pubs.acs.org/Langmuir © 2010 American Chemical Society
Hysteresis and Reversibility of a Superhydrophobic Photopatternable Silicone Elastomer Gerald Blanco-Gomez,† Leonard M. Flendrig,‡ and Jonathan M. Cooper*,† †
Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8LT, U.K., and ‡ Department FSD, Unilever R&D Vlaardingen, Vlaardingen 3130 AC, the Netherlands Received November 18, 2009. Revised Manuscript Received February 9, 2010
We report upon the wetting property of layers of a micropatterned photodefinable silicon elastomer, PDSE, repetitively and alternatively treated with oxygen plasma and temperature cycles. At low power plasma treatments, we observed a hysteresis in terms of contact angle between phases lowering the contact angle and phases of recovery. As opposed to high power plasma for which we show that by generating fissures on the surface, the structure can be cycled between superhydrophobic and superhydrophilic states. The plasma-generated diffusion paths were characterized by electron microscopy and were found to be directly related to the recovery of the wetting properties of the plasma treated layers of PDSE. The cycling between the superhydrophobic and superhydrophilic states was dependent on the power of the applied plasma as well as the condition during the contact angle recovery amplified by a temperature-controlled baking step.
Introduction The cosine of the contact angle, θ, is usually defined as the ratio between interfacial forces (γls: solid-liquid; γsv: solid-vapor; γlv: liquid-vapor) acting upon a droplet on a specific solid surface in air:1 cos θ ¼
γsv - γsl γlv
ð1Þ
Superhydrophobic surfaces are generally acknowledged as having a contact angle with water above 150° in air, together with a low hysteresis (150°) and is primarily explained by the trapping of small pockets of air below the droplet (within the surface roughness), as described by eq 2, first formulated by Cassie and Baxter, for a heterogeneous surface of two materials (numbered 1 and 2), cos θCB ¼ f1 cos θ1 þ f2 cos θ2
ð2Þ
where θ1 and θ2 are the contact angles of each material, and f1 and f2 are the fraction of the droplet on material 1 and 2, respectively.2 As the fraction f2 increases, the overall contact angle of droplet comes closer to 180° (indeed, the contact angle of water on air is 180°). In addition, materials called ultrahydrophobic typically show contact angles between 120° and 150° and are also defined as being metastable. Man-made superhydrophobicity dates back to early twentieth century as first shown by Ollivier and Cassie-Baxter.2,3 Subsequently, controlling a material’s wetting property has been a subject of considerable interest for many technologies such as those associated with the textile, automotive, electronics, and biotechnology industries. For instance, interestingly, the attach*Corresponding author: e-mail
[email protected].
(1) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. (2) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (3) Ollivier, H. Ann. Chim. Phys. 1907, 10, 229.
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ment of cells on a surface depends, in part, on the contact angle of that surface. When maximum adhesion is found, the contact angle is between 45° and 75° (with the threshold between hydrophilic and hydrophobic being at 65°).4 At these values, the contact angle corresponds to the maximum adsorption of proteins within the extracellular matrix (ECM) associated with cell adhesion. Indeed, surfaces presenting extreme hydrophilic or hydrophobic behaviors tend to generally hamper the adhesion of the proteins of the ECM and resist cell adhesion.4 Similarly, manufacturers of the car industry aim to control the wetting property of windscreens so that water condensation does not impede the driver’s vision. Recently, there has been a significant interest in materials showing the ability to reversibly switch their wetting properties between superhydrophobic and superhydrophilic. Such a property could have applications in controlling fluid movement in a microfluidic channel (making a reversible valve) or be used for drug delivery mechanism. There are a number of different sources of external power that can cause a switch to this wetting property, including electrical, thermal, mechanical, and light-mediated mechanisms.5 As stated, flow control components for microfluidic platforms can benefit from materials that can present tunable extreme wetting properties. For example, a hydrophilic section of a microfluidic channel can be filled with water due to the capillary forces, whereas hydrophobic regions can cause back-pressure and prevent liquid from flowing, as a valve.6 Light-sensitive oxide materials such as TiO2, Fe2O3, SnO2, ZnO, and V2O5 show reversible switching between superhydrophobic and superhydrophilic surfaces upon the application of UV light (where the surface roughness has been enhanced enough to generate reliable composite states).7-10 In addition, a number of other molecules (4) Vogler, E. A. Adv. Colloid Interface Sci. 1998, 74, 69. (5) Xia, F.; Zhu, Y.; Feng, L.; Jiang, L. Soft Matter 2009, 5, 275. (6) Blanco-Gomez, G.; Glidle, A.; Flendrig, L. M.; Cooper, J. M. Anal. Chem. 2009, 81, 1365. (7) Borras, A.; Barranco, A.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 8021. (8) Hou, W.; Wang, Q. Langmuir 2009, 25, 6875. (9) Zhang, X. T.; Jin, M.; Liu, Z. Y.; Tryk, D. A.; Nishimoto, S.; Murakami, T.; Fujishima J. Phys. Chem. C 2007, 111, 14521. (10) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. B. Langmuir 2004, 20, 5659.
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undergo conformational changes upon exposure to UV light, inducing an increase of their wettability. The superhydrophobic property of such materials can be recovered if stored in the dark for one to several weeks.11 Similarly, materials made of conducting polymers such as poly(aniline) or poly(pyrrole) can be electrochemically switched between the two extreme wettability states.12 Reversible switching over a wide range of contact angles has also been observed using mechanical forces involving the stretching and unstretching polyamide and polyester fibers.13 In other studies, Song et al. explored the effect of argon ion plasma duration on the wettability of commercial polyester used to culture cells.14 Thermal treatment of modified poly(N-isopropylacrylamide) and poly(caprolactone) can also exhibit a useful change of wettability.15 Lindqvist et al. have described how modified cellulose materials, whose surface energy is responsive to both pH and temperature, change (even though no attempt was made to amplify their wettability switch by inducing an additional roughness).16 Electrowetting has been used to dynamically and reversibly switch the contact angle of water droplets in air from superhydrophobic to hydrophobic/hydrophilic.17 For instance, Verplanck et al. reversibly switched the contact angle of silicon nanowires between 137° and 160° (water in air).18 However, the physical study of the different wetting states observed (including e.g. Cassie-Baxter or Wenzel) as a transitional and composite, metastable state can explain contact angle values between 130° and 140°. Reversible electrowetting behavior was also observed using water mixed with glycerol and potassium chloride droplet in dodecane.19 Krupenkin et al. have also showed that the switching between a pinned state and a composite state can be carried out as a consequence of the evaporation of a thin portion of the droplet in contact with the substrate roughness (in this case made of silicon nanotubes).20 The mechanism, although producing locally very high temperatures, was used to trigger the transition between hydrophobic and superhydrophobic states, which initiated an efficient convective transport mechanism inside the droplet.21 Spontaneous and random transitions of droplets from pinned to composite state have also been observed under rather specific conditions.22 Microfluidic channels are often made of poly(dimethylsiloxane) (PDMS), either bonded to itself or to harder surfaces such as glass. Alternative silicon-based materials such as silicon rubber23 or photodefinable silicon elastomer (PDSE) have also (11) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M. H.; Cho, K. J. Am. Chem. Soc. 2006, 128, 14458. (12) Xu, L. B.; Chen, Z. W.; Chen, W.; Mulchandani, A.; Yan, Y. S. Macromol. Rapid Commun. 2008, 29, 832. (13) Wonjae, C.; Anish, T.; Shreerang, C.; Joseph, M. M.; Robert, E. C.; Gareth, H. M. Adv. Mater. 2009, 21, 2190. (14) Song, W. L.; Veiga, D. D.; Custodio, C. A.; Mano, J. F. Adv. Mater. 2009, 21, 1830. (15) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Minko, S. Adv. Mater. 2008, 20, 200. (16) Lindqvist, J.; Nystrom, D.; Ostmark, E.; Antoni, P.; Carlmark, A.; Johansson, M.; Hult, A.; Malmstrom, E. Biomacromolecules 2008, 9, 2139. (17) Krupenkin, T. N.; Taylor, J. A.; Kolodner, P.; Hodes, M. Bell Labs Tech. J. 2005, 10, 161. (18) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V.; Coffinier, Y.; Boukherrou, R. Nano Lett. 2007, 7, 813. (19) Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J.; Rack, P. D.; Fowlkes, J. D.; Doktycz, M. J.; Melechko, A. V.; Simpson, M. L. Langmuir 2006, 22, 9030. (20) Krupenkin, T. N.; Taylor, J. A.; Wang, E. N.; Kolodner, P.; Hodes, M.; Salamon, T. R. Langmuir 2007, 23, 9128. (21) Wang, E. N.; Bucaro, M. A.; Taylor, J. A.; Kolodner, P.; Aizenberg, J.; Krupenkin, T. Microfluid. Nanofluid. 2009, 7, 137. (22) Dorrer, C.; Ruhe, J. Adv. Mater. 2008, 20, 159. (23) Yasukawa, T.; Glidle, A.; Nomura, M.; Cooper, J. M. J. Microelectromech. Syst. 2005, 14, 839.
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been used to fabricate fluidic microstructures.24,25 Lowering the hydrophobicity of PDMS is generally performed through a change in the chemical composition of the surface, using plasma treatment in a carrier gas such as oxygen.26 It is also known that the migration of low molecular weight chains travel from the polymer bulk to the surface, partly explaining the contact angle recovery.26-29 Eddington et al. suggested that the diffusion of low molecular weight chains can be suppressed by overcuring (by several days) polysiloxane samples.27 Likewise, Owen et al. concluded that diffusion of low molecular weight molecules (which are responsible for the increase in contact angle) could benefit from additional thermal steps. Several studies have already reported the integration of superhydrophobic materials30 in microfluidic platforms. In particular, switching properties using superhydrophobic material can be reversed acting directly on the flow, for instance upon the application of a thermal step or light exposure.31 Plasma treatment has previously been used with various gases on siloxane-based polymer.32 In this paper we show that, by carefully adapting the dose of plasma and heating steps on samples, we were able to cycle the wettability of PDSE between superhydrophobic and superhydrophilic. Further, we demonstrate the effect leads to a hysteresis, a phenonemon which refers to there being a difference between the contact angles measured after successive low power oxygen plasma treatments. The effects, including the reversibility of the wetting and the hysteresis, were enhanced by inducing a designed roughness (as posts).
Materials and Methods Microscopy. Scanning electron micrographs (SEM) were obtained using the S-4700 and S-3000 from Hitachi. Prior to imaging, a thin conducting gold layer was sputtered on polymer surfaces. SEM images were taken, unless otherwise stated, at an accelerating voltage of 10 kV, at a working distance of 12 mm, using appropriate magnifications. Chemicals and Fabrication. PDSE (product no. WL-5351) and hexamethyldisiloxane (HMDSO) were purchased from Dow Corning. Glycidyl ether bisphenol A novolac (SU-8) (product no. 2002 and 2050) were from Microchem. The 4 in. silicon prime wafers were from University Wafer. The fabrication of the multilayers structure was carried out by successively coating a 2 μm thin layer of SU-8 2002, a 35 μm thick layer of SU-8 2050, and a 5 μm thin coating of WL-5351 organized in 5 mm 5 mm arrays on silicon substrate. The three layers were processed following manufacturer’s guidelines including substrate cleaning, resist coating, soft bake, exposure to i-line high-power source, development for SU-8 layers, and postexposure bake. Hard bakes were carried out as prescribed. All resists were developed in appropriate solvents and rinsed. In detail, SU-8 2002 resist layers were spun at 500 rpm for 5 s (acceleration: 100 rpm/s), then at 3000 rpm for 30 s (300 rpm/s), soft-baked at 95 °C for 2 min (hot plate), exposed to UV light for 9.7 s (70 mJ cm-2), postexposure baked at 95 °C for 2 min, and hard baked for 1 min at 180 °C. (24) Desai, S. P.; Taff, B. A.; Voldman, J. Langmuir 2008, 24, 575. (25) Blanco-Gomez, G.; Glidle, A.; Flendrig, L. M.; Cooper, J. M. Microelectron. Eng. 2009, 86, 1325. (26) Owen, M. J.; Smith, P. J. J. Adhes. Sci. Technol. 1994, 8, 1063. (27) Eddington, D. T.; Puccinelli, J. P.; Beebe, D. J. Sens. Actuators, B 2006, 114, 170. (28) Bodas, D.; Khan-Malek, C. Sens. Actuators, B 2007, 123, 368. (29) Fritz, J. L.; Owen, M. J. J. Adhes. 1995, 54, 33. (30) Lu, C.; Xie, Y.; Yang, Y.; Cheng, M. M. C.; Koh, C.-G.; Bai, Y.; Lee, L. J.; Juang, Y.-J. Anal. Chem. 2007, 79, 994. (31) Mumm, F.; van Helvoort, A. T. J.; Sikorski, P. ACS Nano 2009, 3, 2647. (32) Hollahan, J. R.; Carlson, G. L. J. Appl. Polym. Sci. 1970, 14, 2499.
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Figure 1. Roughness of the material was organized in arrays of pillars of widths W and period P; scale bar 250 μm. W was fixed at 50 μm and P was 112 μm, giving a ratio spacing/width of 2.2. Therefore, the percentage of the material in contact with droplets (assuming a composite wetting state) was 20%.
Figure 2. (A) Cosine of the contact angle for water droplets on PDSE, treated by different powers of oxygen plasma (6 s duration, with powers ranging from 20 to 200 W). (B) Pictures were taken 44 h after the third plasma treatment, with results displayed in Figure 3. We note here that droplets totally wetted PDSE surfaces treated with oxygen plasma of 90 and 200 W. SU-8 2050 resist layers were spun at 500 rpm for 10 s (100 rpm/ s), then at 4000 rpm for 45 s (300 rpm/s), soft-baked at 95 °C for 6 min (hot plate), exposed for 22.2 s (160 mJ cm-2), postexposure baked at 95 °C for 6 min. WL-5351 resist layers were spun at 5000 rpm for 3 min (500 rpm/s), soft-baked at 115 °C for 2 min (hot plate), exposed for 120 s (864 mJ cm-2), and postexposure baked at 135 °C for 2 min. Layers of PDSE were developed in some cases as indicated, using HMDSO. Features in SU-8 were patterned through an appropriate photolithographic mask using a MA-6 from SUSS MicroTec mask aligner (7.2 mW cm-2). Micropillars were designed with fixed periods and widths in order to fabricate surfaces with a solid percentage of 20% and a surface roughness of 1.27 (Figure 1A). Micrographs taken after each step of the described fabrication process using SEM and are available in the Supporting Information. For information, the surface solid percentage was defined as the ratio between W2 and P2 (see Figure 1 for details). Design. The lithographic process was carried out using a highresolution mask, designed with the Software L-Edit from Tanner 7250 DOI: 10.1021/la904374g
Figure 3. (A) Contact angle as a function of recovery time in air for plasma-treated samples performed at 20 (b), 30 (O), 40 (1), 50 (3), 60 (9), and 70 W (0). (B) Recovery in air for samples which were not immersed in HMDSO and were plasma treated at 20 (b), 30 (O), 40 (1), 50 (3), 60 (9), and 70 W (0). (C, D) Cycling wettability change upon the alternating application of oxygen plasma and recovery (storage plus bake). (E) Net CA recovery measured 284 h after plasma treatment for undeveloped (black bars) and developed (gray bars) samples. Langmuir 2010, 26(10), 7248–7253
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Figure 4. Net recoveries of the water CA were monitored for samples which had undergone 200 W oxygen power plasma for 1 min (A, B), 3 min, and 5 min (B) after each of the first three cycles plasma-bake. and defined with an electron beam lithographic tool EBPG5 HR 100 from Vistec. The geometry of the mask is show in Figure 1. Contact Angle. The Easy Drop from KRUSS GmbH was used for contact angle measurements. The contact angle of droplets with a volume of 4-6 μL were measured using ultrapure water (18 MΩ 3 cm) and calculated using the sessile drop method and Young-Laplace fitting after the steady state was reached. Plasma Treatment. Oxygen plasmas were carried out using Plasma Fab 5 from Gala Instrument running at 0.2 mbar under various powers. Treatment of PDSE by an oxygen plasma modulated the contact angle of the material, between a mild change of hydrophobicity (a few degrees at 20 W power from an original contact angle of 106°) to a surface presenting superhydrophilic behavior (total wetting at a power of 100 W and above) (Figure 2). Thin layers of PDSE were patterned on 1 1 cm2 silicon substrate and underwent low-power oxygen plasma treatment (between 20 and 70 W) for 6 min, before the recovery of their wetting properties were screened by recording their contact angle with water in air. High-power plasma treatments were also applied on the silicon elastomer surfaces using 200 W power for 1, 3, and 5 min. Bake Step. A 2 h bake at 180 °C was performed on samples 284 h after plasma treatment to enhance the recovery of contact angles.
Results Screening and Control of Intrinsic Contact Angle Recovery. Surfaces treated at the lowest powers (20, 30, and 40 W) did not show any significant contact angle recovery, as their contact angle remained at 107°, 100°, 86° (undeveloped) and 98°, 96°, 81° (developed), respectively (Figure 3A,C,E). Low-power plasma treatments do not show any fissures (which may favor the diffusion of short polymer chains, responsible for the recovery of their wetting properties).26-29 The amplitude of the recovery was also assessed as a function of the power of oxygen plasma treatment used (Figure 3E). Interestingly, the development step in HMDSO was found to impede the wetting property recovery (Figure 3A,B). Samples that were not processed in the developer solution showed less ability to recover their wetting properties than those which went through the development step. We assumed that immersion in the developer solution removed the short siloxane chains which had not polymerized and therefore remained within the polymer structure as free molecules to take part in the diffusion processes related to the wetting recovery. The relationship between the presence and contamination of such short chain siloxane polymers, their diffusion in resist matrix, and the effect (33) Thibault, C.; Sverac, C.; Mingotaud, A. F.; Vieu, C.; Mauzac, M. Langmuir 2007, 23, 10706.
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on the contact angle recovery after oxygen plasma treatments have previously been discussed in detail.26-29,33 The net recovery in terms of contact angle was also dependent on the power of the oxygen plasma used ranging from 0° for 20 W to 72° for 70 W (undeveloped) (Figure 3E). After 284 h (∼2 weeks), the same samples were repetitively (three times) exposed to the same power oxygen plasma treatment, for the same length of time in order to measure the recovery obtained, in terms of the contact angle. We observed a quantitative hysteresis between the first recovery and the one thereafter (for power 40-70 W), that is, after the baking step and subsequent plasma treatment (Figure 3C,D). As stated, for low-power treated samples, the recovery of the contact angles on high-power treated surfaces were screened over a 2 week period. The values taken directly after the plasma treatment revealed a state of total wetting with a contact angle close to 0°. Measurements of contact angle collected between 20 and 300 h after successive plasma treatments and baking steps were related, as can be seen as Figure 4A. The degree of contact angle recovery value (Figure 4B) was represented as the net difference between the contact angle measured after ∼300 h of recovery in air (plus bake step) and the resulting contact angle taken directly after high-power plasma treatment (∼0°). The net contact angle change after 284 h showed a much larger recovery (than for samples treated with a low power) as well as a better reversibility (no hysteresis) for successive plasma treatments (Figure 4A,B). All samples that underwent 200 W oxygen plasma treatments showed a recovery that exceeded 85°, which was enough to form composite states as shown on roughness enhanced surfaces. Electron Microscopy. No network of fissures was detected on untreated surfaces, as freshly processed samples presented smooth and homogeneous surfaces (see Supporting Information). Diffusion paths, which could be observed using SEM, were created during high-power oxygen plasma only. SEM investigations were carried out in the fissures developed during high-power plasma treatments. Networks of shallow fissures, hundreds of micrometers long and ∼1 μm wide, were observed together with narrower ones (typically below 150 nm) for samples treated for 1 and 3 min at high powers. Figure 5A,B shows three sizes of grooves widths, 1.2 μm, 120 nm, and 50 nm in size. As the duration of the plasma increased, the widths and the frequency of the appearance of fissures also increased. Samples treated for 5 min typically show widths of fissures >1.5 μm together with artifacts with widths of ca. 200 nm. For instance, in Figure 5C,D fissures had widths of 2.7 and 2.0 μm, and artifacts showed widths around 200 nm. In Figure 5E,F, we can also see the depth of the plasma generated fissures ranged from a few hundred nanometers to several micrometers in range. No DOI: 10.1021/la904374g
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Figure 5. (A) SEM picture, example of a fissure appearing after a 1 min plasma treatment; scale bar: 1 μm. (B) Fissures and artifacts observed after a 3 min plasma treatment; scale bar 50 μm and 500 nm, respectively. (C) Network of fissures which were observed after 5 min of plasma treatment; scale bar 200 μm, accelerating voltage 2 kV. (D) Interesection between two fissures sharing a continuity of artifacts; scale bars are 4 μm and 500 nm, respectively, accelerating voltage 5 kV. (E, F) Intersection of two fissures with a tilt angle of 77°; scale bars are 50 μm (E) and 3 μm (F), respectively, accelerating voltage 12 kV.
Figure 6. (A) Contact angle measurements of water droplets on microstructured SU-8/PDSE surfaces (solid percentages of 20%) after each of the four successive oxygen plasma-bake cycles (mean and SE for n = 3). The effect of 180 °C baking steps on the same plasma-treated PDSE/SU-8 was observed. (B) Picture of contact angle = 152° on surface of solid percentage of 20% before plasma treatment and after 180 °C bake. (C) Picture of contact angle = 0° on RS = 0.2 after plasma treatment (200 W, 3 min). 7252 DOI: 10.1021/la904374g
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obvious sign of fissures were found on low-power treated samples using SEM, implying that the magnitude of the wetting recovery was correlated to the dimension of the diffusion paths produced within the material during treatments by oxygen plasma. Indeed, we propose that fissures can either provide paths through the polymer, helping the diffusion of both oxidized chains from the layer surface to the bulk and nonoxidized chains from the bulk to the surface for recovery of the wetting properties. Surface Roughness Amplification of Wetting. We tested the recovery scheme of high-power treated samples to roughness enhanced surfaces. Oxygen plasma treatments at a power of 200 W for 3 min duration were applied on rough siloxane surfaces, presenting solid percentages of 20% and a roughness factor of 1.27. The plasma treatments induced a loss of superhydrophobicity, with a water contact angle after this step measured around 0° (similar to wettability observed on flat PDSE surfaces) (Figure 6A). After this oxygen plasma treatment, rough materials were left to relax in air. The recovery process was further investigated by measuring their contact angles after storing the substrates in air prior to a bake step (Figure 6A). During this process, when the contact angles of PDSE were 40°, droplets can be observed as spherical and relied on the Wenzel’s model on wetting, as previously described.34 Baking steps were found to accelerate and enhance the wetting recovery process of high-power plasma treated samples, as also observed for smooth PDSE surfaces (Figure 6A). When compared with surfaces coated with low surface energy polymers such as fluoro- or alkylchlorosilane to produce superhydrophobic surfaces on silicon or SU-8 pillars, one could expect that a self-assembled monolayer (∼15 A˚) would be irreversibly damaged after a similar oxygen plasma treatment (either low or high power), resulting in the irreversible loss of superhydrophobicity. Reproducibility of Successive Oxidations and Recoveries. Oxygen plasma treatments and baking steps were alternatively (34) Bico, J.; Tordeaux, C.; Quere, D. Europhys. Lett. 2001, 55, 214.
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cycled four times to observe that the superhydrophobic ([150°, 152°], Figure 6B) and superhydrophilic (≈0°, Figure 6C) states can be switched back and forth as observed (Figure 6A). The recoveries associated with high-power plasma-treated PDSE material were further enhanced by deliberately inducing a specific roughness, fabricated as a network of SU-8. Using eq 2, and relating this to the recoveries observed, one would have expected the contact angles to reach 143° (f1 = 0.2, θ1 = 85°). Fissures noticed by electron microscopy may also play a role as providing a second level of roughness in addition to the array of SU-8 microposts, resulting in underestimating the parameter f2 in eq 2 (Figure 5).
Conclusion We present a study of a silicon elastomer resist, PDSE, whose wettability can be finely adjusted, by controlling the power of oxygen plasma treatments. We show that PDSE wettability can be tailored over a wide range of contact angles. In doing so, the materials exhibit functionalities which include interesting stabilities (e.g., after treatment with a low-power plasma) and reversible superhydrophobic switching (e.g., after treatment with a high-power plasma), providing contact angles between 0° and 150°. We show that the recovery of the contact angle, during such state-switching depends upon the extent of the formation of fissures within the resist. This superhydrophobic recovery was achieved after successive and repetitive oxidations of the material, indicating that PDSE can, in future, be used as a smart material for controlling the surface energy. For example, applications such as those involved in cell engineering or microcontact printing may exploit the possibility of finely controlling the wetting property of silicon elastomer sheet or stamp. Moreover, stamps for microcontact printing can benefit from the control over the amount of contamination particles (development step). Acknowledgment. The authors acknowledge support from the Scottish Consortium in Integrated Microphotonics System and Unilever Vlaardingen Research Centre. Supporting Information Available: Pictures of two fabrication steps of the device. This material is available free of charge via the Internet at http://pubs.acs.org.
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