-
School of Biomedical Engineering, Science, and Health
Systems
Drexel E-Repository and Archive (iDEA)
http://idea.library.drexel.edu/
Drexel University Libraries www.library.drexel.edu
The following item is made available as a courtesy to scholars
by the author(s) and Drexel University Library and may contain
materials and content, including computer code and tags, artwork,
text, graphics, images, and illustrations (Material) which may be
protected by copyright law. Unless otherwise noted, the Material is
made available for non profit and educational purposes, such as
research, teaching and private study. For these limited purposes,
you may reproduce (print, download or make copies) the Material
without prior permission. All copies must include any copyright
notice originally included with the Material. You must seek
permission from the authors or copyright owners for all uses that
are not allowed by fair use and other provisions of the U.S.
Copyright Law. The responsibility for making an independent legal
assessment and securing any necessary permission rests with persons
desiring to reproduce or use the Material.
Please direct questions to [email protected]
-
1400 ieee transactions on ultrasonics, ferroelectrics, and
frequency control, vol. 53, no. 8, august 2006
Letters
Characterization of Superhydrophobic MaterialsUsing
Multiresonance Acoustic Shear Wave
Sensors
Sun Jong Kwoun, Ryszard M. Lec, Richard A.Cairncross, Pratik
Shah, and C. Jerey Brinker
AbstractVarious superhydrophobic (SH) surfaces, withenhanced
superhydrophobicity achieved by the use ofnanoparticles, were
characterized by a new acoustic sens-ing technique using
multiresonance thickness-shear mode(MTSM) sensors. The MTSM sensors
were capable of dif-ferentiating SH properties created by
nano-scale surfacefeatures for lm, exhibiting similar macroscopic
contact an-gles.
I. Introduction
In recent years, a variety of synthetic approaches havebeen
developed to create so-called superhydrophobic(SH) surfaces
characterized normally by high static con-tact angles of water
(> 150) [1][4]. Superhydrophobicitydepends on surface roughness
and surface chemistry, butto date rigorous structure-property
relationships have notbeen established, especially the relationship
between staticand dynamic properties and how superhydrophobicity
isinuenced by nanoscale structural features. In this pa-per we use
high-frequency shear acoustic waves generatedby a piezoelectric
quartz resonator thickness-shear mode(TSM) sensor to interrogate SH
surfaces loaded with liquidmedia. For the TSM operating in the
frequency range of 1to 100 MHz, the depth of penetration is on the
order oftens to thousands of nanometers [5]; therefore, these
sen-sors are sensitive to nanoscale interfacial phenomena
andprocesses. Moreover, because the depth of penetration de-creases
with increasing TSM frequency, a multiresonanceexcitation of the
sensor allows spatial interrogation of theinterface with
controllable interrogation depth. The pur-pose of this
investigation is to evaluate the multiresonanceTSM sensing
technique to study the dynamic behavior ofthe SH/H2O interface and
to correlate the multiresonanceTSM (MTSM) response with microscopic
and nanoscopicfeatures of SH surfaces. Although all the SH surfaces
in thisstudy had similar macroscopic wettability (optical
contactangle 150), MTSM showed dierent responses, depend-ing on the
surface treatments and lm morphology. Thus,MTSM sensing may provide
a new means of probing thefunctional behavior of SH lms and help
establish neededstructure-property relationships.
Manuscript received January 10, 2006; accepted February 16,
2006.S. J. Kwoun, R. M. Lec, and R. A. Cairncross are with
Drexel
University, Philadelphia, PA 19104 (e-mail:
[email protected]).P. Shah and C. J. Brinker are with the
University of New Mexico,
Albuquerque, NM 87131.
II. Experiment
The TSM sensors were 10 MHz fundamental resonantfrequency quartz
crystals AT-cut and coated initially withgold electrodes on both
sides. Five samples of varying hy-drophobicity were prepared by
coating the sensors andsubjecting the coating to various surface
treatments (Ta-ble I summarizes the sample preparation). Sample 1
wasa bare TSM sensor and Samples 2 to 4A were coated withSH
coatings of increasing hydrophobicity. Sample 2 wascoated with
TFPTMOS (triuoropropyltrimethoxysilane)to produce a low-surface,
free energy with submicron-scaleroughness [4]. Sample 3 also had a
TFPTMOS coating butwas further treated with HMDS
(hexamethyldisilizane) toderivatize any remaining hydroxyl groups
with hydropho-bic trimethyl silyl Si(CH3)3 groups on the surface.
Sample4A was prepared by the same techniques as Sample 3 butwith
the addition of silica nanoparticles (2% by weight)to the TFPTMOS
coating, followed by treatment withHMDS. Sample 4B is the same MTSM
sensor and coatingas Sample 4A but after exposure to ultraviolet
(UV)/ozoneto reduce its hydrophobicity. All the SH samples were
pre-pared in the laboratory at the University of New Mexico.
The samples were characterized for macroscopic con-tact angle
and surface roughness. Contact angle was mea-sured by the sessile
drop method [6]. The surface topologyand roughness of samples was
measured using atomic forcemicroscopy (AFM) [7]. The AFM images of
the SH sur-faces were obtained in 3 m 3 m areas and are shownin
Fig. 1. The AFM software enables analysis of relativechanges of
surface area (Table I).
III. Results and Discussion
The measured macroscopic optical contact angles of wa-ter on
each of the samples are reported in Table I alongwith images of a
water droplet resting on the sample sur-face. The contact angle
increases from approximately 80
on the bare MTSM sensor to 140 to 155 for the SH coat-ings;
Ultraviolet/ozone treatment reduces the contact an-gle to
approximately 90 by changing the surface energywithout aecting
morphology. Samples 2, 3, and 4A areall SH with contact angles of
140 or more. The additionof silica nanoparticles in Sample 4A does
not signicantlyincrease the contact angle compared to Sample 3.
The surface area of the samples, as measured by AFM(Fig. 1),
increases steadily from Sample 1 to 4A. The sur-face area of Sample
4A was approximately 46% larger thanthe initial surface area
because of roughness resulting fromthe surface treatment. Samples 3
and 4A had similar sur-face areas and exhibited high contact angles
characteristicof SH surfaces. So according to standard static
character-ization protocols, both Samples 3 and 4 were equally
SH.
08853010/$20.00 c 2006 IEEE
-
et al.: characterization of superhydrophobic materials 1401
TABLE IPhysical Properties of SH-Fil ms Deposited on MTSM
Sensors.
Water drop onFilms and surface treatment 5 A 6 Schematic TSM
sensors
1. Gold, no surface treatment 80 0%
2. SH1 lm, no surface treatment 140 17%
3. SH1 lm + HMDS 2 surface treatment 155 43%
4A. NP 3 mixed SH 1 lm + HMDS 2 surface treatment 155 46%
4B. NP 3 mixed SH 1 lm + HMDS 2 + UV 4 treatment 90 These
pictures show a water droplet on the various SH surfaces studied;
however, for the MTSM experiments thesensors were completely
covered by a 4-mm layer of DI water. SH 1 lm:, TFPTMOS. HMDS 2 ,
(Y) hexamethyldisilizane,enhances hydrophobicity by replacing
hydroxyl groups with trimethyl silyl groups. NP 3 , () silica nano
particles (sizesbetween 2266 nm) increase the surface roughness and
geometrical SH mechanism. UV 4 , Ultraviolet light treatmentreduces
the water contact angle by removal of trimethyl silyl and
triuoropropyl groups. 5 , contact angle measuredoptically. A 6 ,
increase in surface area relative to bare surface measured by AFM
(%).
A network analyzer measurement system was used tomonitor the
frequency response of the MTSM sensor ex-posed to air or submersed
in 200 l (about 4-mm depth)of deionized (DI) water [5]. All
measurements were per-formed in an air-ow controlled chemical hood
at roomtemperature (approximately 25 C 0.1 C). The multi-harmonic
frequency response characteristics (i.e., resonantfrequency and
attenuation at rst, third, fth, and seventhharmonics) of coated
MTSM sensors were measured whenthe sensors were exposed to air
(dry) and when coveredwith a 4-mm layer of water (wet). The
relative changesof resonant frequency of the wet and dry sensors [f
rel.(1)] are plotted in Fig. 2(a):
f rel. =f dry f wet
f dry, (1)
where f dry and f wet indicate resonant frequencies of dryand
wet conditions, respectively. Changes in attenuationalso were
measured but are not reported here.
Two obvious phenomena can be extracted fromFig. 2(a):
Sample 4A always shows smaller f rel. than the othersamples at
all harmonics.
At higher harmonics (fth and seventh), f rel. of Sam-ples 2 and
3 are greater than Sample 1, and f rel. ofSample 4A is still
smaller than Sample 1.
Sample 4A exhibits much less response to water loadingat all the
tested harmonics.These trends can be explained
based on the hypothesis that, although the macroscopi-cally
observed contact angle of water on the SH surfacesare similar, the
mechanics of interaction near the water-SHcoating interface dier.
It is commonly understood that onrough SH surfaces, water doesnot
wet the entire surfaceat the microscopic or nanoscopic level [4].
Rather at thelevel of the roughness, water contacts the peaks
protrudingfrom the surface but does not penetrate into the
valleys,which are lled with air or vapor. For the SH lms
testedhere, the actual penetration depth of the water layer intothe
valleys of the rough SH surface are dependent on theconditions
(surface wettability) of the SH surface, withSample 4A exhibiting
much less interaction (i.e., less wa-ter penetration into
roughness). At the macroscopic scale,this reduced interaction could
be interpreted as eectiveslip between the liquid and SH surface due
to the reductionof the eective contact area [4], [8], [9].
The MTSM responses displayed in Fig. 2(a) show thatSH surfaces
with similar contact angles can exhibit dier-ent mechanical
interactions with water. The contact angleof water droplets on the
surface of Samples 2, 3, and 4Aare similar with approximately 150 ,
but the response ofMTSM sensors to DI water loading of each SH
sampleare dierent and are dependent on the harmonics. Dier-ent
harmonics probe dierent a coustic penetration depthsinto the
liquid, with higher harmonics more sensitive toliquid trapped in
submicron valleys.
Sample 4B was produced from Sample 4A by treatmentwith UV light
for about 30 minutes. This UV/ozone treat-
-
1402 ieee transactions on ultrasonics, ferroelectrics, and
frequency control, vol. 53, no. 8, august 2006
Fig. 1. AFM images of SH lms on MTSM sensors. (a) Sample 1,
surface of gold electrode on the MTSM sensor; (b) Sample 2; (c)
Sample3; and (d) Sample 4A.
ment does not change the surface morphology; rather it re-places
hydrophobic CH3 and CF3 groups with hydrophilichydroxyl groups via
an ozone mediated, photo-oxidativeprocess. The contact angle of
water on Sample 4B wasapproximately 90 (see Table I). Again, the
frequency re-sponses of Sample 4B were monitored and compared
withthose of the SH lm before UV treatment (Sample 4A)and the bare
MTSM (Sample 1). As shown in Fig. 2(b),the relative changes of
resonant frequency (frel.) of theUV treated sample (Sample 4B) in
response to water load-ing approaches that of the bare MTSM (Sample
1). Thefrel. curves for Samples 4B and 1 overlap, except for therst
harmonic. Although the morphology of the surface ofSample 4B is
virtually identical to that of 4A, the surfacefree energy of Sample
4B is higher than 4A due to the UVtreatment as evidenced by the
much lower contact angle ofwater. Higher surface free energy allows
water to penetratedeeper into the valleys. Sample 4B senses the
additionalmass eect from the trapped water and additional
viscousdamping from the water load on the SH lms. Sample 4Bshows a
similar response to Sample 1 at larger values ofresonant
frequency.
IV. Summary and Conclusions
Three superhydrophobic (SH) and two control surfaceswere
fabricated by ve dierent processes to produce sur-
faces with varying wettability and submicron scale rough-ness.
The surfaces were characterized with three methods:the acoustic
response of MTSM sensors to water loading,nano-scale surface
morphology by AFM, and optical mea-surements of contact angle of
water droplets. The AFMmeasured increases in surface area of 17 to
46% due toroughness of surface treatments. The dierences in
datawere next supported by MTSM results, which also showedthe
signicant dierences between those coatings. How-ever, the optical
measurements of macroscopic contact an-gles did not detect
dierences between the SH coatings.Specically, the three SH coatings
(Samples 2, 3, and 4A),fabricated with and without nanoparticles
and with dier-ent chemical treatments showed similar macroscopic
con-tact angles; the optical method was capable of only mea-suring
the dierence between less hydrophobic Samples 1and 4B and more
hydrophobic Samples 2, 3, and 4A.
Thus, although the contact angles of water droplets aresimilar
on all SH lms (samples 2, 3, and 4A), the acous-tic method using
MTSM sensors clearly exhibited dierentresponses in each sample. It
is interesting to notice thatthe MTSM responses were dependent on
the harmonic fre-quency. Sample 4A showed a much smaller frequency
shiftunder water loading than the other samples; this lm
in-corporated three techniques to provide SH: low free sur-face
energy TFPTMOS coating, HMDS surface treatment,
-
et al.: characterization of superhydrophobic materials 1403
Fig. 2. Relative changes in f for (a) MTSM-SH sensors at
har-monics, and (b) Sample 4A (before UV), Sample 4B (after UV),
andSample 1 (bare MTSM).
and addition of nano-particles in TFPTMOS. The combi-nation of
these factors produces a sample SH coating withthe optimized SH
properties. These MTSM results can be
interpreted as the presence of eective slip on the
texturedsurface [4].
Acknowledgments
The authors are pleased to acknowledge the fundingsupport from
DOE-Basic Energy Sciences, the Air ForceOce of Scientic Research,
the Army Research Oce,Sandia National Laboratorys LDRD Program, and
theNational Science Foundation under Grant DBI-0242622.
References
[1] Z. Yoshimitsu, A. Nakajima, T. Watanabe, and K.
Hashimoto,Eects of surface structure on the hydrophobicity and
slidingbehavior of water droplets, Langmuir, vol. 18, pp.
58185822,2002.
[2] J. Kim and C. Kim, Nanostructured surfaces for dramatic
re-duction of ow resistance in droplet-based microuidics, inProc.
IEEE Conf. MEMS, 2002, pp. 479482.
[3] L. Zhai, F. C. Cebeci, R. E. Cohen, and M. F. Rubner,
Sta-ble superhydrophobic coatings from polyelectrolyte
multilay-ers, Nano Lett., vol. 4, pp. 13491353, 2004.
[4] S. Gogte, P. Vorobie, R. Truesdell, A. Mammoli, F. van
Swol,P. Shah, and C. J. Brinker, Eective slip on textured
superhy-drophobic surfaces, Phys. Fluids, vol. 17, pp.
051701-1051701-4, 2005.
[5] G. L. Cote, R. M. Lec, and M. V. Pishko, Emerging
biomedicalsensing technologies and their applications, IEEE Sens.
J., vol.3, pp. 251266, 2003.
[6] K. Rogers, E. Takacs, and M. R. Thompson, Contact
anglemeasurement of select compatibilizers for polymer-silicate
layernanocomposites, Polymer Testing, vol. 24, pp. 423427,
2005.
[7] H. Yang, H. An, G. Feng, and Y. Li, Visualization and
quantita-tive roughness analysis of peach skin by atomic force
microscopyunder storage, LWTFood Sci. Technol., vol. 38, pp.
571577,2005.
[8] G. McHale, R. Lucklum, M. I. Newton, and J. A. Cowen,
In-uence of viscoelasticity and interfacial slip on acoustic
wavesensors, J. Appl. Phys., vol. 88, pp. 73047312, 2000.
[9] G. McHale and M. I. Newton, Surface roughness and
interfacialslip boundary condition for quartz crystal
microbalances, J.Appl. Phys., vol. 95, pp. 373380, 2004.