-
A HIGHLY FLEXIBLE SUPERHYDROPHOBIC MICROLENS ARRAY WITH SMALL
CONTACT ANGLE HYSTERESIS
FOR DROPLET-BASED MICROFLUIDICS Maesoon Im1, Dong-Haan Kim1,
Xing-Jiu Huang2,
Joo-Hyung Lee1, Jun-Bo Yoon1, and Yang-Kyu Choi1 1Korea Advanced
Institute of Science and Technology (KAIST), Daejeon, KOREA
2University of Oxford, Oxford, UK ABSTRACT
This paper reports a highly flexible superhydrophobic and
superhydrorepellent microlens array substrate with very low flow
resistance. Even though the microlens array has no nanostructures,
it shows hydrophobic property due solely to its geometrical effect.
A contact angle of 165° and hysteresis of 3° are achieved on a
flexible polydimethylsiloxane (PDMS) microlens array substrate with
a Teflon (polytetrafluoroethylene) coating. Moreover,
double-layered metals (Cr/Au) are sandwiched between the PDMS and
Teflon layers for electrostatic or electrowetting-on-dielectric
(EWOD) actuation. Due to its low flow resistance and
superhydrophobicity, the array can be used as a microfluidic
component that reduces external pressure and power consumption for
mobility. INTRODUCTION
Random [1-3] or ordered [4-7] nanostructures and microstructures
can be used to realize superhydrophobic surfaces. To introduce
nanoscaled rough surfaces, researchers have reported numerous
approaches, including crystal growth [1], catalyzed growth [1],
electrospraying [2], plasma treatment [3], dry etching [4], and
replica molding with a porous anodic aluminum oxide template [5,
6]. Various materials have been used such as polydimethylsiloxane
(PDMS) [3], silicon [4], carbon nanotube arrays [7, 8], and
nanowires [9].
In addition to those previous works, we have reported a
perfectly ordered microbowl array [10] that makes large-area
superhydrophobic surfaces without nanostructures. A photoresist
microbowl array fabricated by means of three-dimensional diffuser
lithography [11] has extremely superhydrophobic features [10].
Recently, we fabricated a microbowl array and a microlens array on
a flexible polymer substrate [12] with a soft lithography replica
molding method.
A liquid droplet on a microbowl or microlens array follows the
wetting behavior of a Cassie-Baxter model [13]. Because air is
trapped among adjacent microlenses, the surface of the microlens
array is more hydrophobic than a flat surface made of the same
material. In Figure 1, a shape of the droplet on the microlens
array is shown.
One helpful way of manipulating liquid droplets in microfluidic
systems is to utilize electrostatic force or
electrowetting-on-dielectric (EWOD) actuation; however, a few
issues should be addressed in relation to the aforementioned
structures before these methods can be used in flexible
applications [14]. First, the low flow resistance is crucial. It is
noticeable that the high contact angle does not guarantee small
contact angle hysteresis [15]. Second, a thin metal film for the
electrostatic or EWOD actuation should be conformal, uniform, and
reliable on the substrate.
When the PDMS is used as a structural material, the microlens
array has a smaller contact angle than the microbowl array [12],
even though the microlens array is also hydrophobic and has a
simpler fabrication process. Moreover, the high adhesive force of a
PDMS microlens array is a fatal disadvantage for a microfluidic
component because the adhesive force impedes the transportation of
liquid samples to a designated location. By decreasing the adhesive
force on the surface of the PDMS microlens array, we can ensure
that the delivery of liquid samples with reduced power consumption
is possible in a microfluidic system, especially as a microfluidic
channel and as a form of droplet manipulation with electrostatic
force or EWOD actuation.
In the case of nanostructured superhydrophobic surfaces, a
process of filling gaps between nanostructures to form metal and
dielectric layers may degrade the hydrophobic property that
originates from nanoscaled geometrical shapes. Although metal
electrodes can be integrated underneath nanostructures, higher
operating voltages are needed on account of the thick
nanostructured materials required for hydrophobicity [16]. On the
surface of a microlens array, a metal layer can be deposited with
conformal coverage due to its convex shape. While keeping up this
advantage of the microlens array, we attain a lower flow resistance
and a higher contact angle in this work.
Liquid droplet
Microlens array
Figure 1: Schematic of a droplet shape on a microlens array in a
Cassie-Baxter regime FABRICATION PROCESS
The fabrication process of a microlens array with reduced flow
resistance is shown in Figure 2. A thick positive-type photoresist
(AZ9260, Clariant Co. Ltd.) is spin-coated on a silicon wafer.
Three-dimensional diffuser lithography [11] is then applied to the
photoresist to create microbowl patterns. As shown Figure 2(a), the
direction of UV light is randomized by a sandblasted diffuser plate
(F43-725, Edmund Optics Co. Ltd.) on a photomask, resulting in
microlens-shape exposure profiles. The fabrication conditions of
the photoresist microbowl array are well described in the
literature [10].
978-1-4244-2978-3/09/$25.00 ©2009 IEEE 475
-
D
Diameter
Height
Silicon wafer
UV
AZ9260 photoresist
Diffuser Photomask
Silicon waferMicrobowl array
Poured PDMS
PDMS microlens array
Teflon
SuperhydrophobicPDMS microlens array
Cr/Au
(a) (b)
(c) (d) Figure 2: Fabrication process of the proposed
hydrorepellent superhydrophobic PDMS microlens array (a)
three-dimensional diffuser lithography; (b) the pouring of PDMS
onto the fabricated microbowl array (photoresist mold); (c) the
peeling of the PDMS from the photoresist mold; (d) Teflon (500 nm)
coating on the sputtered Cr/Au (20 nm/300 nm)
Before the replica is formed, the surface of the photoresist
microbowl array is passivated with the vapor phase of a silanizing
agent (tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane) to
help release the PDMS. A PDMS prepolymer is prepared by thorough
mixing of the PDMS base and the curing agent (Sylgard 184 Silicone
Elastomer Kit, Dow Corning, Midland, MI) in a 10:1 (base:agent)
weight ratio. The prepolymer is then poured onto the mold of the
photoresist microbowl array, and air bubbles created during the
mixing and pouring process are removed in a low pressure chamber.
After that, the PDMS prepolymer is solidified at room temperature
for a day or at 80°C for an hour in a convection oven.
After peeling off the PDMS from the microbowl mold, we sputtered
double-layered metals (Cr/Au) on it for electrostatic or EWOD
actuation. Finally, a 500 nm Teflon layer is formed by the
spin-coating of 2 wt% Teflon AF2400 (amorphous fluoropolymer,
DuPont, Wilmington, DE) solution in FC-40 (perfluorocarbon, 3M, St.
Paul, MN), which evaporates overnight at room temperature.
Figure 3 shows scanning electron microscope (SEM) images of the
fabricated photoresist microbowl array and the PDMS microlens
array, which is a replica of the microbowl array. As shown in
Figures 3(a) to 3(d), the arrays are formed uniformly in a large
area. The microlens has a diameter of 10 μm and a height of 13 μm.
Figures 3(e) and 3(f) clearly show that metal layers are deposited
over the microlens array with conformal coverage.
Photographs of the fabricated samples in Figure 4 demonstrate
good flexibility and superhydrophobicity. The area of the PDMS
microlens array sample is 1 cm2, and it can be enlarged to wafer
size or even further. On account of the flexible PDMS substrate and
the very thin
metal layer, the superhydrophobic sample is highly flexible.
Additionally, a spherical water droplet is sustained on the
superhydrophobic surface, which assures a high contact angle. (a)
(b)
(c) (d)
(e) (f)
Figure 3: SEM images of fabricated sample (a) top view of a
photoresist microbowl array mold fabricated by means of diffuser
lithography; (b) tilted view of the photoresist microbowl array
mold; (c) top view of a PDMS microlens array fabricated from a
photoresist microbowl array mold; (d) tilted view of a PDMS
microlens array; (e) cross section of a PDMS microlens array; (f)
cross section of a PDMS microlens array after Cr/Au deposition
1cm
1cm
(a) (b)
(c) Figure 4: Photographs of fabricated samples (a) a PDMS
microlens array; (b) flexibility test after Cr/Au deposition and
Teflon coating on a PDMS microlens array; (c) a water droplet
(approximately 10μl) on the fabricated superhydrophobic sample
476
-
EXPERIMENTAL RESULTS The wettability on the surface of the
fabricated
microlens array is in the Cassie-Baxter regime [13] because of
the air trapped between the microstructures, as on the photoresist
microbowl array [10]. With consideration given to the surface
roughness, the contact angle (θCB) of the surfaces governed by the
Cassie-Baxter model is expressed [1] as follows:
cos θCB = rf (cos θFLAT)+ f −1 (1) where r is the roughness
factor of the surface, f is the fraction of area that supports the
liquid droplet, and θFLAT is the contact angle on a flat
surface.
As shown in Figure 5, the contact angle of the flat Teflon
surface (θFLAT, Teflon) is slightly higher than that of the flat
PDMS surface (θFLAT, PDMS). Together with the geometrical effect (r
and f) on hydrophobicity, the Teflon coating on the PDMS microlens
array enhances the contact angle so that the angle is comparable to
that of the microbowl array [10]. Figure 6 shows the contact angles
before and after the Teflon coating. Note that the hydrophobic
surface becomes superhydrophobic (θC>150°) when the Teflon layer
is introduced.
θC
(a) (b)
θC
Figure 5: Contact angles of flat surfaces (a) PDMS on a silicon
wafer (θFLAT, PDMS=116°) and (b) a Teflon-coated silicon wafer
(θFLAT, Teflon=122°)
(a) (b)
θC θC
Figure 6: Contact angles of the microlens array (a) a PDMS-only
microlens array (θC=141°) and (b) a Teflon-coated PDMS microlens
array (θC=165°)
(a) (b)
θR
θAθR θA
Figure 7: Contact angle hysteresis of the microlens array on a
tilted plate (a) a PDMS microlens array (θADV=154°, θREC=117°;
θHYS=37°) and (b) a Teflon-coated PDMS microlens array (θADV=165°,
θREC=162°; θHYS=3°)
Reduction of the flow resistance on the Teflon-coated surfaces
is confirmed by measurement of the contact angle hysteresis (θHYS),
which is the difference between an advancing and receding contact
angle. The tilting plate method is used to analyze the contact
angle hysteresis, as shown in Figure 7. The hysteresis of the
proposed structure (Teflon/Au/Cr/microlens) is reduced remarkably
from 37° to 3°.
The small contact angle hysteresis means that a liquid droplet
can roll off easily due to the negligible static friction force
(Ff), which is calculated as follows [16]:
Ff = 2γlaw(cos θADV − cos θREC) (2) where γla is the liquid-air
interfacial energy, w is the width of the droplet, θADV is the
advancing contact angle, and θREC is the receding contact
angle.
We can estimate the reduction of static friction force by the
above equation (2). The friction force on the proposed structure is
reduced enormously to just 3.3% of that on the initial PDMS
microlens array. This level of reduction reveals that droplet
manipulation is feasible on the proposed surface with low power
consumption.
The contact angle and its hysteresis of the PDMS microlens array
are strongly dependent on the aspect ratio of the microlens [12].
Therefore, by controlling the aspect ratio, we can adjust those
parameters to satisfy the demands of end-users.
To demonstrate the superhydrorepellency of the fabricated
sample, we recorded a video clip of a water droplet rolling off.
Figure 8 shows the captured images with a time interval of 1/30 s.
A deionized water droplet of 15 μl is dispensed by a micropipette
on the sample, which is placed on a slide glass tilted about 6°.
This demonstration clearly shows the exceedingly slippery surface
characteristic, which is a very significant aspect of a
self-cleaning application.
#1 #2 #3 #4
#5 #6 #7 #8
Figure 8: Captured images to show rolling off characteristics of
a water droplet (approximately 15μl) on the fabricated sample with
6° tilted angle. The time interval between adjacent frames is 1/30
s.
An endurance test is carried out with a vortex mixer to give the
cycled bending stress to the fabricated sample. It should be noted
that the fabricated sample showed no significant degradation of the
contact angle or electrical resistance (RAB) of the metal layer
after being bent more
477
-
than 105 times as shown in Figure 9. This result is critical in
applications involving liquid transportation on a flexible
substrate for an arbitrarily curved shape [14]. The fact that the
electrical connection is guaranteed after the repetitive bending
highlights the potential use of the fabricated sample as a
substrate for droplet movements by electrostatic force or EWOD
actuation in droplet-based microfluidics.
100 101 102 103 104 1050
20406080
100120140160180
0
2
4
6
8
10
Con
tact
ang
le (°
)
Number of bending
Res
ista
nce,
RAB
(Ω)
A
B
Figure 9: Endurance of the contact angle and the electrical
resistance after the cycled bending stress CONCLUSIONS
In this work, substrates with the characteristics of
superhydrophobicity, superhydrorepellency, and flexibility were
demonstrated with the aid of a Teflon-coated PDMS microlens array.
The array consists of a unit microlens with a diameter of 10 μm and
a height of 13 μm. The contact angle improvement from 141° to 165°
ensures the attainment of a satisfactory level of hydrophobicity.
In addition, the fact that the contact angle hysteresis is reduced
from 37° to 3° ensures that a satisfactory level of hydrorepellency
is achieved with the aid of Teflon coating and a three-dimensional
microlens structure.
For potential droplet manipulation on the fabricated sample by
electrostatic force or EWOD actuation, double-layered metals were
integrated between the PDMS and the Teflon layer. The endurance of
electrical continuity on the same flexible substrate was
characterized in a cyclic bending test. The results confirm that
the reduced small contact angle hysteresis can decrease any
external pressure and power consumption in droplet manipulation for
transportation of liquid samples with warranted reliability.
The proposed structure is expected to be utilized in
applications for droplet-based microfluidics and for the
self-cleaning of arbitrarily curved surfaces such as a swimsuit,
goggles for swimmers, anti-fog glasses, and the windshield of a
car. ACKNOWLEDGEMENTS
This work was partially supported by a grant from the National
Research Laboratory (NRL) program (No. R0A-2007-000-20028-0) of the
Korea Science and Engineering Foundation (KOSEF), which is funded
by the Korean Ministry of Education, Science and Technology (MEST).
It was also partially supported by the National Research and
Development Program (NRDP, 2005-01274) for the development of
biomedical function monitoring biosensors; this program is also
sponsored by the Korean Ministry of Education, Science and
Technology.
REFERENCES
[1] P. Roach, N. J. Shirtcliffe, and M. I. Newton, “Progress in
superhydrophobic surface development,” Soft Matter, vol. 4, pp.
224-240, 2008. [2] B. Burkarter, C. K. Saul, F. Thomazi, N. C.
Cruz, L. S. Roman, and W. H. Schreiner, “Superhydrophobic
electrosprayed PTFE,” Surf. Coat. Tech., vol. 202, pp. 194-198,
2007. [3] A. D. Tserepi, M.-E. Vlachopoulou, and E. Gogolides,
“Nanotexturing of poly(dimethylsiloxane) in plasmas for creating
robust super-hydrophobic surfaces,” Nanotech., vol. 17, pp.
3977-3983, 2006. [4] T. N. Krupenkin, J. A. Taylor, T. M.
Schneider, and S. Yang, “From rolling ball to complete wetting: the
dynamic tuning of liquids on nanostructured surfaces,” Langmuir,
vol. 20, pp. 3824-3827, 2004. [5] L. Zhang, Z. Zhou, B. Cheng, J.
M. DeSimone, and E. T. Samulski, “Superhydrophobic behavior of a
perfluoropolyether lotus-leaf-like topography,” Langmuir, vol. 22,
pp. 8576-8580, 2006. [6] M. Kim, K. Kim, N. Y. Lee, K. Shin, and Y.
S. Kim, “A simple fabrication route to a highly transparent
super-hydrophobic surface with a poly(dimethylsiloxane) coated
flexible mold,” Chem. Commun., vol. 22, pp. 2237-2239, 2008. [7] K.
K. S. Lau, J. Bico, K. B. K. Teo, M. Chhowalla, G. A. J.
Amaratunga, W. I. Milne, G. H. McKinley, and K. K. Gleason,
“Superhydrophobic carbon nanotube forests,” Nano Lett., vol. 3, no.
12, pp.1701-1705, 2003. [8] L. Ci, R. Vajtai, and P. M. Ajayan,
“Vertically aligned large-diameter double-walled carbon nanotube
arrays having ultralow density,” J. Phys. Chem. C, vol. 111, pp.
9077-9080, 2007. [9] J. Yuan, X. Liu, O. Akbulut, J. Hu, S. L.
Suib, J. Kong, and F. Stellacci, “Superwetting nanowire membranes
for selective absorption,” Nature Nanotech., vol. 3, pp. 332-336,
2008. [10] X.-J. Huang, J.-H. Lee, J.-W. Lee, J.-B. Yoon, and Y.-K.
Choi, “A one-step route to a perfectly ordered wafer-scale
microbowl array for size-dependent superhydrophobicity,” small,
vol. 2, pp. 211-216, 2008. [11] S.-I. Chang, and J.-B. Yoon,
“Shape-controlled, high fill-factor microlens arrays fabricated by
a 3D diffuser lithography and plastic replication method,” Optics
Express, vol. 12, no. 25, pp. 6366-6371, 2004. [12] X.-J. Huang,
D.-H. Kim, M. Im, J.-H. Lee, J.-B. Yoon, and Y.-K. Choi,
“‘Lock-and-key’ geometry effect of patterned surfaces: the
wettability and the switching of adhesive force,” small, accepted
to be published. [13] A. B. D. Cassie, and S. Baxter, “Wettability
of porous surfaces,” Trans. Faraday Soc., vol. 40, pp. 546-551,
1944. [14] M. Abdelgawad, S. L. S. Freire, H. Yang, and A. R.
Wheeler, “All-terrain droplet actuation,” Lab Chip, vol. 8, pp.
672-677, 2008. [15] J. Kim, and C.-J. Kim, “Nanostructured surfaces
for dramatic reduction of flow resistance in droplet-based
microfluidics,” in Digest Tech. Papers IEEE MEMS 2002 Conference,
Las Vegas, NV, Jan. 20-24, 2002, pp. 479-482. [16] K.-S. Yun, and
C.-J. Kim, “Low-voltage electrostatic actuation of droplet on thin
superhydrophobic nanoturf,” in Digest Tech. Papers IEEE MEMS 2007
Conference, Kobe, Japan, Jan. 21-25, 2007, pp. 139-142.
478
MAIN MENUGo to Previous DocumentCD/DVD HelpSearch CD/DVDSearch
ResultsPrint
/ColorImageDict > /JPEG2000ColorACSImageDict >
/JPEG2000ColorImageDict > /AntiAliasGrayImages false
/CropGrayImages true /GrayImageMinResolution 200
/GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true
/GrayImageDownsampleType /Bicubic /GrayImageResolution 300
/GrayImageDepth -1 /GrayImageMinDownsampleDepth 2
/GrayImageDownsampleThreshold 2.00333 /EncodeGrayImages true
/GrayImageFilter /DCTEncode /AutoFilterGrayImages true
/GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict >
/GrayImageDict > /JPEG2000GrayACSImageDict >
/JPEG2000GrayImageDict > /AntiAliasMonoImages false
/CropMonoImages true /MonoImageMinResolution 400
/MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true
/MonoImageDownsampleType /Bicubic /MonoImageResolution 600
/MonoImageDepth -1 /MonoImageDownsampleThreshold 1.00167
/EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode
/MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None
] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false
/PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000
0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true
/PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ]
/PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier ()
/PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped
/False
/CreateJDFFile false /Description > /Namespace [ (Adobe)
(Common) (1.0) ] /OtherNamespaces [ > /FormElements false
/GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks
false /IncludeInteractive false /IncludeLayers false
/IncludeProfiles true /MultimediaHandling /UseObjectSettings
/Namespace [ (Adobe) (CreativeSuite) (2.0) ]
/PDFXOutputIntentProfileSelector /NA /PreserveEditing false
/UntaggedCMYKHandling /UseDocumentProfile /UntaggedRGBHandling
/UseDocumentProfile /UseDocumentBleed false >> ]>>
setdistillerparams> setpagedevice