High-Aspect-Ratio Ridge Structures Induced by Plastic Deformation as

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High-Aspect-Ratio Ridge Structures Induced by Plastic Deformation as a Novel Microfabrication Technique Atsushi Takei,*,†,# Lihua Jin,‡,§ and Hiroyuki Fujita† A. Takei and H. Fujita †

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan

Lihua Jin ‡

Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States § Department of Civil & Environmental Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: Wrinkles on thin film/elastomer bilayer systems provide functional surfaces. The aspect ratio of these wrinkles is critical to their functionality. Much effort has been dedicated to creating high-aspect-ratio structures on the surface of bilayer systems. A highly prestretched elastomer attached to a thin film has recently been shown to form a high-aspect-ratio structure, called a ridge structure, due to a large strain induced in the elastomer. However, the prestretch requirements of the elastomer during thin film attachment are not compatible with conventional thin film deposition methods, such as spin coating, dip coating, and chemical vapor deposition (CVD). Thus, the fabrication method is complex, and ridge structure formation is limited to planar surfaces. This paper presents a new and simple method for constructing ridge structures on a nonplanar surface using a plastic thin film/elastomer bilayer system. A plastic thin film is attached to a stressfree elastomer, and the resulting bilayer system is highly stretched one- or twodimensionally. Upon the release of the stretch load, the deformation of the elastomer is reversible, while the plastically deformed thin film stays elongated. The combination of the length mismatch and the large strain induced in the elastomer generates ridge structures. The morphology of the plastic thin film/elastomer bilayer system is experimentally studied by varying the physical parameters, and the functionality and the applicability to a nonplanar surface are demonstrated. Finally, we simulate the effect of plasticity on morphology. This study presents a new technique for generating microscale high-aspect-ratio structures and its potential for functional surfaces. KEYWORDS: thin-film wrinkling, wrinkling, postwrinkling, bifurcation, plastic deformation, microfabrication

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INTRODUCTION

To overcome this limitation, a new method of creating highaspect-ratio structures was recently proposed. In this method, a stiff thin film is attached onto a highly stretched elastomer, and compressive strain is then applied by gradually releasing the stretch; as a result, high-aspect-ratio structures are formed on the surface as postwrinkling bifurcations. This mode of instability was first proposed14 based on numerical simulations, where the high-aspect-ratio structures were referred to as “ridges”. Both one-dimensional13,15−17 and two-dimensional18,19 high-aspect-ratio ridge patterns have been observed experimentally; in particular, the patterned surface of two-

The morphological instability of a bilayer system composed of a thin stiff layer and a thick elastomeric substrate has been studied both experimentally and theoretically. Under one- or two-dimensional compression, different wrinkling patterns form on the surface of such a system. These wrinkling patterns have been used to produce functional surfaces such as smart adhesion surfaces,1 hydrophobic surfaces,2−4 optical structures,5−8 and cell culture templates.9−12 Previous work has primarily focused on bilayer systems under low levels of compression in which the aspect ratio of the wavelength to the amplitude of the wrinkling pattern is typically on the order of 0.1.13 The functionality of such a patterned surface is relatively poor because of this small aspect ratio; therefore, the utility of the bilayer approach in practical applications remains limited. © 2016 American Chemical Society

Received: June 30, 2016 Accepted: August 25, 2016 Published: August 25, 2016 24230

DOI: 10.1021/acsami.6b07957 ACS Appl. Mater. Interfaces 2016, 8, 24230−24237

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ACS Applied Materials & Interfaces dimensional ridges provides high functionality for hydrophobic surfaces or cell culture templates. The ridge structures formed using a highly stretched elastomer indicate a promising method that can be exploited for preparing functional surfaces. However, the applications of these ridge structures are still limited because mechanical stretchers are required for attaching the film. Because of this requirement for stretchers, conventional thin-film deposition methods (e.g., spin coating, dip coating, or chemical vapor deposition (CVD)) are not suitable for creating such high-aspect-ratio structures. The high-aspectratio structures presented in previous studies were achieved only on flat surfaces, whereas the buckling patterns of bilayer systems under small compression have been achieved on nonplanar surfaces, such as cylinders20−22 and spheres.23−29 The present paper reports a method for creating ridge structures without the need for stretchers during film attachment by exploiting the plastic deformation of the film. Using our proposed method, we obtained ridge structures in a simple manner because no stretching was necessary during attachment. This paper presents the ridge formation of bilayer systems using plastic deformation for the first time, while the use of plastic deformation in thin film/elastomer bilayer systems is limited in the literature (e.g., under small tensile strain30 and under small or large compressive strain31). Furthermore, our method simplifies the process of fabricating functional surfaces. In previous studies, additional treatment was typically required to enhance the functionality of the structured surfaces. For example, to enhance the surface’s hydrophobicity, silica nanoparticles3 or trichlorosilane19 were added after creating a structured surface. In contrast, with use of our method, a functionalized plastic material can be easily coated onto the substrate via spin coating or dip coating. Using a functionalized plastic material (e.g., a material possessing both plasticity and hydrophobicity), we can obtain a highly enhanced functional surface without additional treatment. In the literature, Ahmed et al. achieved high-aspect-ratio wrinkles by depositing a carbon film at incident angle of 75° onto a polydimethylsiloxane (PDMS) surface.32 With use of this method, the amplitude/wavelength ratio of the resulting wrinkles reached 2.5 at the maximum. However, because of the incident-angle deposition, the method is not suitable for nonplanar structures. In contrast, with use of our method, the applicability of our method is not limited to planar structures. The plastic film can be coated onto nonplanar surfaces or the inner walls of narrow channels, such as tubes and microfluidic channels. When the bilayer systems are stretched, ridge structures can then be formed on these nonplanar surfaces and inner walls. Furthermore, in this study, we also performed finite element simulations to study the influence of the plastic film on the surface morphology.

Figure 1. (a) Schematic illustrations of the one- and two-dimensional surface patterns produced via plastic deformation. A plastic film was attached to an elastomeric substrate, and localized deformations were produced on the surface by large-magnitude stretching of the bilayer system and the subsequent release of the applied stretch. (b) Image of a typical one-dimensional pattern induced by the plastic deformation technique. The surface profile of the pattern is embedded, and the ridge structure is localized, with a flat neighboring region. (c) SEM image of a typical two-dimensional pattern; the deformation is convex and localized.

dimensional ridges can be produced by applying uniaxial or biaxial stretching, respectively. Figure 1b,c presents the typical morphologies of the one- and two-dimensional ridges produced using the proposed method, respectively. In this section, the morphology of the ridges is analyzed through experiments, and the applications of the ridges produced using our method are demonstrated. Here, the stretch ratio λ is defined as the ratio of the maximum length after stretching Ls to the original length L0, and the compressive strain ε is defined as ε = 1 − λ′/λ, where λ′ is the current stretch. We first quantify the geometry of the one-dimensional ridges created using strips of plastic/elastic bilayers. The bilayer strips were stretched up to λ = 2.4 and then released, with plastic deformation persisting in the film. In this experiment, Parylene C (plastic material, Specialty Coating Systems, Inc., Indianapolis, IN, USA) and PDMS (Sylgard 184, Dow Corning, Corp., Midland, MI, USA) were used as the plastic film and elastic substrate, respectively. As previously reported,18 when Parylene film forms on a PDMS substrate using CVD, the film is strongly bonded to the substrate and does not delaminate from the substrate during ridge formation. In addition to the strong adhesion, Parylene film can be formed uniformly on sample surfaces, even if their structures are not planar. Furthermore, as demonstrated later in this paper, Parylene film can be easily formed on the inner walls of a narrow channel using CVD. For the sample preparation, Parylene C was

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EXPERIMENTS AND RESULTS As shown in a schematic drawing of our proposed method in Figure 1a, a thin layer of plastic material is attached to a nonstretched elastomeric substrate, and the bilayer system is then stretched beyond the yield strength of the plastic material. During the elongation of the bilayer system, the thin layer is deformed plastically, whereas the substrate is deformed elastically. When the stretch is released, the length of the elastomeric substrate returns to its initial value, whereas the plastic layer remains longer than its initial length. Thus, this situation is similar to that of a nonstretched thin layer deposited on a highly prestretched elastomeric substrate. One- or two24231

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thin film is considered at the end of this paper based on a finite element analysis. The formation of two-dimensional ridges in a bilayer system is presented in Figure 3. The bilayer system was subjected to equibiaxial stretching and compression. Cytop (amorphous hydrophobic material, Asahi Glass Co. Ltd., Tokyo, Japan) was used as the plastic film in this experiment. The initial Young’s modulus of the Cytop was 1.4 GPa,35 and its initial shear modulus was 0.47 GPa; Cytop is considered to be incompressible. PDMS with a base:cross-linker ratio of 30:1 and a shear modulus of 0.037 MPa was used in the experiment. The thicknesses of the plastic film hf and the thickness of the elastomeric substrate hs were 500 nm and 500 μm, respectively. The stress−strain curve of the PDMS used in this experiment is shown in Figure S1. An equibiaxial stretch ratio of λ = 3 was applied, and a compressive strain of up to ε = 66% was exerted on the film via pressurization on the bottom surface of the bilayer. The details of the experiment are provided in the Experimental Methods section and the Supporting Information (Figure S3). The evolution of the two-dimensional ridges is presented in Figure 3. When ε < 5%, the surface remains flat (Figure 3a). As the compression increases (ε ∼ 7%), a ridge clearly emerges on the surface (Figure 3b). When the compressive strain reaches ε > 12%, additional ridges appear, forming a network (Figure 3c). This network grows increasingly dense as the compression continues to increase (ε > 20%) (Figures 3d−f). The surface profile is presented in Figure 3g. The representative ridge width and height are approximately 500 nm and 1 μm, respectively, giving an aspect ratio of 2; more understanding will be discussed in the Numerical Analysis section. As shown in Figure 3a−f, the surface roughness increases as the compressive strain ε increases, and a rough surface can behave as a superhydrophobic surface. In previous studies,3,19 structured surfaces were hydrophobized with nanoparticles or trichlorosilane to enhance their hydrophobicity. In contrast, the structured surface produced using our proposed method did not require any additional treatment because the material used to form the two-dimensional ridge structure, Cytop, is highly hydrophobic. The contact angle of a water droplet on a flat Cytop surface is 110°,35 and Figure 3h shows the contact angle of the water droplet on a prepared surface as a function of the compressive strain. The advancing and receding angles were measured by setting a nozzle 2 mm above the surface and then injecting and withdrawing a water droplet. In this experiment, the bilayer was first stretched to λ = 3, and a compressive strain was then induced by releasing the stretch. In the absence of compressive strain, the initial advancing and receding angles were 110° and 100°, respectively. As the compressive strain increased (ε ∼ 0.1), the advancing angle increased to 140°, whereas the receding angle did not change significantly (95°). As the compressive strain increased further (ε ∼ 0.5), the receding angle increased to 125°; thus, the difference between the advancing and receding angles decreased to 25°. The transition of the contact angles (advancing and receding angle) and the hysteresis is due to the variation of wettability with respect to the surface roughness, similar to previous studies.3,36 In our case, we assume that as the surface roughness increases with ε, the water droplet tends to be pinned at the low-density ridges. As a result, the advancing angle and the contact angle hysteresis increases (the so-called Wenzel state37). As the surface roughness increases further, air is trapped between the droplet and the high-density ridges. Consequently, the pinning

deposited onto a 300-μm-thick PDMS elastomeric substrate. The Parylene C layer had a thickness of hf = 0.9−7.3 μm and an initial shear modulus of μf = 0.6 GPa. The shear modulus of the PDMS, μs, was tuned by modifying the base:cross-linker ratio (0.46 MPa for a ratio of 5:1 and 0.27 MPa for a ratio of 10:1). The details of the sample preparation procedure are presented in the Experimental Methods section. The stress−strain curves of the PDMS and Parylene C are shown in Figure S1 of the Supporting Information. As shown in Figure 1b, the ridge formed by the gradual release of the stretch and the deformation was localized. The amplitude A and width w of each ridge were measured, and the results are shown in Figure 2. The findings indicate that the ridge width w is proportional

Figure 2. Measured ridge width w as a function of the film thickness hf, the stretch λ, and the stiffness ratio μf/μs. A plot of the aspect ratio of the width w and the amplitude A as a function of the stretch is shown in an inset; the definitions of width and amplitude are also illustrated in another inset. The equation of the fitting line is w = 0.85hf/λ1/2(μf/ μs)1/3. For this experiment, 5:1 PDMS and 10:1 PDMS specimens were used. We also varied the film thickness hf from 0.9 to 7.3 μm and varied the stretch λ from 1.4 to 2.4.

to (hf/λ1/2)(μf/μs)1/3. The literature has shown that both the wavelength of the wrinkles33,34 and the width of the ridges18,19 are proportional to the film thickness hf. In the case of a bilayer system with a plastic film, if we assume that the film is incompressible and the yield strain of the film is negligible, the effective thickness of the film under the stress-free state decreases from hf to hf/λ1/2 as a result of the Poisson effect due to the stretching. The ratio of the ridge amplitude to the width A/w as a function of λ is also presented in an inset in Figure 2. The aspect ratio A/w increases with increasing stretch ratio λ. These trends are consistent with the previously presented results.19 In the experiment, the aspect ratio of the ridges reached a maximum of 0.5, which is higher than that previously reported for wrinkled structures (approximately 0.1). In this analysis, we considered the effective shear modulus of a yielded film to be identical to the initial value μf, which appears to be an appropriate assumption based on the following experimental finding: As presented in Figure S2, we performed a simple experiment to study the effective stiffness of a yielded thin film by comparing the wavelengths of the wrinkles in bilayer systems consisting of an elastomeric substrate combined with a yielded thin film or a nonyielded thin film. The experiment indicated that the effective stiffness of a thin film for wrinkle formation is not significantly affected by yielding, whereas the change in thickness after stretching due to the Poisson effect affects the wrinkle wavelength. The elasto-plastic behavior of a 24232

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Figure 3. Evolution of a typical two-dimensional pattern induced by plastic deformation. (a) The surface remains flat up to a compressive strain of approximately ε = 5%. (b) A ridge is clearly formed at a compressive strain of ε ∼ 7%. (c−e) With further increases in the compressive strain, the ridges form a network, and the density of the network progressively increases. (f) When the stretch is further released, the structure is packed. (g) The surface profile of the two-dimensional pattern, where (g1, g2, g3, and g4) correspond to the surfaces presented in (b), (c), (d), and (e), respectively. (h) The advancing angle and receding angle of a water droplet on the surface as a function of the compressive strain. Images of the droplet advancing (top) and receding (bottom) on the surface at ε = 66% are presented as insets in the graph.

unlikely occurs and the contact angle hysteresis decreases (the so-called Cassie state38). The structured surface produced using the proposed technique therefore exhibited low hysteresis. With use of our proposed method, a highly functional surface can be achieved in a simple manner. In previous studies, ridges were formed only on planar surfaces because the requirement for stretching during film deposition limited the sample geometry. However, with use of CVD, a plastic material can be coated onto nonplanar surfaces or the inner walls of a narrow channel. During deposition, no mechanical displacement is required, overcoming the previous geometric limitations. A structured surface can be formed on a nonplanar surface or an inner wall by simply stretching the sample after film deposition. Here, we present a silicone rubber tube and a PDMS microfluidic channel with structured surfaces produced using the proposed method. First, using the CVD process, we coated 2-μm-thick Parylene N onto a silicone rubber tube that initially had smooth exterior and interior surfaces. The inner and outer diameters of the tube were 0.5 and 1.0 mm, respectively, and plastic films were formed on the exterior and interior surfaces of the tube. By application of a stretch ratio of λ = 2 in the axial direction and then release of the stretch completely, ridge structures with a width on the order of 10 μm were formed on both surfaces. Figure 4a,b shows SEM images of the exterior and interior surfaces of the silicone tube, respectively. (The smooth exterior and interior

surfaces in the initial state are shown in insets of (a) and (b), respectively, of Figure 4. During deposition and stretching, the tube remained uncut. The tube was then cut in half for observation. In this case, the ridge structure has a wavy form because the one-dimensional stretch was released completely. With use of our proposed method, structured surfaces can be formed on both nonplanar surfaces and interior surfaces. When a plastic film is coated onto a substrate and the structure stretched, micrometer-scale patterns can be formed on the surface. Our method represents a new approach for fabricating a microfluidic channel with a complex structure on its surface, as illustrated in Figure 4c. In the conventional micromachining method, microfluidic channels are formed using molds. These molds are created via lithography, and the surfaces of the channels are generally flat. The fabrication of a microfluidic channel with a structured surface involves complex processes, such as multiple UV exposures.39 In contrast, with use of our method, microstructures can be generated on the surface of a microchannel that was produced using the conventional method (i.e., lithography and molds). Therefore, the proposed method has the advantage of being compatible with conventional micromachining techniques. Using CVD, we deposited 2-μm-thick Parylene N film onto the surface of the inner wall of a microfluidic channel made of 20:1 PDMS. The width and height of the rectangular channel were 400 and 250 μm, respectively. After film deposition, the closed microfluidic 24233

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Figure 4. Surface patterning on nonplanar structures and inner walls. Patterned surfaces were formed on (a) the exterior surface and (b) the interior surface of a silicone rubber tube; (a2) and (b2) are magnified views of (a1) and (b1), respectively. The inner and outer diameters of the tube were 0.5 and 1 mm, respectively. The smooth exterior and interior surfaces are presented as the insets of (a1) and (b1), respectively. The scale bars in the insets correspond to 500 μm. (c) Schematic image of a microchannel with an inside wall patterned via plastic deformation. Such a patterned surface can be formed by coating a plastic film onto the surface of the inside wall and then stretching and releasing the microchannel. The fabricated channel had an H-shape. The channel possessed four inlets, one at each tip. Through stretching and releasing, a patterned surface was formed in the center part of the channel. (d) The microchannel stretched parallel to its long axis; (d1), (d2), and (d3) present an SEM image of the microchannel, the surface profile, and a magnified SEM image, respectively. The schematic presented in the inset of (d1) illustrates the direction of stretching; the region we observed is indicated by a dashed circle in the inset. The images represent the surface of the center part of the channel. The wavelength and height of the wavy pattern were approximately 20 and 6 μm, respectively. A hierarchical pattern can be observed in the wavy structure. (e) The microchannel was stretched along a direction oriented at an angle of 45° to the long axis; (e1), (e2), and (e3) present an SEM image of the microchannel, the surface profile, and a magnified SEM image, respectively. The schematic presented in the inset of (e1) illustrates the direction of stretching; the region we observed is indicated by a dashed circle. A pattern was formed on all walls at an inclination with respect to the axial direction. The wavelength and height of the wavy pattern were approximately 30 and 10 μm, respectively. In (d2) and (e2), the front and rear planes of the depicted boxes are parallel to the side wall of the channel.

channel was stretched to λ = 1.5 along its long axis, and the stretch was then released completely. A periodic wavy pattern was formed perpendicular to the long axis of the microchannel, as shown in Figure 4d1. The wavelength and height of this periodic pattern were 20 and 6 μm, respectively, as shown in the surface profile presented in Figure 4d2. In Figure 4d3, hierarchical patterns can be observed in addition to the wavy pattern. These hierarchical patterns were likely formed due to the combination of two- and one-dimensional patterns; a similar pattern was previously reported on the surface of a metal/elastomer bilayer system.19 As shown in Figure S4, although Parylene N was deposited onto stress-free PDMS, two-dimensional patterns formed on the surface. When the Parylene was deposited onto the PDMS, residual stress formed in the film.18 This equibiaxial residual stress likely induced the two-dimensional pattern. The hierarchical patterns observed on the inner wall of the microchannel arose from the combination of the two- and one-dimensional patterns caused by this residual stress and the stretching, respectively. In contrast to the case previously reported,19 the use of a mechanical stretcher during deposition was not required to obtain a hierarchical pattern. In the cases shown in Figures 1 and 2, 5:1 and 10:1 PDMS were used, respectively, and hierarchical patterns were not formed. This result implies that the hierarchy of the pattern can be controlled.

The wavy patterns shown in Figure 4d were formed perpendicularly to the axial direction of the microchannel. However, the angle at which a pattern can be created is not limited to the direction perpendicular to the long axis. When the structure is stretched in a direction that is inclined with respect to the long axis, a pattern can be formed in that inclined direction, thereby endowing the microchannel with unique functionality. In previous studies, periodically patterned grooves were formed at 63.5°39 or 45°40 with respect to the long axis of a channel. When two different liquids flow through such a microchannel, the inclined patterns induce a rotational motion of the flow, thereby enhancing the mixing of the liquids. A channel with a similar structure can be fabricated using our method. We deposited 1.3-μm-thick Parylene N onto a microchannel and then stretched the microchannel to λ = 2 along a direction 45° from the long axis. Upon release of the stretch completely, a wavy pattern was formed on the inner wall of the channel (Figure 4e1). The wavelength and height of this periodic pattern were 30 and 10 μm, respectively, as indicated by the surface profile shown in Figure 4e2. A wavy pattern also formed on the side wall (Figure 4e3). Thus, with use of our proposed method, a functional microfluidic channel can be prepared in a simple manner. A quantitative analysis of liquid flow in the proposed microchannel will be presented in a future study. 24234

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Figure 5. Numerical simulations of the ridges in a bilayer system consisting of a plastic film on a neo-Hookean substrate. The film is stress-free when the substrate is prestretched to λ = 2 under plane-strain conditions. A compressive strain is applied to the film and then released. (a1) Ridge morphology at a strain of 0.045 in a linear elastic film. (a2) Contour plot of the maximal principal strain in the vicinity of the ridge tip. (a3) Vertical displacement V after subtracting the displacement under homogeneous deformation V0 and normalized by the film thickness hf as a function of the strain for two different wrinkle peaks: one that becomes a ridge under higher strain and one that becomes its neighboring peak. (b1) and (c1) Ridge morphologies at a strain of 0.045; (b2) and (c2) contour plots of the maximal principal strain; and (b3) and (c3) normalized vertical displacements (V − V0)/hf as a function of strain for bilinear elasto-plastic films with yield strains of εy = 0.05 and εy = 0.02, respectively. Images of the film morphologies when the compression is completely released are presented in the insets of (b3) and (c3). In all images, the black bars indicate the relative size of the simulation results.

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NUMERICAL ANALYSIS Finally, finite element simulations were performed to study the morphology of ridges in bilayer systems with plastic films. In our previous studies,17,18 we treated a thin stiff film as a neoHookean material. Here, we modeled such a film as a bilinear elasto-plastic material exhibiting isotropic hardening under the J2 flow theory. We set the Young’s modulus of the film to 1.8 GPa, and we set the tangent modulus to 90 MPa beyond the yield strain εy. The substrate was still modeled as an incompressible neo-Hookean material with a shear modulus of 0.5 MPa. We assumed that the film was stress-free when the substrate was under a plane-strain stretch of λ = 2. We simulated the gradual release of the substrate by applying a compressive strain of ε = 0.045 to the film and then restretching the substrate back to λ = 2, all under plane-strain conditions. Numerical damping was added to the static simulation to stabilize the snap-through between the wrinkle state and the ridge state.17 The simulation modeled a bilayer system composed of a plastic film and an elastomeric substrate. The experiment presented in Figure S2 indicated that the effective stiffness of the plastic film during elastic unloading should be of the same order of magnitude as the initial stiffness of the nonyielded film (i.e., the Young’s modulus should be 1.8 GPa). The tangent modulus beyond the yield strain was estimated based on the stress−strain curve of the plastic film presented in Figure S1, and we set its value to 90 MPa for simplicity (1.8 GPa/90 MPa = 20). To understand the effect of plasticity on the wrinkle-to-ridge transition, we varied the yield strains of the stiff film among εy = 0.02, 0.05 and infinity (i.e., a linear elastic material). For all three ratios of the modulus of the film to that of the substrate, wrinkles were initiated at a strain of ε = 0.008, which is smaller than the yield strain in any of the cases. Therefore, immediately after the initiation of wrinkling, the wrinkle morphology is the same in all three cases. For the case of the linear elastic film, the

wrinkles remain stable up to a strain of approximately 0.0225, beyond which the wrinkles snap into a localized ridge. Figure 5a1 shows the morphology of such a ridge at a strain of 0.045, with the film in yellow and the substrate in blue. The morphology appears similar to that in the case of a neoHookean film.17 Figure 5a2 shows the contour plot of the maximal principal strain in the film in the vicinity of the ridge. Under an external strain of 0.045, the local strain at the tip of the ridge can be as high as 0.142, whereas the strain far away from the ridge can be as low as 0.002. Figure 5a3 plots the vertical displacements of two wrinkle peaks V after subtracting the vertical displacement under homogeneous deformation V0 and normalized by the film thickness hf as a function of the strain. Here, one peak becomes a ridge under higher strain, and the other becomes its neighboring peak; the two peaks are labeled “ridge” and “wrinkle”, respectively. The values of (V − V0)/hf for both peaks become nonzero upon wrinkle initiation, and they first deviate from each other at the wrinkle-to-ridge transition. The ridge peak snaps to a larger amplitude, whereas the neighboring peak snaps to a smaller amplitude. During unloading, the ridge snaps back into wrinkles under a strain of approximately 0.0205. The bilayer returns to its original shape when ε = 0. There is a slight discrepancy in the values of (V − V0)/hf for the wrinkle states observed during loading and unloading because of the numerical damping in the simulation. For the case of εy = 0.05, the wrinkles snap into a localized ridge at the same strain as that in the linear elastic case, 0.0225, prior to which the entire film is in the elastic state. However, after the snapping occurs, the film in the vicinity of the ridge peak becomes plastic, and the shape of the ridge becomes sharper than that observed in the linear elastic case, with a larger curvature in the ridge peak due to the ease with which the plastic flow localizes the strain in the ridge peak. Figure 5b1 shows the ridge morphology at a strain of 0.045. The aspect ratio of the ridge is higher than that in the case of the elastic 24235

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ACS Applied Materials & Interfaces film. The maximal principal strain at the tip of the ridge reaches 0.775, which is much higher than that in the linear elastic case, whereas the strain far away from the ridge is even smaller, with a value of 0.001 (Figure 5b2). During unloading, the ridge peak gradually shrinks without transitioning back into the wrinkle state (Figure 5b3). When the sample is unloaded to ε = 0, a residual ridge remains because of the plastic deformation of the film, as shown in the inset of Figure 5b3. For the case of εy = 0.02, the ridge morphology at a strain of 0.045 is presented in Figure 5c1. The wrinkles snap into a localized ridge at a smaller strain of approximately 0.0113 (Figure 5c3), prior to which the entire film is still in the elastic state. In the case of εy = 0.05 (Figure 5c2), the shape of the ridge becomes even sharper, and the maximal principal strain at the tip of the ridge is even larger. A similar residual ridge remains when the sample is unloaded to ε = 0 (inset of Figure 5c3). The influence of the film plasticity on the ridge morphology can be clearly observed. When the yield strain is smaller, the ridges have a higher aspect ratio, which is an important factor for functional surfaces.

applied to the PDMS surface to enhance the adhesion between the thin film and the substrate. Cytop was spin-coated onto the substrate at 6000 rpm and then cured for 30 min at 180 °C to form a thin film. The bilayer system was removed from the glass substrate, and equibiaxial stretching was then induced by applying pressure to the bottom surface of the bilayer system (see the details in the Supporting Information). The surface profiles of the one- and two-dimensional ridges were measured using a surface profiler (VX series, Keyence Corp., Osaka, Japan). Structured Microchannels. First, an open microfluidic channel was fabricated using a mold, and the inlet holes were formed using a biopsy punch. The open microfluidic channel was bonded to a PDMS plate to form a closed microfluidic channel. Parylene N was coated onto the inside surface of the microchannel via CVD. To obtain the pattern in the microchannel presented in Figure 4d, the prepared channel was stretched while maintaining a closed microchannel structure. To obtain the pattern in the microchannel presented in Figure 4e, the bonded PDMS plate was removed before the microchannel was stretched.

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S Supporting Information *

CONCLUSION This article presents a method for creating high-aspect-ratio surface structures by exploiting postwrinkling ridge instability. By inducing plastic deformation of the thin film in a bilayer system, we can easily create convex patterns on a surface using simple fabrication techniques. The bilayer system can be highly stretched either uniaxially or biaxially, generating one- or twodimensional high-aspect-ratio structures, respectively. Using a functionalized plastic material, we demonstrated that a highly hydrophobic surface could be achieved without further treatment through the creation of a structured surface. We also demonstrated that our method can be applied to nonplanar surfaces such as the exterior and interior surfaces of tubes and microfluidic channels. The details of the structures created in this manner were experimentally studied by varying the prestretching conditions, thicknesses, and stiffnesses of the film. We also studied the effect of the film plasticity on the morphology of the resulting structure using finite element modeling. The simulation results demonstrated that the existence of plasticity sharpens the ridges, resulting in a higher aspect ratio of the structures. We conclude that the formation of surface structures induced by plastic deformation represents a new opportunity for the microfabrication of novel devices.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07957. Stress−strain curves of PDMS and Parylene C, measurements of the effective stiffness of the plastic film, the method used to apply equibiaxial stretching and compression, SEM images of the silicone tube, and the surface image and profile of a Parylene/PDMS bilayer system (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address #

Center for Soft Matter Physics, Ochanomizu University, 2-1-1 Otsuka, Bunkyo, Tokyo 112-8610, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for JSPS Fellows (Grant Number 2310841).

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EXPERIMENTAL METHODS

REFERENCES

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Typical Ridge Morphology. In the experiment presented in Figure 1, Parylene C and Cytop were used to produce one- and twodimensional ridge structures, respectively. In both cases, the films were attached to a PDMS substrate. The surface profiles of the one- and two-dimensional ridges were measured using a surface profiler (VK series, Keyence Corp., Osaka, Japan) and SEM, respectively. One-Dimensional Ridges. For the preparation of the bilayer system used in the experiment presented in Figure 2, PDMS was first spin-coated onto a glass substrate and then cured for 2 h at 75 °C; then, Parylene C was deposited onto the PDMS. The resulting bilayer system was cut into 10-mm-wide strips, and the bilayer strips were removed from the glass substrate. The edges of each bilayer strip were clamped by two mechanical stretchers separated by a distance L0 of 25 mm. The surface profiles and images of the ridges were obtained using a VK-series surface profiler. Two-Dimensional Ridges. As in the one-dimensional case, to prepare the bilayer system used in the experiment presented in Figure 3, PDMS was first spin-coated onto a glass substrate. Before thin-film deposition, oxygen plasma (at 100 sccm, 20 W, 20 Pa, and 30 s) was 24236

DOI: 10.1021/acsami.6b07957 ACS Appl. Mater. Interfaces 2016, 8, 24230−24237

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b07957 ACS Appl. Mater. Interfaces 2016, 8, 24230−24237