Lock-and-Key Geometry Effect of Patterned Surfaces: Wettability and Switching of Adhesive Force**

Microlithography Lock-and-Key Geometry Effect of Patterned Surfaces: Wettability and Switching of Adhesive Force** Xing-Jiu Huang,* Dong-Haan Kim, Maesoon Im, Joo-Hyung Lee, Jun-Bo Yoon, and Yang-Kyu Choi* A rough surface can be a regular (engineered surface), a random (irregular rough surface), or an intermediate case (hierarchical rough surface). [1] Whichever case is used for wettability, a truly superhydrophobic surface exhibits not only a high contact angle (>150 8) but also a low-contact-angle hysteresis (sliding angle). [2] Quéré et al. theoretically described how contact-angle hysteresis generates an adhesive force and the contact angle and hysteresis can be controlled by tailoring the surface topography of the solid substrate. [3] Researchers have since attempted to capture these properties in synthetic materials with nanoscale surface features [4] or changes of surface topography. [5,6] For flexibility in adapting to rough surfaces, poly (dimethylsiloxane) (PDMS) elastomer, an ideal elastic material in terms of its stress strain response, has attracted great attention. Particular interest has focused on introducing nanoscale structures onto microscale surfaces using surface treatments to reach a hydrophobic state, such as mechanically assembled monolayers, [7] CO 2 pulsed-laser etching, [8 10] UV/ ozone surface treatments, [11] SF 6 plasmas, [12] oxygen plasma and chemical surface treatments, [13,14] and laser etching. [15] Besides these, Hang et al. created an artificial lotus leaf by [] Dr. X.-J. Huang, Prof. Y.-K. Choi, M. Im Nano-Oriented Bio-Electronic Lab School of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology Daejeon, 305-701 (South Korea) E-mail: xingjiuhuang@hotmail.com; ykchoi@ee.kaist.ac.kr D.-H. Kim, J.-H. Lee, Prof. J.-B. Yoon 3D Micro-Nano Structures Lab School of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology Daejeon, 305-701 (South Korea) [] X.-J.H. would like to express appreciation for the financial support of the Brain Korea 21 project, the school of Information Technology, and the Korea Advanced Institute of Science and Technology in 2007. This work was also partially supported by the NRL program of the Korea Science and Engineering Foundation grant funded by the Korea government (MOST) (No.R0A-2007-000- 20028-0). : Supporting Information is available on the WWW under http:// www.small-journal.com or from the author. DOI: 10.1002/smll.200800649 nanocasting [16] and Lee et al. fabricated PDMS micropillar structures. [17] Patterned surfaces thus exhibit their unique advantages in hydrophobicity due to their large-scale surface uniformity. However, the adhesive force still exists at some interfaces. Inspired by the lock-and-key model, a new type of patterned surface consisting of dense arrays of microfabricated PDMS lenses (lock) and bowls (key) for the wettability and switching of the adhesive force is presented here. The dimension of the arrayed PDMS microlens is studied to optimize the hydrophobicity and adhesive force. The imprinted microbowl-arrayed surface exhibits a superhydrophobicity with a high contact angle (approximately 164.6 8) and a low adhesive force (the work of adhesion decreased nearly one-tenth in comparison with microlens arrays). This approach provides a simple method to investigate the transformation of an adhesive force into an anti-adhesive force. Significantly, the anti-adhesive properties of materials with microbowl arrays should be very useful in a wide range of biomedical applications, including blood pumps, cardiac pacemaker leads, and other PDMS-based medical devices, due to their good biocompatibility and flexibility. [18] A superhydrophobic surface is created by close-packing trifluoromethyl groups to decrease the surface energy. The strategy in this study employed microlens and microbowl arrays without a thin fluorinated carbon film coating (Figure 1). First, an AZ9260 positive photoresist (PR, Clariant Co. Ltd.) layer with a thickness of 80 mm was spin coated onto a Si substrate at 1500 rpm for 0.5 s. Using the threedimensional (3D) diffuser lithography that has been reported previously, [19] a highly ordered array of PR microbowls was formed on the substrate. For the microlens array, the primary design criteria included the diameters of the heights of the lens. The microlens array is supported on a continuous PDMS (Sylgard 184, Dow Corning) or h-pdms (KE-1606, Shin-Etsu, Japan) film (2 3 mm in thickness). A microlens array of 10-mm diameter and 6-mm height (10 6), a 10 2 microlens array, and a flat PDMS were also tested for adhesive force. To comparably check the wettability and switching of adhesive force, 10 6 PDMS microbowl arrays, with imprinted inverse microlens structure following the lock-and-key domains) were fabricated. A second replication of PDMS was performed on the h-pdms microlens-arrayed template in the same manner as the first process. Before casting, an antistick monolayer 90 ß 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2009, 5, No. 1, 90 94

Figure 1. Rational design and fabrication of lens and bowl microarrays based on a lock-andkey principle. A three-dimensional diffuser lithograph was used to create an array of microbowls in a photoresist (PR) thin film supported on Si (PR/Si master). PDMS or h-pdms casting onto the master was followed by curing, and the lift-off resulted in the microlens array containing the lock aspect. Finally, an anti-stick monolayer was coated onto the fabricated h-pdms microlens array (used as a template). A second replication of PDMS was then performed on the microlens array and peeled off, resulting in the matching key PDMS microbowl array. Figure 2. A 408 view of the fabricated PDMS microlens arrays with different magnifications. A uniform feature is clearly observed at low magnification (a, b). The microlens-arrayed surface morphology is apparent at high magnification (c, d). (vapor-phase tridecafluoro-1,1,2,2-tetrahydrooctyltricholrosilane (CF 3 -(CF 2 ) 5 (CH 2 ) 2 -SiCl 3, Fluka) was evaporated on the PDMS microlens array surface as a release agent for 30 s, and the PDMS solidification was performed at 85 8C for 60 min. The PDMS film was then peeled off, which imprinted the inverse microlens array structures. Scanning electron microscopy (SEM, Philips XL 30 AFEG Eindhoven, The Netherlands) images are shown in Figure 2 to demonstrate the features of the fabricated microlens arrays. A uniform feature is clearly observed at low magnification (Figure 2a and b). The surface exhibits a high flatness that was inherited from the supporting PR microbowl-arrayed substrate. No distortion was observed on the array surface, suggesting that this approach eliminates significant drawbacks (e.g., small areas, non-homogeneity, nonuniform distribution) encountered with conventional bottom-up techniques. Looking at the surface properties, it can be deduced that the highly uniform surface with very few defects contributes significantly to the smallest sliding angle, an important characteristic of the rough surfaces. In contrast, it was observed that the structure exhibits hydrophobic properties with a high-contact-angle hysteresis, as will be considered in more detail in the following section. Figure 2c and d shows high-magnification images of the PDMS microlens, and the convex shape and smooth exterior surface of the lens can be clearly seen. The first attempt at measuring the contact angle (u) of a droplet was undertaken to find the geometrical effect and dimension-dependent hydrophobicity of the perfectly ordered microlens surface (u was measured on a Dataphysics OCA20 CA system at ambient temperature; the droplet volume used in the experiments was 6 ml). Arrays of PDMS microlens 5, 10, and 20 mm in diameter and 2 mm in height, as well as 10 mm in diameter and 6, 7, 8, and 12 mm in height, were successfully fabricated using a 3D diffuser lithograph. Figure 3a clearly shows the comparison results of a flat surface and the microlensarrayed surface. The water-contact angle of the flat surface of PDMS was approximately 108.3 8 (Figure 3b). A heightdependent hydrophobicity was also found (Figure 3a, violet square and fitted line); the apparent contact angle noticeably increased with an increasing height and the surface changed from moderately hydrophobic (which is always the case with water where the contact angles never exceed 120 8) [20] close to a superhydrophobic state (150 8). This is a very interesting result: it is entirely different from previous reports on microfabricated pillars or post arrays that show a highly hydrophobic state if given the smallest possible values of a/h (a: pillar size; H: height; the contact angle drastically increased small 2009, 5, No. 1, 90 94 ß 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 91

Figure 3. The diameter- and height-dependent hydrophobic behavior of a microlens- arrayed surface. a) The comparison between the wettability of a flat surface and the microlens array with different dimensions. The inset is a simple model of a water droplet pinned on a microlens-arrayed surface; b d) Micrographic pictures of a droplet on a flat surface and microlens array with different diameters and heights. with increasing H). [21] Therefore, the fact can be explained by considering the unique structure of the microlens and the adhesive force between the PDMS microlens and water droplet, which results in a heterogeneous contact. As shown in Figure 3a, the variation of the contact angles measured at different sites further implies that the PDMS microlens-arrayed surface has hydrophobicity with a high adhesive force. Again, the wettability of the microlens-arrayed surfaces with different diameters was investigated. As highlighted by the black circle in Figure 3a (black square), a good diameter-dependent wettability is evident; that is, the contact angle decreased as the diameter of the microlens increased (Figure 3c). In particular, the contact angle approaches 110.5 8 for the 20 2 microlens-arrayed surface, which is very close to that of the flat surface (108.3 8). It is not difficult to understand this situation. If the structure is too low and too wide, the microlens-arrayed surface becomes almost flat and the droplet is able to wet the surface and enters the Wenzel regime [22] that is based on the hypothesis of a saturated surface. Therefore, it was concluded that, in these situations, a perfectly ordered microlens array geometrically and crucially influences the hydrophobic behavior of the surfaces. Furthermore, the diameter and height of the microlens are crucial parameters that govern the hydrophobicity. Besides, this type of patterned surface is very useful for quantitative studies of the Figure 4. Theadhesiveforceof a microlensarray. a,b) Behaviorof awater droplet on a 10 6 microlens-arrayed surface with tilt angles of 90 8 and 180 8, respectively. c) The work of adhesion (open symbol) and contactangle(solidsymbol) asafunctionofthedropvolumeonthepdms with flat surface (square), 10 2 array (circle),and 10 6 array (triangle). The work of adhesion was obtained directly from the contact-angle measurements. Theinsetsshowa cross-sectional viewof thesem images of the microlens array with different heights. d) Schematic image of thepossiblestaticanddynamicbehaviorofadropletatmicrolens-arrayed interfaces according to the observations. equilibrium configurations of droplets on rough substrates [9] and therefore avoids the indiscriminate problems that occur with randomly rough surfaces. Figure 4a and b shows the behavior of the water droplet when the PDMS microlens-arrayed substrate is tilted vertically or turned upside down. It was noted that the water droplet does not slide and the contact angle is larger at the front (advancing angle, u A ) than at the rear (receding angle, u R ; Figure 4a). This contact-angle hysteresis (u A u R ) generates a force that opposes the weight of the drop (and is able, if the drop is small enough, to balance it): the liquid is pinned. [23] As indicated above, even though the height of the microlens was 92 www.small-journal.com ß 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2009, 5, No. 1, 90 94

increased, the surface could not reach a state in which the contact angle was larger than 150 8; this is the case where the contact-angle hysteresis (i.e., a strong adhesive force) might be influenced in the transition of a surface from a hydrophobic to a superhydrophobic. Here, it is ascribed to the Wenzel model in which the droplet is able to wet the surface, thus resulting in a high-contact-angle hysteresis (Figure 4d). The microlensarrayed surface thus becomes sticky. The work of adhesion is a method used to investigate the physical force (i.e., adhesive force or adhesion) at interfaces between solid substances and water. [24] It is generally defined by the following equation: [25] W ¼ g LV ð1 þ cos uþ where W is the work of adhesion, and g LV and u refer to the surface tension of the liquid vapor and apparent contact angle, respectively. Therefore, the work of adhesion and contact angle of the PDMS flat and microlens-arrayed surfaces was measured, and the comparison is shown in Figure 4c. The work of adhesion decreased with an increase of the contact angle for these three types of surface. The structure of the microlens array improved the adhesion at the interfaces. The work of adhesion was obviously decreased by introducing microlens arrays onto the surface. Additionally, the adhesive force on the 10 6 microlens-arrayed surface was smaller than that on the 10 2 surface but it was still strong. It was then concluded that the flat PDMS showed a higher work of adhesion and the surface topography could greatly influence the adhesive force. For a nanotube film and an artificial lotus-leaf surface, it has been suggested that the high adhesive force results from the nanotube density [26] and the force acting between the micro-orifices and wall roughness. [27] It is proposed here that the high adhesive force results from the microlens-shaped array geometry. Considering that the contact angle and contact-angle hysteresis can be controlled by tailoring the surface topography of the solid substrate, PDMS microbowl arrays were fabricated using PDMS microlens arrays as a templates (lock-and-key domains). Figure 5a shows the surface topography of microbowl arrays. The bowl-like shape and nanoscale wall notches are clearly visible. The bowls are arranged in a regular hexagonal array. This is a perfect holearrayed surface with numerous notches and sharp nano-apexes (this is why the influence of release agent on the wettability is ignored). The substrate is mainly composed of air, which eventually leads to a strong reduction or elimination of the contact-angle hysteresis. The inset in Figure 5a indicates that the bowl-shaped structure has a hollow that looks like a capillary tube. For this geometry, its size-dependent superhydrophobic behavior has been demonstrated both experimentally and theoretically using a photoresist material as an example. [19] Given reasonable dimensions, the wettability follows the Cassie Baxter model in which the air trapping mode is dominant. [28] As expected, the 10 6 PDMS microbowl-arrayed surface exhibits perfect superhydrophobicity with a contact angle of approximately 164.6 8 (Figure 5b) owing to the hollow structures in the surface. It is much better than that of the AZ9260 photoresist microbowl-arrayed surface with the same dimensions (143.5 8). [19] This is because Figure 5. The superhydrophobic behavior of a 10 6 PDMS microbowlarrayed surface. a) SEM image of a 10 6 PDMS surface morphology with a microbowl-arrayed structure. The inset shows a high-magnification SEM image of the bowl-shaped structure. The scale bar is 2 mm. b) The shape of a water droplet on the microbowl-arrayed surface, indicating its superhydrophobicity with a contact angle of 164.6 8. c) The contact angle measured at different locations for a PDMS microbowl-arrayed sample, and the comparison of adhesive force obtained directly from contactangle measurements from a PDMS microlens-arrayed surface. d) Schematic image of the possible static and dynamic behavior of a droplet at microbowl-arrayed interfaces according to the observations. the AZ9260 photoresist is a weak hydrophilic material, u Y ¼ 78.5 8. Since the contact-angle hysteresis originates in the defects of the solid substrate, only a very small hysteresis is expected on this microtextured superhydrophobic surface. Next, in order to determine the contact-angle hysteresis of the surfaces, the sample was fixed on an optical bench below the syringe, which could be moved freely. A droplet was first formed and maintained with the syringe. When the droplet was in the exact contact state, the substrate was slowly moved with a micrometric screw. It is noted that the water droplet does not come to rest and can be slipped effortlessly on this surface using a syringe; the receding contact angle is almost equal to the advancing contact angle (Figure S1), indicating a very small contact-angle hysteresis or sliding angle. Based on the small 2009, 5, No. 1, 90 94 ß 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 93

understanding that the contact-angle hysteresis or sliding angle is a direct measure of the adhesive force, the adhesive force of a PDMS microbowl-arrayed structure should be small and a droplet should easily roll off the surface. This situation is rationalized by considering the trapped air below the droplet (Cassie Baxter state), which has an elastic effect on the water drop (Figure 5d). The slippage is reinforced; thus, the microbowl-arrayed surface becomes slippy. Further confirmation that the switching of adhesive force is caused by the geometry of the microtextures on a solid is found by measuring the sliding angle of an h-pdms microlens-arrayed surface, that is, the same geometry but using different materials (Figure S2). A water drop could not be separated from the surface of the sample. The shape of the water droplet changed significantly, implying a very high contact-angle hysteresis. Figure 5c shows the contact angle measured in different locations for a PDMS microbowl-arrayed sample and a comparison of the adhesive force with a PDMS microlensarrayed surface obtained directly from the contact-angle measurements. Firstly, there were no obvious variations, even when the contact angles were measured numerous times at twelve sites, further demonstrating the uniform surface. Secondly, Figure 5c clearly confirms the small adhesion of the PDMS microbowl-structured surface. The work of adhesion at a PDMS microbowl-arrayed intersurface is onetenth of that at a PDMS microlens-arrayed intersurface. As discussed above, it is important to design textures that not only induce air trapping, but that also create a more stable state than the Wenzel state. In summary, the transition between hydrophobic and superhydrophobic and the switching of the strong adhesive force and anti-adhesive force were experimentally realized using the lock-and-key geometry effect of patterned surfaces without silanization. The microlens-arrayed surfaces showed a dimension-dependent hydrophobic behavior. Their wettability showed a low contact angle and a high adhesive force following the Wenzel state: the liquid droplet retains contact at all points with the solid surface below it. The microbowlarrayed surfaces exhibited a high contact angle and antiadhesive behavior following the Cassie Baxter state where a drop rests on the peaks of the surface protrusions and bridges the air gaps in between. The contact-angle hysteresis microscopically results in the adhesive force, while the contact angle microscopically results from such a force. Thus, it is unlikely that a patterned surface can be constructed with both a high contact angle and a high adhesive force. 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