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].

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(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

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(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

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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)

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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

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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.

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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

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