purposes,
the author sprayed fluorescent microspheres onto a glass slide placed beneath the PT. Figure
4.3(a) shows the 3D profilometry of a glass substrate spray-coated by 1 μm red fluorescing
polymer microspheres (Duke Scientific Corp.) diluted in an aqueous solution of 50 mM NaCl.
The spray was operated at 20 Vamp AC input voltage for 70 seconds. The surface roughness of
was measured to be 0.05 μm over an area of 15 mm × 5 mm using a profilometer (Bruker from
Lafayette Instrument Co.). This roughness is negligible when compared with the 1 µm bead
diameter, and indicates uniform coating. Figure 4.3(b) provides a scanning electron microscope
(SEM) image of the deposited beads, showing fairly uniform coverage.
64
Figure 4.3: 3D profilometry of a glass slide coated by 1 µm beads. The roughness was determined
to be 0.05 μm over an area of 15 mm × 5 mm. (b) SEM image of as deposited particles.
4.2.4 Volumetric Flow Estimation
The author measured the weight (and hence volume) of the collected spray using a
digital lab scale as a function of time to estimate the volumetric flow rate. The PT was mounted
above the sensitive plate of a precise weight balance (Ohaus Adventurer Pro Digital Balance)
connected to a PC and a Matlab-based program for data acquisition (Appendix A). The
generated spray was directly collected in a container on the balance, and the weight was
averaged over 30 s at a rate of 10 samples/s.
(b)
50 µm
10 µm
65
Figure 4.4: Generated spray volume for 5 mM NaCl solution in DI water versus time. The slope of the
curve fit indicates a constant volumetric flow rate of 20 µl/min over 5 min of applied constant 19 Vamp AC
input voltage. The dash lines represent the standard deviation in the spray volume from three repeated
experiments.
The entire experiment was conducted in a glass box to limit the influence of random
drafts in the laboratory and limit the influence of evaporation. Figure4.4 shows the measured
volume as a function of time for a duration of 5 minutes, illustrating the overall stability of the
spray.
The author estimated that the volumetric flow rate is ~12-30 µL/min, depending on the
exact experimental conditions (input voltage, spray solution). For comparison, most mass
spectrometry applications use rates between 0.1-10 L/min, and most spray coating
applications have rates of 10-600 L/min10,11.
4.2.5 Linear Flow Speed Visualization
To confirm that the flow was not simply due to capillary action, the author also tracked
the solution front through the paper wick using red dye. The pumping effect of the PT-driven
spray system was investigated by tracking the flow front of red dye through the paper (Fig.
4.5).
66
Figure 4.5: Front tracking of the red dye during spray generation.
Using videography, the flow front was visualized and a frame-by-frame analysis was
used to analyze the flow.
Figure 4.6 shows the front distance as a function of time, and the linear relationship
indicates a constant flow speed of ~0.12 mm/s. In contrast, the distance-time relationship for
strict capillary flow is non-linear (d ~ t1/2), and the author measured that wicking alone over 10
min caused the front to progress approximately 3 mm . The solution front progresses linearly
with time, indicating a constant speed through the paper wick. This evidence indicates that the
PT is actively pumping the fluid during the spray process faster than capillary action alone.
Figure 4.6: Distance of the front of red dye through the paper wick as a function of time with
applied constant 20 Vamp AC input voltage at 59.91 KHz. The dotted line is a linear curve fit indicating a
constant speed.
67
4.2.6 Bulk Motion of Droplets Directly on the PT Device
To assess whether this phenomenon was similar to other conventional PT based spray
formation mechanisms, two 10 ml droplets of DI water were placed on the surface, one near
the center and the other at the corner of the PT device as shown in Figure 4.7. Upon application
of a 13 Vamp, 59.9 kHz input, no bulk translation was observed for the droplet that was placed
at the center, but the one closer to the corner was translated to the edge of the PT. As shown in
Figure 4.7, the droplet subsequently wetted the edge of the PT crystal, however no spray was
formed. Further, for droplet volumes as small as 200 nL, no spray was generated and for
droplets greater than 15 mL, no bulk motion of the droplet or atomization was observed. This
suggests that the mechanism is not vibrational, but rather electrical.
Figure 4.7: Time evolution of droplet motion under a 13 Vamp, 59.9 kHz input to the PT. The
droplet nearest the corner eventually translated to the edge of the PT, wetting it, but no spray was
formed. The more central droplet only exhibited vibrational motion, but no bulk translation.
4.2.4 Measurement of Output Voltage
Prior to behavioral studies of this novel approach of spray generation, the author first
confirmed that the output voltage was of sufficient magnitude to generate an electrospray.
The output voltage of the transformer is proportional to the applied input voltage,
crystal properties, and device geometry. It can be determined using12
t
68
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛∝ 𝑘23𝑘33𝑄
𝐿
𝑇(4.1)
where Vin is the input voltage, k23 and k33 are the transverse and longitudinal vibration
coupling coefficients, Q is the crystal loss factor, and L and T are the length and thickness of
the crystal, respectively. For a 128° Y-cut LiNbO3 crystal, Q and the product k23 k33 are
reported to be approximately 104 and 0.3, respectively12. In these studies, the crystal length is
100 mm and the crystal thickness is 0.5 mm, allowing the voltage gain to be estimated as
1.5×105.
To attempt to confirm this gain, the piezoelectric transformer output voltage was
measured via a piece of wire mounted on the tip of the PT and connected to an oscilloscope, as
shown in Figure4.8(a). As shown in the plot (Figure 4.8(b)), the output voltage increases
linearly with input voltage, and a curve fit shows a voltage gain of about 124. This is
significantly lower than that predicted by Eq. (4.1). However, this can be attributed to the
inherent challenge of measuring the output voltage. Discharges tend to form at triple points and
the output wire connected to the PT forms such triple points. These discharges, which were
observed visually, generated a significant voltage drop, reducing the value measured from the
wire itself. Also, the output wire and oscilloscope were changing the load resistance, which
was suggesting the replacement of a super conductive wire could increase the measured gain.
This experiment, however, does confirm that the gain is at least 102 for this configuration. Thus,
even for an input of 10 Vamp, an output voltage of about 1240 V is expected, which is more
than sufficient to generate an electrospray.
69
Figure 4.8: (a) PT output voltage measurement configuration. (b) PT output voltage as a function
of the applied input voltage at a frequency of 60 kHz.
(a)
70
4.2.5 Effect of Liquid Conductivity on Spray Current
To explore whether the spray behaves similar to a conventional electrospray, a series
of studies were conducted to investigate the effect of the electrical conductivity and surface
tension of the solution on spray production. Based on electrospray theory, the author expects
to see that the output spray current ispray increases non-linearly with the liquid conductivity and
surface tension following
𝑖𝑠𝑝𝑟𝑎𝑦 ∝ (𝛾𝜆𝑚𝑜 𝐶)𝑛 (4.2)
where 𝛾 is the surface tension, 𝜆𝑚𝑜 is the limiting molar conductivity, C is the
concentration of the electrolyte, and n is an empirical constant between 0.2 13 and 0.5 14.
To study the effect of the electrolyte conductivity, aqueous solutions of sodium chloride
(NaCl) and hydrochloric acid (HCl) were used for testing due to the considerable difference in
their limiting molar conductivity. The limiting molar conductivity of (Na+) in water is
50 Ω-1cm2mol-1, whereas for H+ it is 350 Ω-1cm2mol-1, due to the high mobility of the proton13.
To study the effect of the conductivity of the liquid on the generated spray current, the
output current was measured by placing a grounded collecting electrode beneath the PT
connected to a picoammeter and a PC containing a LabVIEW base program for data acquisition
and further analysis (Figure 4.9).
Figure 4.9: PT driven generated spray current measurement configuration.
Ground
71
Solutions of 5-20 mM NaCl and HCl in DI water were tested, corresponding to
conductivities of 0.48-2.3 mS/cm and 0.77-5.2 mS/cm for NaCl and HCl, respectively. Sprays
were generated with a constant 18 Vamp AC input voltage.
As expected, the output current increased monotonically with solution conductivity as
shown in Figure4.10(a). When the limiting molar conductivity is accounted for, by multiplying
with the solution concentration, all the data is plotted on a logarithmic scale and all the data for
the most part collapse, as shown Figure 4.10(b). A value of n = 0.35 can be extracted from a
linear curve fit, consistent with the expectations according to Eq. (4.2). Curve fits of the
individual solution data yield values of n = 0.45 and 0.37 for NaCl and HCl, respectively, which
are also consistent with Eq. (4.2).
Figure 4.10: (a) Logarithmic plot of output spray current for different (a) concentrations and (b)
conductivity of HCl and NaCl in DI water with an applied voltage of 18 Vamp. The dash line in (b) is a
linear curve fit with a coefficient of determination equal to r2 =0.76.
72
4.2.7 Analysis of the Load Resistance and the Input Current
One notable by-product of increasing the solution conductivity is that the input current
to the PT also increased. The input current applied to the PT was measured using an
oscilloscope (Tektronix DPO 2024B) for spray of HCl and NaCl solutions, each using an 18
Vamp input voltage. Figure 4.1 shows the input current as a function of the solution
concentration, so that, the input current increases monotonically with solution concentration
and thus conductivity. The author attributed this to the fact that increasing the conductivity
effectively decreases the output load resistance.
Figure 4.11: Input current for different concentrations of HCl and NaCl in DI water for an input
voltage of 18 Vamp and slightly different frequency around 60 KHz dependent on the liquid.
The author simulated equivalent circuit for the piezoelectric transformer15 using LT
Spice software (Figure 4.12(a)), and the effect of the change in the load resistance was
investigated. Results showed that for smaller values of load resistance, both the input and
output current of the piezoelectric transformer were increased, as shown in Figure 4.12(b).
According to these results, decreasing the output load resistance increases the input current and
total power consumed by the PT, which is consistent with increasing the conductivity of the
solution (reducing the load resistance) as discussed before.
73
Figure 4.12: (a) PT equivalent circuit. (b) Simulated input and output current (I in , I out) as a
function of load resistance (RL).
4.2.6 Effect of Liquid Conductivity and Surface Tension on the Onset
Voltage
The increase in the input current (and thus power) with conductivity for a fixed input
voltage of 18 Vamp potentially suggested that the minimum input voltage to onset the spray,
Vin,onset, is also a strong function of the conductivity. The onset voltage is set by the Maxwell
pressure induced by the electric field at the end of the PT, which must overcome the surface
tension, 𝛾, of the liquid film to induce a spray16. If it is assumed that the output voltage at the
edge of the PT is linearly proportional15 to the input voltage Vin, as suggested by the
measurements of the output voltage, it is anticipated that
𝑉𝑖𝑛,𝑜𝑛𝑠𝑒𝑡 ∝ 𝛾0.5 ( 𝜆𝑚𝑜 𝐶)−0.5 (4.3)
(a)
74
Figure 4.13(a) confirms that the onset voltage does decrease non-linearly with ionic
concentration. It is important to note that, for all of these studies, the surface tension was
measured to be 73 dyn/cm, when the conductivity was varied, which precludes the possibility
that surface tension was causing the variations in current and voltage that were measured. The
surface tension of solutions was measured using an FTA 100 Angstroms instrument. A 4 mL
syringe was used to inject the solution through the device, and results were obtained by
calculating the average of three different sequential measurements.
Interestingly, the exponent factor was measured to be -0.32, which is slightly lower
than the expected relationship from a simplified balance of Maxwell pressure and surface
tension or electrostatic and capillary pressure17. While this could be attributed to the effect of
mechanical vibration on the atomization, further study is required to resolve this completely.
Nevertheless, the general trend is consistent with the argument that the electric field induced
by the PT is primarily responsible for inducing the spray.
Figure 4.13: a) Logarithmic plot of the onset voltage for various concentration of NaCl in DI
water. The dash line is the linear curve fit with a slope equal to -0.32. b) Onset voltage as a function of
surface tension for various concentration of glycerol in DI water.
To explore the effect of surface tension, the polar solvent glycerol was added to aqueous
solutions of NaCl (0.53 mS/m) to reduce the surface tension from γ = 73 to 69 dyn/cm
(corresponding to glycerol concentrations from 0.01-200 mM). As expected, the onset voltage
75
increases as γ increases (Figure 4.13(b)). However, unexpectedly, both the spray output current
and PT input current were unaffected as the surface tension was varied for a constant input
voltage. For the same input voltage (21 Vamp), the input current (irms) for all the samples was
essentially 84 mA, and the spray current (ispray) was 20 nA. As the surface tension does not
impact the output load resistance, it is not surprising that the PT input current was not affected.
But Eq. (4.2) shows that the spray current should be proportional to a positive power of the
surface tension. The author attributed this anomaly to the fact that the resolution to detect the
expected minute changes in current (~0.0001%) was not sufficient as predicted by Eq. (4.2) for
the small range of surface tension values explored here. But it is possible that there may be
more than conventional electrospray mechanisms at play, altering the dependence of these
parameters on surface tension.
4.2.7 Chemical Solution Preparation
Sodium chloride (NaCl), glycerol and Hydrochloric acid (HCl) were purchased from
Sigma Aldrich. Aqueous solutions at different concentrations were made by diluting using
deionized (DI) water (18 MΩ). Solutions of 5-20 mM NaCl and HCl in DI water were prepared,
which corresponds to conductivities of 0.48-2.3 mS/cm and 0.77-5.2 mS/cm for NaCl and HCl,
respectively.
4.4 Summary
This chapter provided a novel approach to create a spray generation system using a
piezoelectric device. A detailed report of this work including the piezoelectric transformer and
stand design, spray generation, spray coating profilometry, volumetric flow estimation, linear
flow speed visualization, bulk motion of the droplet directly on a PT device, measurement of
output voltage, effect of solution conductivity on the output current, analysis of the load
76
resistance and the input current, effect of liquid conductivity and surface tension on the onset
voltage and chemical solution preparation has been presented.
To summarize, the author successfully generated a continuous spray by pinning a
128°YX LiNbO3 piezoelectric crystal in the standing wave mode and providing the desired
liquid film using a paper wick. The paper covers the PT width which enabled a final spray that
was as wide as the PT device. Profilometry of the coated area by the generated spray
demonstrated a uniform coating. Volumetric flow rate of the produced spray was measured to
be 12-30 µL/min, depending on the exact experimental conditions (input voltage, spray
solution) and the front dye tracking proved the pumping effect of the PT device. To verify
whether electrical or mechanical feature of the piezo electric crystal is the dominant reason for
the spray generation, bulk motion of the droplet was recorded.
The output voltage gain of the PT was measured to be 124 and compared to the
calculated value. Different concentrations of HCl and NaCl in DI water were used for spray
generation and results showed that the liquid conductivity and the spray current are
monolithically related as it is predicted in a conventional electrospray system. The simulations
done by the author also indicated that the input and output current increased when the liquid
conductivity was elevated. The minimum voltage for liquid film breakage was shown to
decrease as the liquid surface tension reduced.
4.5 References
1 Dobre, M., and L. Bolle. "Practical design of ultrasonic spray devices: experimental
testing of several atomizer geometries." Experimental Thermal and Fluid Science 26, no. 2
(2002): 205-211.
77
2 Rezk, Amgad R., Leslie Y. Yeo, and James R. Friend. "Poloidal flow and toroidal
particle ring formation in a sessile drop driven by megahertz order vibration." Langmuir 30,
no. 37 (2014): 11243-11247.
3 Yeo, Leslie Y., and James R. Friend. "Surface acoustic wave microfluidics." Annual
Review of Fluid Mechanics 46 (2014): 379-406.
4 VanGordon, J. A., B. B. Gall, S. D. Kovaleski, E. A. Baxter, R. Almeida, and J. W.
Kwon. "High voltage production from shaped piezoelectric transformers and piezoelectric
transformer based circuits." In 2010 IEEE International Power Modulator and High Voltage
Conference, pp. 334-337. IEEE, 2010.
5 Ho, Jenny, Ming K. Tan, David B. Go, Leslie Y. Yeo, James R. Friend, and Hsueh-
Chia Chang. "Paper-based microfluidic surface acoustic wave sample delivery and ionization
source for rapid and sensitive ambient mass spectrometry." Analytical chemistry 83, no. 9
(2011): 3260-3266.
6 Kim, Hyun, Albrecht Brockhaus, and Jürgen Engemann. "Atmospheric pressure
argon plasma jet using a cylindrical piezoelectric transformer." Applied Physics Letters 95, no.
21 (2009): 211501.
7 Johnson, Michael J., and David B. Go. "Ferroelectric crystals for the low-voltage
operation of surface dielectric barrier discharges." Applied Physics Letters 105, no. 26 (2014):
264102.
8 Itoh, Haruo, K. Teranishi, and S. Suzuki. "Discharge plasmas generated by
piezoelectric transformers and their applications." Plasma Sources Science and Technology 15,
no. 2 (2006): S51.
9 Sleighter, Rachel L., and Patrick G. Hatcher. "The application of electrospray
ionization coupled to ultrahigh resolution mass spectrometry for the molecular characterization
of natural organic matter." Journal of Mass Spectrometry 42, no. 5 (2007): 559-574.
78
10 Tait, Jeffrey G., Barry P. Rand, and Paul Heremans. "Concurrently pumped
ultrasonic spray coating for donor: acceptor and thickness optimization of organic solar cells."
Organic Electronics 14, no. 3 (2013): 1002-1008.
11 Mahmood, Khalid, Bhabani Sankar Swain, and Hyun Suk Jung. "Controlling the
surface nanostructure of ZnO and Al-doped ZnO thin films using electrostatic spraying for
their application in 12% efficient perovskite solar cells." Nanoscale 6, no. 15 (2014): 9127-
9138..
12 Nakamura, Kiyoshi, and Yoshinori Adachi. "Piezoelectric transformers using
LiNbO3 single crystals." Electronics and Communications in Japan (Part III: Fundamental
Electronic Science) 81, no. 7 (1998): 1-6.
13 Tang, Liang, and Paul Kebarle. "Effect of the conductivity of the electrosprayed
solution on the electrospray current. Factors determining analyte sensitivity in electrospray
mass spectrometry." Analytical Chemistry 63, no. 23 (1991): 2709-2715.
14 Ganan-Calvo, A. M., J. Davila, and A. Barrero. "Current and droplet size in the
electrospraying of liquids. Scaling laws." Journal of Aerosol Science 28, no. 2 (1997): 249-
275.
15 Alonso, J. Marcos, Carlos Ordiz, Marco A. Dalla Costa, Javier Ribas, and JesÚs
Cardesin. "High-voltage power supply for ozone generation based on piezoelectric
transformer." IEEE Transactions on Industry applications 45, no. 4 (2009): 1513-1523.
16 Guerrero, I., R. Bocanegra, F. J. Higuera, and J. Fernández De La Mora. "Ion
evaporation from Taylor cones of propylene carbonate mixed with ionic liquids." Journal of
Fluid Mechanics 591 (2007): 437-459.
17 Fernández de La Mora, Juan. "The fluid dynamics of Taylor cones." Annu. Rev. Fluid
Mech. 39 (2007): 217-243.
79
CHAPTER V
PIEZOELECTRIC TRANSFORMER BASED SPRAY GENERATION FOR
THIN FILM MEMBRANE COATING
5.1 Introduction
Layer-by-layer (LbL) deposition of multilayer films has been widely used in the
fabrication of various sensors including gas, humidity, and bio sensors1. The LbL deposition
method entails alternating deposition of oppositely charged ions to form stable multilayer films
via electrostatic interactions or hydrogen bonding2 . There are many approaches of building
multilayer films such as dip coating3, inkjet printing4, spin-assisted assembly and electrophoretic
deposition 5 . However, scalability and thin-film uniformity are still challenging for
commercialization of these techniques.
5.1.1 Spray Coating Techniques
Spray technologies have been investigated for their potential capabilities of material
coating. There are various methods of breaking the liquid droplet down to the spray. One of the
simplest and widely used approach is a pneumatic spray which does not contain a precise or
uniform droplet size and hence is not a proper choice for use in membrane coating. Among the
vibrational approach of spray generation, surface acoustic wave atomizers have been utilized
for material coating. Although this technique provides a uniform coating, the complicated
procedure to fabricate this device along with the 100 V range of required input voltage can
undermine their efficiency6.
80
Electrosprays are well known to form a reliable continuous spray from a liquid sample.
However, the need of high input voltages (kV), along with costly equipment, does not make it
an apt choice in large scale sensor fabrication processes. Moreover, the principal of the
electrospray phenomenon is based on the Taylor cone formation and droplet plumes produce a
circular injected area which is not ideal for uniform material coating7.
5.1.2. Membrane Coating Materials
Among the other coating methods, polymer coating techniques have been witnessing a
rising demand due to the vast range of applications for polymer coated membranes in fuel cells8,
solar cells9, isopropanol dehydration10,surface treatment of metallic coronary stents11 and the
next generation of drug delivery systems12.
The key factor in membrane coating is to find a balance between flux and separation. In
other words, the liquid flow rate through the membrane needs to stay significant, while the target
ions are blocked by the coated membrane. It has been proved that alternating coating of a
membrane with opposite charge polymers can produce a thin film13. Layer by layer deposition
of anionic and cationic polyelectrolytes such as poly (allylamine hydrochloride) (PAH) and
poly(styrenesulfonate) (PSS) on a solid membrane was shown to be successful in the separation
of ethanol and water mixtures14.
Ouyang et al. 15 reported that adding a layer of poly(diallyl-dimethylammonium
chloride) (PDADMAC) to the ion rejection membrane, coated with other polymers, enhances
the liquid flux trough the membrane by increasing the swelling of films contacting the
PDADMAC.
Here, the author demonstrated an innovative procedure for LbL thin film deposition
using a piezoelectric driven spray generator. As a proof-of-concept, some of the previously
81
used polymers for membrane coating such as poly (allylamine hydrochloride) (PAH),
poly(styrenesulfonate) (PSS), poly(diallyl-dimethylammonium chloride) and poly(diallyl-
dimethylammonium chloride) (PDADMAC) were sprayed using the novel PT-driven spray
formation technology introduced by the author in chapter 4. A polycarbonate track-etched
(PCTE) membrane was spray coated by the polymer mixtures and water permeability and ion
rejection ratio of the final PT spray coated membrane were measured. As a proper solvent for
polymer based mixtures, dimethyl sulfoxide (DMSO) has been widely used due to its
nontoxicity, polymeric and water solubility capabilities, hence the author included this chemical
to the spray formation list to examine the ability of PT spray formation technology for different
probable polymer mixtures16. This method has several advantages due to its cost efficiency,
quick response and low material consumption when compared with the traditional thin film
deposition methods.
5.2 Experimental
5.2.1 Spray Generation Setup
Spray formation was achieved via the same PT driven setup mentioned in previous
chapter. A 15 mm (or 30 mm) × 100 mm × 1.5 mm (w/l/t) 128°YX lithium niobate (LiNbO3)
crystal was purchased, and bottom and top electrodes were painted on the PT surface using
silver paint (conductive silver paint from SPI company). A PT stand was designed and
fabricated using a 3D printer to mount and pin the PT at two locations corresponding to nodes
in the displacement wave related to second resonant frequency, which was around 60 KHz and
63 KHz for 15 mm and 30 mm width PT devices, respectively. A signal generator and a radio
frequency amplifier provide the required input current at the desired frequency. A liquid sample
82
was delivered to the surface of the PT by a paper wick placed between a liquid reservoir for
coating material and the PT surface.
5.2.2 Material Preparation
PAH (15 kDa), PSS (70 kDa) and Poly(diallyldimethylammonium chloride)
(PDADMAC) ( 20 wt. % in H2O) in liquid form were purchased from Sigma-Aldrich. Aqueous
solutions of the polyelectrolytes at 20 mM (based on repeat unit molecular weight) were
prepared. The pH of the PAH and PSS solution were unadjusted. A PCTE membrane (pore
diameter: 50 nm, membrane thickness: 10 μm, porosity: 3×108 pores/cm2, Whatman) was used
as a template.
5.2.3 Moving Stage (Lazy Susan)
To provide a moving stage for high throughput membrane coating, a circular, slow
spinning stage (lazy susan) was designed and made by the author (Figure 5.1).
A DC gear box reduction motor was used to produce 5 RPM vertical spinning motion
by applying 12 V input voltage delivered by a DC power supply. To change the motion to a
horizontal direction, a vertical shaft worm gear box was connected to the motor. This shaft also
reduced the speed of spinning 30 times due to the size difference. A gearbox ratio of 30:1
means that the output side is about 30 times faster than the input side. This gear box works
using a 30:1 ratio worm drive, which is able to hold the position when the power is not applied
to prevent any interference in the experiment result when any electrical interruptions occurs.
A 10 inch diameter plexiglass sheet was used as the moving stage connected to the
motor shaft. A simple variable power supply driver motor was also used to change the power
delivered to the motor by a resistor based voltage divider. The author used this configuration
83
to control the motor speed, which leads to the variable plexiglass speed of spinning. This
enabled the exposure of the membrane with the coating spray for different time durations. The
whole structure was mounted on an aluminum sheet for support and stability.
Figure 5.1: Sequential PT driven Spray coating of membrane using PAH and PSS by placing the
membranes on a Lazy Susan.
5.2.4 Water Permeability and Salt Rejection Tests
The PT- driven spray coated membrane was placed in a water filled cell (Amicon model
8003) and stirred. A pressure of 4 bar was applied to drive water through the membrane, which
was collected in a small beaker. The collected mass was measured using a scale, which was
connected to a PC with a LabVIEW™ based program for data acquisition and recording the
change in mass over time. The hydraulic permeability of the membrane was calculated using
the slope of this graph.
In salt rejection tests, 10 mM solutions of single salts (i.e., KCl, MgCl2) were used as
the feed solutions and a pressure of 4 bar was applied to drive the flow. The solution was
collected in a glass reservoir. Ion chromatography (Dionex ICS-5000) was used to analyze the
84
concentration of ions in the feed (cf) and permeate (cp) solutions. The percent rejection, R, was
calculated according to
𝑅(%) = (1 −𝑐𝑃
𝑐𝐹) ×100 (5.1)
5.3. Results
5.3.1. Bilayer coating of PAH and PSS
A paper was placed between a reservoir which contained PAH or PSS solution and PT
surface. Autor found it useful to soak the paper in the reservoir in the beginning of the
experiment to provide a better contact between the paper and the PT edge which can enhance
the uniformity of the generated spray. The PCTE template membrane was placed 3 cm beneath
the PT on the moving stage. By introducing around 15 Vamp AC input voltage in 59.8 KHz
frequency, the spray was generated. The spray was formed along the width of the PT, which is
15 mm in this experiment, and the spray droplets are of sufficient size that fall to the surface
beneath in the PT (in a manner analogous to a shower). As such, the spray deposits over an
area of 15 mm by 0.5 mm. Membrane was exposed to PAH spray for 30 seconds and after
drying up, which took almost 5 minutes, PSS was sprayed over the membrane for 30 second to
from the first bilayer coating of the membrane.
The process was repeated to form multiple bilayers and to coat the PCTE membrane
completely. Repeated spray coating of the membrane using PAH and then PSS resulted in the
fabrication of multilayer thin films on the membrane surface. As shown in Figure 5.2, scanning
electron microscope (SEM) images revealed that the native 50 nm diameter pores of the bare
PCTE membrane were coated by the sprayed polymers until total coverage was achieved.
85
Figure 5.2: SEM images of PCTE membranes sprayed with different numbers of bilayers.
Different numbers of PAH/PSS bilayers were sprayed onto a 50 nm PCTE membrane. The sprayed
membranes were taped onto a SEM grid, sputtered with iridium (2 nm), and visualized in SEM.
Table 5.1 shows the results for water permeability performed for bare, 1 bilayer, 2
bilayers, and 3 bilayers coated membranes using 30 s spray durations for each layer. The water
permeability decreased rapidly with increasing number of bilayers, suggesting that a successful
LbL thin film was sprayed on the PCTE membrane. The sprayed LbL thin film also showed
higher salt rejection at a higher number of sprayed bilayers. The water permeability of the
membrane sprayed with three bilayers was not measurable, indicating the pores of the
membrane were completely blocked by the sprayed films.
86
Table 5.1: Water permeability and ion rejection results for PAH/PSS PT_ spray coating.
Number of
bilayer
Water
permeability
L/m2/hours/bar
Ion rejection
ratio
Bare 425
1 bilayer 41±5
2 bilayers 3.5 25% for 10 mM NaCl
32% for 10 mM MgCl2
3 bilayers Too slow
In some cases, it is desirable to prepare thin films with a high number of bilayers while
maintaining membrane permeability. This condition can be achieved by shortening the PT
spray durations. A duration of 10 s was used instead of 30 s to alternatingly spray PAH/PSS
bilayers. Figure 5.3 shows a logarithmic plot of how the water permeability changes with
different number of bilayers. The water permeability decreased from 425 L m-2 hr-1 bar-1 for
the bare PCTE membrane to 0.9 L m-2 hr-1 bar-1 with 5 bilayers. The water permeability was
unmeasurable for the membrane with 6 bilayers. The results show that by shortening the
spraying duration, a thinner film is deposited for each bilayer. The small values of the water
permeability of the coated membrane suggest the absence of big defects in the LbL films.
87
Figure 5.3: Logarithmic plot of the normalized water permeability vs. number of bilayers. A 50
nm PCTE membrane was used as the substrate membrane with a different number of sprayed PAH/PSS
bilayers with 10 s durations. The water permeability test was measured in a dead-end filter cell. The error
bars meant three measurements with the same membrane and were barely seen since they were smaller
than the symbols.
Figure 5.4: Salt rejection vs. number of bilayers. A different number of PAH/PSS bilayers were
sprayed alternatingly with 10 s duration on a 50 nm PCTE membrane. The salt rejection test was carried
out in a dead-end filter cell filled with either 10 mM KCl or 10 mM MgCl2.
Figure 5.4 shows how the rejection of 10 mM KCl and MgCl2 changes with the number
of bilayers sprayed on the PCTE membrane. The results illustrate that by increasing the number
of bilayers, a higher salt rejection is achieved for the sprayed membrane.
Since there is no waste of coating material, unlike dip coating approach, the amount of
the required polymer solutions for membrane coating is considerably decreasing. The
membrane diameter is 2.5 cm while the moving stage diameter is 25cm, which makes it
possible to mount 30 membranes on the moving stage simultaneously. If the speed of spinning
88
in moving stage is set to expose each membrane with the PT spray for 20 second, bilayer
coating of 30 membranes is possible within 26 minutes. This can prove the capability of this
PT driven spray generation approach as a high throughput membrane coating system.
89
5.3.2. PDADMAC Spray Coating
To prove the ability of this novel spray formation technology for polymer coating of
the membrane, PDADMAC was tried.
The membrane was placed on the Lazy susan, at a distance of 3 cm below the PT device.
A DC motor was reconfigured to provide a 0.1 cm/s spinning speed for moving the stage. The
author exposed the membrane with PT driven spray of PDADMAC for 25 s. A mixture of 0.1
Molar of PDADMAC and 1% poly vinyl alcohol (PVA) was diluted 33% with DI water to
provide the coating material for the membrane. Table 5.2 shows the results for this experiment.
Table 5.2: Water permeability and ion rejection results for PDADMAC PT_spray coating.
Number of
layer
Water
permeability
lit/m2/hours/bar
Ion rejection ratio
1 layer 300
3 layers 300
6 layers 300
20 layers 50 66% ± 2 for 10 mM KCl
The results represent the capability of the system for spray production. Applying just
12 Vamp to PT was sufficient to generate a uniform continues spray from the PDADMAC
solution. The water permeability was almost as high as a bare membrane with no coating layer
in the first 5 layers coating. The author attributed this to the dilution factor used for the coating
mixture. The results suggest that the percentage of the polymer per sprayed droplet was very
low in the first stages of the coating.
90
5.4 Summary
A piezoelectric transformer was used to generate a uniform spray by applying 15 to 25
Vamp to the crystal at its second resonant frequency. A continuous supply of solution from a
paper wick was delivered to the surface of the PT, without the need for an external pump. Layer-
by-layer spraying of multilayer thin films with PAH/PSS and PDADMAC were used as a model
system to demonstrate the PT-driven spray technique as a material-deposition method. Results
show that the spray duration and number of bilayers have a significant impact on the water
permeability and salt rejection of the sprayed membrane.
5.5 References
1 R. Article, D. Rawtani, and Y. K. Agrawal, “Emerging Strategies and Applications of
layer-by-layer self-assembly,” Nanobiomedicine, pp. 1–15, 2014.
2 S. Rajesh, Y. Yan, H. Chang, H. Gao, and W. A. Phillip, “Mixed mosaic membranes
prepared by layer-by-layer assembly for ionic separations,” ACS Nano, vol. 8, pp. 12338–
12345, 2014.
3 B. W. Stanton, J. J. Harris, M. D. Miller, and M. L. Bruening, “Ultrathin, multilayered
polyelectrolyte films as nanofiltration membranes,” Langmuir, vol. 19, pp. 7038–7042, 2003.
4 P. Gao, A. Hunter, S. Benavides, M. J. Summe, F. Gao, and W. A. Phillip, “Template
synthesis of nanostructured polymeric membranes by inkjet printing,” ACS Appl. Mater.
Interfaces, vol. 8, pp. 3386–3395, 2016
5 J. L. Lutkenhaus and P. T. Hammond, “Electrochemically enabled polyelectrolyte
multilayer devices: from fuel cells to sensors,” Soft Matter, vol. 3, p. 804, 2007.
91
6 A. Yabe, Y. Hamate, M. Hara, H. Oguchi, S. Nagasawa, and H. Kuwano, “A self-
converging atomized mist spray device using surface acoustic wave,” Microfluidics and
Nanofluidics, vol. 17, pp. 701-710, 2014.
7 A. Ganan-Calvo, J. Davila, and A. Barrero, “Current and droplet size in the
electrospraying of liquids. Scaling laws,” Journal of Aerosol Science vol. 28, pp. 249-275,
1997.
8 Kurozumi, Tomohiro, Yoshiki Okajima, Hidekazu Nagai, and Masao Sudoh.
"Transport Properties of Plasma Polymerized Anion Exchange Membrane for Direct Methanol
Alkaline Fuel Cells." ECS Transactions 50, no. 2 (2013): 2109-2118.
9 Krebs, Frederik C. "Fabrication and processing of polymer solar cells: a review of
printing and coating techniques." Solar energy materials and solar cells 93, no. 4 (2009): 394-
412.
10 Albo, Jonathan, Jinhui Wang, and Toshinori Tsuru. "Application of interfacially
polymerized polyamide composite membranes to isopropanol dehydration: Effect of
membrane pre-treatment and temperature." Journal of Membrane Science 453 (2014): 384-
393.
11 Lewis, A. L., L. A. Tolhurst, and P. W. Stratford. "Analysis of a phosphorylcholine-
based polymer coating on a coronary stent pre-and post-implantation." Biomaterials 23, no. 7
(2002): 1697-1706.
12 Sahatjian, Ronald. "Drug delivery system making use of a hydrogel polymer
coating." U.S. Patent 5,304,121, issued April 19, 1994.
13 Decher, Gero. "Fuzzy nanoassemblies: toward layered polymeric
multicomposites." science 277, no. 5330 (1997): 1232-1237.
92
14 Krasemann, Lutz, Ali Toutianoush, and Bernd Tieke. "Self-assembled
polyelectrolyte multilayer membranes with highly improved pervaporation separation of
ethanol/water mixtures." Journal of Membrane Science 181, no. 2 (2001): 221-228.
15 Ouyang, Lu, Ramamoorthy Malaisamy, and Merlin L. Bruening. "Multilayer
polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent
cations." Journal of Membrane Science 310, no. 1 (2008): 76-84.
16 Anderson, Daniel G., David M. Lynn, and Robert Langer. "Semi‐automated
synthesis and screening of a large library of degradable cationic polymers for gene
delivery." Angewandte Chemie 115, no. 27 (2003): 3261-3266.
93
CHAPTER VI
CONCLUSION
6.1 Conclusion
In this dissertation, the author has successfully developed a piezoelectric based system
for fluid manipulation. Piezoelectric devices produce mechanical strain under applied electrical
field, which results in propagating mechanical vibration. This acoustic wave has been used to
manipulate the liquid medium in sensing technologies due to their benefits such as small
volume of required sample, high sensitivity, bio-compatibility and non-invasive procedures.
The high sensitivity arises from the large electromechanical coupling factor of the piezoelectric
crystal.
Moreover, the aforementioned features make piezoelectric devices also apt choices in
actuating systems such as spray formation. The mechanical vibration along with the large
polarization produced on the surface of the piezoelectric crystal can be used to overcome the
liquid film surface tension and break it into the droplets.
In this work, a piezoelectric based sensor, which employs photolithographically
patterned electrodes was used for sensing applications. In addition, the author explored the
possibility of using piezoelectric transformers to generate a continuous uniform spray by
applying low input voltages when compared to the conventional spray production techniques.
The dissertation was organized and pursued in three projects in order to achieve the
research outcomes.
94
In the first project, an efficient piezoelectric based detection sensing system for the
detection of heavy metal compounds was developed. A photolithographically fabricated
piezoelectric sensor, which incorporates Au IDEs was used for the detection of various heavy
metal compounds. Proper sensitive layers were chosen for the selective detection of the heavy
metal compounds including lead, cadmium, nickel and mercury nitrate. Based on the results
from the piezoelectric sensor, the author concluded the capability of the developed piezo
electric based system to detect heavy metal compounds at pico molar concentration levels using
the frequency shift.
The author was responsible for design and fabrication of the SAW device and finding
proper sensing layer. Experiment setup and controlling the environmental factors in order to
preserve the experiment repeatability was also done by author.
In the second project, the author designed and fabricated a novel piezoelectric
transformer for fluid manipulation and spray generation. A micro fiber glass paper was used to
wick the liquid film out of the contained reservoir. Based on the imaging results, a large area
uniform spray was generated along the width of the piezoelectric transformer. The author
demonstrated that bringing a thin-film of liquid to the surface of a PT via a wick generated a
spray of droplets. The author’s studies strongly suggest that the spray mechanism is
electrospray in nature, and thus parameters such as solution conductivity and surface tension
can be used to manipulate the spray behavior. By spraying directly from the wick/PT interface,
the nebulization in many ways resembles a free-surface electrospray rather than capillary
electrospray. In the author’s opinion, the broad area nature of the spray makes it very appealing
for emerging applications in materials synthesis and coating, were uniform deposition may be
possible over large substrates simply by scaling the size of the PT crystal.
95
The author was responsible for design and fabrication of the device and stand. Proper
experiments were designed and conducted by author to study the key factors of this
phenomenon.
In the third project, the author tried to use the PT spray generation system for coating
purposes. Some previously proven polymeric materials for membrane coating were chosen:
poly(allylamine hydrochloride) (PAH), poly(styrene sulfonate) (PSS) and (PDADMAC) were
sprayed on to a polycarbonate track-etched (PCTE) membrane. SEM results illustrated the
polymer thin film formation on the membrane. Water permeability and ion rejection ratio of
the coated membrane were measured. The author concluded that the number of layers has an
optimum number for each desired polymer coating material and it occurs where the polymer
film is uniformly coated on the membrane surface to result in a good ion rejection ratio, while
the polymer film is still not too thick to block the liquid flow through the coated membrane.
There are several possibilities to improve the current projects which are covered in this
dissertation. The author has some suggestions for further studies:
• The ability to integrate the SH-SAW sensor into a hand-held and
portable detection system.
• Improve the selectivity of the SH-SAW sensor to exposed
chemicals by studying target to sensing layer binding conditions.
• Enhance the experiment repeatability by exploring the other
methods of the sensing layer deposition which results in a uniform film.
• The effect of the piezoelectric transformer geometry on the
discharge points of the edge of the PT device can be investigated using software
simulations such as COMSOL.
• The resultant aerosols size and dispersity can be measured using
video graphic techniques.
96
• More suitable approach of liquid delivery to the PT surface needs
to be found to increase the produced spray uniformity along the width of the PT.
• The dependency of the spray formation towards the liquid
conductivity and surface tension can be the focus of the further studies to
investigate the capability of the system for spray generation from the
nonconductive solutions.
• The influence of the spray duration, temperature and PT to
membrane distance on the ion rejection ration can be investigated to find the
optimized condition.
• Since the positive affect of washing the membrane proven in the
conventional LBL membrane coating on the membrane function has already
been proven, a washing step can be added in between the two spray stations
above the moving stage.
• PT-driven spray technique can be extended to the layer-by-layer
deposition of a large variety of polymer materials for the rapid, scalable synthesis
of ion-exchange and ion-rejection membrane sensors.
97
Appendix A
List of publications
A.1 Inventions
A.1.1 Intellectual Property
[1] D. B. Go, Z. Ramshani, W. A. Phillip, P. Gao, M. Z. Atashbar “Multi-
Functional Thin Film Coating by Piezoelectric Driven Spray,” University of Notre Dame,
Intellectual Property Disclosure, UND CASE# 17-001.
[2] D. B. Go, Z. Ramshani, M. J. Johnson, M. Z. Atashbar “Piezoelectric Transfer
Driven Transformer Device,” University of Notre Dame, Intellectual Property Disclosure,
UND CASE# 16-048.
A.1.2 Patent
[1] D. B. Go, Z. Ramshani, W. A. Phillip, P. Gao, M. Z. Atashbar “Apparatus and
Method for Atomization of Fluid, Thin Film Synthesis and Coating,” U.S. Provisional Patent
62/414,317, filed 10/26/2016.
[2] D. B. Go, Z. Ramshani, M. J. Johnson, M. Z. Atashbar “Piezoelectric
Transformer-Driven Electrospray Device,” U.S. Provisional Patent 62/319,775, filed
04/07/2016.
A.2 Journal Papers
[1] Ramshani, Zeinab, Michael J. Johnson, Massood Z. Atashbar, and David B.
Go. "A broad area electrospray generated by a piezoelectric transformer." Applied Physics
Letters 109, no. 4 (2016): 044103.
98
[2] Ramshani, Zeinab, Avuthu SG Reddy, Binu B. Narakathu, Jared T. Wabeke,
Sherine O. Obare, and Massood Z. Atashbar. "SH-SAW sensor based microfluidic system for
the detection of heavy metal compounds in liquid environments." Sensors and Actuators B:
Chemical 217 (2015): 72-77.
[3] Go, David, Massood Atashbar, Zeinab Ramshani, Huse Chia Chang. “Surface
acoustic wave devices for chemical sensing and microfluidics: A review and perspective”.
Analytical Methods, 2017, (in print)
A.3 Conference Papers
[1] Ramshani, Zeinab, Binu B. Narakathu, Sepehr Emamian, Avuthu SG Reddy,
and Massood Z. Atashbar. "Investigation of SH-SAW sensors for toxic heavy metal detection."
SENSORS, 2013 IEEE, pp. 1-4. IEEE, 2013.
[2] Ramshani, Zeinab, Binu B. Narakathu, Avuthu SG Reddy, Massood Z.
Atashbar, Jared T. Wabeke, and Sherine O. Obare. "SH-SAW-based sensor for heavy metal
ion detection." Frequency Control Symposium & the European Frequency and Time Forum
(FCS), 2015 Joint Conference of the IEEE International, pp. 536-540. IEEE, 2015.
[4] Eshkeiti, A., Z. Ramshani, S. Emamian, B. B. Narakathu, S. G. R. Avuthu, M.
M. Ali, A. Chlaihawi, M. K. Joyce, and M. Z. Atashbar. "A stretchable and wearable printed
sensor for human body motion monitoring." SENSORS, 2015 IEEE, pp. 1-4. IEEE, 2015.
[5] Ramshani, Zeinab., B. Narakathu, Massood Z. Atashbar, Kapseong Ro, J-C.
Song, Daewoo Lee, and C. H. Lee. "Design of a novel 3D localization and identification system
using surface acoustic (SAW) devices." Electro/Information Technology (EIT), 2013 IEEE
International Conference on, pp. 1-4. IEEE, 2013.
[6] Ramshani, Zeinab, Binu B. Narakathu, Avuthu SG Reddy, Massood Z.
Atashbar. "Heavy Metal Detection using Shear Horizontal Surface Acoustic Wave (SH-SAW)
Sensors. International Meeting on Chemical Sensors (IMCS), pp. 88, 2014.
99
[7] Ramshani, Zeinab, Gao Peng, William A. Phillip, Massood Z. Atashbar, and
David B. Go. "Piezoelectric Transformer-Driven Spray Coating for Membrane Sensor
Fabrication." SENSORS, 2016 IEEE International Conference on, pp. 142-144. IEEE, 2016.
100
Appendix B
Matlab code for scale used to graph the spray volumetric flow rate used in Chapter 4
“
function curpre
obj1(1)=serial('COM5','baudrate',2400,'databits',7,'parity','none','flowcontrol','none','te
rminator','CR/LF'); %from balance user manual
for i = 1:length(obj1)
fopen(obj1(i));
fprintf(obj1(1),'P');
fscanf(obj1(1));
end
t1=timer;
t1.startdelay=1;
t1.startfcn = @starttimer;
t1.TimerFcn = {@collectdata,obj1};
t1.StopFcn = {@stopdata,obj1};
%--------------------------
t1.Period = 0.1; %time between readings in seconds
t1.TasksToExecute = 4000; %number of readings
%-----------------------------
t1.ExecutionMode = 'fixedSpacing';
101
start(t1)
function starttimer(t1,event)
t0=event.Data.time;
data=[0 0];
assignin('base','data',data);
assignin('base','t0',t0);
function collectdata(t1,event,obj1)
fprintf(obj1(1),'P')
ot=fscanf(obj1(1))
out=sscanf(ot,'%f %*s');
tn_ = event.Data.time;
t0_=evalin('base','t0');
assignin('base','data',vertcat(evalin('base','data'),[etime(tn_,t0_) out]))
function stopdata(t1,event,obj1)
for i=1:length(obj1);
fclose(obj1(i));
delete(obj1(i))
clear obj1(i)
end
delete(t1)
clear t1
display('Timer ended');
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