Flexible and Robust Superomniphobic Surfaces Created by

Flexible and Robust Superomniphobic Surfaces Created by Localized Photofluidization of Azopolymer Pillars Jaeho Choi,† Wonhee Jo,† Seung Yeol Lee,† Yeon Sik Jung,‡ Shin-Hyun Kim,*,† and Hee-Tak Kim*,†,§ †

Department of Chemical and Biomolecular Engineering, ‡Materials Science and Engineering, and §KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea S Supporting Information *

ABSTRACT: Springtails, insects which breathe through their skins, possess mushroom-shaped nanostructures. As doubly re-entrant geometry in the mushroom head enhances the resistance against liquid invasion, the springtails have robust, liquid-free omniphobic skins. Although omniphobic surfaces are promising for various applications, it remains an important challenge to mimic the structural feature of springtails. This paper presents a pragmatic method to create doubly re-entrant nanostructures and robust superomniphobic surfaces by exploiting localized photofluidization of azopolymers. Irradiation of circularly polarized light reconfigures azopolymer micropillars to have a mushroom-like head with a doubly re-entrant nanogeometry through protrusion and inward bending of polymer film from the top edge. The light-driven reconfigured micropillars facilitate the pining of triple line as the springtails do. In particular, the unique geometry exhibits superomniphobicity even for liquids whose equilibrium contact angles are almost zero in the presence of a practical level of external pressure. In addition, the simple fabrication process is highly reproducible, scalable, and compatible with various substrate materials including flexible polymeric film. Our results suggest that our photofluidization technology will provide a practical route to develop robust superomniphobic surfaces. KEYWORDS: superomniphobic, doubly re-entrant geometry, localized photofluidization, azopolymer, micro/nanopatterning

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the negative Laplace pressure across the interface and decreasing the pressure gradient within liquid drops. At a certain point on the geometry, the interface becomes flat, at which the invasion is stopped because the pressure gradient is negligible. Re-entrant structures can be prepared by randomly stacking spheres13−16 or fibers2,17 or by producing overhanging structures.1,10,18−20 The previously reported re-entrant structures can pin the air−liquid interface in liquids whose equilibrium contact angle is much smaller than 90° in the absence of any external pressure. However, in practice, the structures are prone to be filled with liquids with a low contact angle due to hydrostatic pressure or environmental perturbation.21 The springtail, a soil-dwelling insect that breathes through the skin, possesses a robust omniphobic property that enables it to maintain dry skin in soil habitats and avoid suffocation.22,23 The essential feature of the evolutionary adapted omniphobic

mniphobic coatings provide liquid-free surfaces by trapping air in the micro- or nanostructured surfaces against most liquids with a wide spectrum of polarity.1 When the heterogeneous surfaces are composed of a high fraction of air mats, the coatings show contact angles larger than 150° and a contact angle hysteresis lower than 10° (or roll-off angle lower than 10°); this property is referred to as superomniphobicity.1 The liquid-free or liquid-repellent surfaces are potentially useful for stain-free fabrics,2−4 nonbiofouling medical tubings,5 corrosion-free surfaces,6 and invariant structural coloration.7 In addition, molecules dissolved in drops can be concentrated at a local area of a surface during evaporation because the surfaces have a low hysteresis; such concentration improves the sensitivity of molecular detection.8 The design rule for omniphobic coatings is to form a structural barrier against liquid invasion into open pores. Although the surface energy of materials is always lowered by wetting with liquids with a contact angle smaller than 90°,9 the invasion into pores can be impeded by re-entrant geometry.10−12 As triple lines proceed through the re-entrant geometry, the negative curvature of the air−liquid interface decreases, thereby reducing © 2017 American Chemical Society

Received: March 14, 2017 Accepted: July 17, 2017 Published: July 17, 2017 7821

DOI: 10.1021/acsnano.7b01783 ACS Nano 2017, 11, 7821−7828

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Figure 1. (a) Photograph of a springtail insect with omniphobic skin, scanning electron microscope (SEM) image showing regular texture in the skin, and transmission electron microscope (TEM) image showing doubly re-entrant nanostructure in the texture. Photograph image courtesy of Brian Valentine and SEM and TEM images are reproduced with permission.22 (b) Schematic illustration showing fabrication procedures for the mushroom-shaped micropillar array: (i) soft-molding of azopolymer to form the micropillar array and (ii) illumination with circularly polarized light to reconfigure the micropillars to have mushroom-like heads. (c) Intensity reduction of the incident beam along the propagation and the resulting structural reconfiguration of micropillars. Only the top slice with a thickness of about 600 nm is under the influence of incident light. The red dots are experimentally measured using the azopolymer films with various thicknesses, and the solid line indicates the Beer−Lambert law. (d−g) SEM images of mushroom head edge (d), cylindrical micropillars (e), mushroom-shaped micropillar (f), and square array of mushroom-like pillars (g).

lithographic or assembly techniques. To the best of our knowledge, there is only one study that has reported the successful preparation of the doubly re-entrant structures using silicon micromachining. The structures prevent invasion even for liquids with an almost zero contact angle, which had never been achieved with simple re-entrant structures.21 Nevertheless, the fabrication procedures were highly delicate, time-consuming, and expensive. More importantly, the material is restricted to an inflexible silicon wafer, which severely limits practical uses. Therefore, a pragmatic route to produce the springtail’s morphology with organic materials remains an important

skin relies on an array of mushroom-like topographies in the cuticles which are referred to as doubly re-entrant structures, as shown in Figure 1a.24,25 The doubly re-entrant structures possess negative geometric angle rather than multiscale roughness. The sharp edge with a small angle in the topologies efficiently pins the triple line and prevents the invasion of liquids, even with very small contact angles in the presence of a practical level of external pressure; the external pressure can be balanced by Laplace pressure exerted by the positive curvature of the interface with triple lines pinned at the edges. Despite the superior omniphobicity of the springtail’s morphology, the structural features are difficult to mimic with conventional 7822


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ACS Nano challenge for providing robust, flexible, and economical omniphobic surfaces. In this paper, we present a pragmatic method to develop robust omniphobic surfaces composed of polymeric mushroom-like microstructures possessing the doubly re-entrant nanogeometry in the mushroom-head. The key to the success is in the field-gradient photofluidization of an azobenzenecontaining polymer. An array of cylindrical posts is prepared by soft molding of the azopolymer, which is then subjected to far-field irradiation with circularly polarized light. The top surfaces of the posts are fluidized under irradiation, while the bodies of the posts remain intact. The fluidized azopolymer protrudes from the top surface and simultaneously bends down as the migration velocity decreases along the depth due to light absorption. Therefore, the cylindrical post is reconfigured to have a mushroom-shaped head with doubly re-entrant nanogeometry. The array that is fluorinated by C4F8 plasma treatment exhibits a high apparent contact angle even for liquids with an almost zero equilibrium contact angle because the sharp edges pin the triple lines and trap air under the heads of the mushrooms. The polymeric omniphobic films can be formed on flexible substrates and transferred onto curved target surfaces. Moreover, the fabrication procedure, which consists of soft molding, photofluidization, and surface fluorination, is highly reproducible and scalable, providing a practical route to creating robust omniphobic surfaces.

cis photoisomerization until the long axis of the azobenzene was perpendicularly aligned to the polarization direction of the incident light.26,27 This photoinduced molecular motion induced the macroscopic motion of the azopolymer, even at room temperature, where the direction of the motion has been demonstrated to be parallel to the polarization direction.28−30 Thus, linearly polarized light results in the deformation of top surface of pillars along the polarization direction, as shown in Figure S4. The circularly polarized light whose linear electric vector being continuously rotated along the wavevector causes a radial deformation, which develops a doubly re-entrant structure along the entire circular top edge.31−33 The fluidization took place locally in a thin surface layer of the azopolymer due to the high absorption efficiency of the azopolymer. To estimate the effective depth for photofluidization, we prepared thin films of azopolymers with various thicknesses on glass substrates by spin-casting and measured the transmittance of the films for a 532 nm-wavelength laser. The intensity of transmitted light, I, exponentially fell off with the film thickness, z, as shown in Figure S5, which is in good agreement with the Beer−Lambert law I(z) = e−z / δp I0

(1)

where I0 is the intensity of the incident light and δp the penetration depth of the light. The value of δp is estimated as 196 nm from the fit of eq 1 to the data. Therefore, only the top slice of the micropillar thinner than a micrometer responded to the incident light as shown in Figure 1c, which led to photofluidization on the top surface, while the body of micropillar remained immobile. This localized, isotropic fluidization caused an effective radial motion of the azopolymers on the top surface of the micropillar. Although many possible mechanisms have been suggested for the photofluidization, including thermal model,34 pressure gradient force model,35 mean-field model,36 optical-field gradient force model,37,38 asymmetric diffusion model,39 and photoinduced molecular diffusion model,40 none of the mechanisms proposed so far can fully explain all phenomena responsible for the movement of azo-materials.41 Nevertheless, it is commonly accepted that higher light intensity causes faster deformation. Therefore, as the intensity of light decreased along the depth, so did the flow velocity. The gradient of velocity possibly causes the fluidized layer to proceed out of the micropillar and simultaneously bent inward as schematically illustrated in Figure 1c, while the surface of the layer remains smooth due to the surface tension; the same folded structures resulted even under the opposite gravity direction as shown in Figure S6, implying no role of gravity in the reconfiguration. As a result, the micropillars were reconfigured to have a mushroom-like head with a structural feature of doubly re-entrant nanogeometry, as shown in Figures 1d−g. The motion of fluidized azopolymers immediately stopped when the illumination was turned off, which allowed the precise control of the mushroomlike structures through adjusting the illumination time. To quantify the structural uniformity of doubly re-entrant structures, we measured the diameters of the structures with top-view scanning electron microscope (SEM) images which are randomly selected from area of 1.33 cm2. The standard deviation (SD) of the diameters measured from 20 pillars is as small as 30 nm, ensuring that the reconfiguration process through photofluidization provided a high structural uniformity.

RESULTS AND DISCUSSION Fabricating a doubly re-entrant geometry that mimics the omniphobic skin of springtails presents a major challenge. To address it, we used localized photofluidization of azopolymers to create micropillars that had a radially protruded film bent downward along the top edge. The production procedure consisted of two steps: (i) solvent-assisted soft-molding of azopolymers to form a micropillar array and (ii) photofluidization of the micropillars through illumination with circularly polarized light to make mushroom-like heads, as illustrated in Figure 1b. The azopolymer, epoxy-based poly(disperse orange) 3 (PDO 3), with a molecular weight of 4700 g mol−1 was synthesized and dissolved in cyclohexanone at a concentration of 30 w/w %. The solution was dropped onto a glass substrate, which was then covered with a polydimethylsiloxane (PDMS) mold. The PDMS mold had a square array of cylindrical holes with a diameter of 17 μm, a depth of 44 μm, and an interhole distance of 40 μm. The solution spontaneously filled the holes in the mold. As the cyclohexanone diffused through the PDMS mold and evaporated, the azopolymer gradually occupied the holes. After the removal of the mold, a square array of cylindrical pillars with the same dimensions as the PDMS mold was prepared, as shown in Figure S1 of the Supporting Information. When the concentration of the azopolymers was lower than 20 w/w %, the holes were incompletely filled, which resulted in short pillars, as shown in Figure S2. By contrast, a concentration higher than 30 w/w % left a thick residual layer on the substrate. The micropillars were reconfigured by photofluidization to have a mushroom-shaped head. For the photofluidization, the micropillars were illuminated by circularly polarized light with a wavelength of 532 nm and intensity of 14.4 mW cm−2 for 4 h from the top. The azopolymers strongly absorbed the light at a wavelength of 532 nm, as demonstrated in Figure S3, and became fluidized. In detail, the azobenzene groups grafted in the main chain of the azopolymer underwent a repetitive trans− 7823


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It is surprising that hexane with θE = 5° was θ* = 150 ± 2° despite the fact that an external pressure was obviously exerted during the drop deposition. This proves that the doubly reentrant nanogeometry in the mushroom structure served as an efficient structural barrier against the liquid invasion. To further confirm the formation of air mats under the heads of mushrooms, we used the Cassie−Baxter model to estimate the apparent contact angles based on the configuration of the flat liquid−vapor interface that was pinned on the edge of the heads of mushrooms. The areal fractions of solid−liquid and liquid−vapor interfaces, f SL and f LV, were calculated from the configuration as 0.0698 and 0.9302, from which the apparent contact angles were estimated:

The doubly re-entrant nanostructures of the mushroomshaped micropillars are beneficial for pinning the triple lines and holding air mats under the structures. To investigate the omniphobic property, we deposited drops of six different liquidshexane (18.4 mN m−1), ethanol (22.1 mN m−1), olive oil (32.0 mN m−1), ethylene glycol (47.7 mN m−1), glycerol (64.0 mN m−1), and water (72.8 mN m−1)on the array of mushroom-shaped micropillars after surface fluorination through C4F8 plasma treatment; the C4F8 plasma treatment yielded a 47 nm-thick CFx layer. The apparent contact angles, θ*, for all the liquids were larger than 150°, as shown in Figure 2a and Figure S7 and denoted with circles: θ* = 150 ± 2°, 153

cos θ* = fSL cos θE − fLV

(2)

The experimental values of θ* for all the liquids with different values of θE were in good agreement with the Cassie−Baxter model, as denoted by the solid line in Figure 2a, indicating that the liquid−vapor interface was prevented from the proceeding to the heads of mushrooms; the configuration is referred to as the Cassie−Baxter state. To study the importance of the mushroom structures, we measured the apparent contact angles on the array of surface-fluorinated plain micropillars for the same set of liquids, as denoted with inverse triangles in Figure 2a. The liquids with θE > 90° had large values of θ*, which was consistent with the Cassie−Baxter model. This was because the plain micropillars with geometric angles of approximately 90° were able to pin the triple lines for liquids with a θE larger than the geometric angle. In contrast, the liquids with θE < 90° spread or had a very small value of θ* as the liquid invaded the interstices between micropillars, forming an air-pocket-free Wenzel state. The Wenzel state can be confirmed by a coincidence between apparent contact angles experimentally measured and estimated with the Wenzel model as denoted by a dotted line in Figure 2a. The Wenzel model uses a roughness, r, defined as the surface area relative to its projection area

cos θ* = r cos θE

(3)

where the value of r is 1.723 for the micropillar array. The stark contrast of wetting property between mushroomlike pillar and cylindrical pillar arrays were further confirmed using a patterned film, as shown in Figure 2b. The cylindrical pillar array was locally irradiated with a laser beam with diameter of 1.3 cm, at which they were reconfigured to mushroom-like structure. The droplets of water, olive oil, and hexane deposited on the irradiated region showed high contact angles as mushroom structures provide Cassie−Baxter state, whereas the ethanol droplet spread on the nonirradiated region as simple cylinders allow the imbibition in Wenzel state. In the Cassie−Baxter state, the areal fraction of liquid−vapor interfaces was as large as 0.9302, indicating high surface homogeneity; that is, the drops were predominantly supported by the air mat. Therefore, low contact angle hysteresis and sliding angle of drops were expected. Advancing and receding contact angles of drops, θA and θR, on the array of mushroom structures were measured during the injection and suction of liquids for hexane, ethanol, and olive oil, as shown in Figure S9. For all three liquids, the contact angle hysteresis, θA − θR, was smaller than 20°, as denoted with green blocks in Figure 2c. The sliding angle at which drops began to roll off from the surface was smaller than 10°, as denoted with yellow blocks; the droplet volume used for the roll-off angle measurement was 3.3 μL. The high apparent contact angles, small hysteresis, and low

Figure 2. (a) Apparent contact angle (θ*) as a function of the equilibrium contact angle (θE) for the mushroom-like pillar array (denoted by the circles) and cylindrical pillar array (inverse triangles). The solid line indicates the Cassie−Baxter model and the dotted line indicates the Wenzel model. Insets show apparent contact angles of six different liquids deposited on the mushroomlike pillar array and cartoons show the Cassie−Baxter state for the mushroom array and the Wenzel state for the cylinder array. (b) Photographs of a film containing azopolymer pillar array that is locally reconfigured to mushroom-like structure as denoted with dotted circles. Droplets of water (transparent), olive oil (yellow), and hexane (blue) are deposited on mushroom-like pillar array and that of ethanol is on cylindrical pillar array. (c) Plots for the advancing angle (θA), receding angle, (θR), contact angle hysteresis (θA − θR), and roll-off angle (θroll‑off) for three liquids deposited on the array of mushroom-like pillars.

± 2°, 154 ± 2°, 155 ± 2°, 158 ± 2°, and 158 ± 2°, respectively. We measured the equilibrium contact angles, θE, by depositing the same liquids on the fluorinated, flat surfaces in the absence of the mushroom-like structures, which were 5°, 19°, 57°, 90°, 95°, and 98°, respectively, as shown in Figure S8. These large contrasts between θ* and θE indicate that the mushroomshaped micropillars prevented the imbibition of all the liquids and maintained the air mat under the heads of the mushrooms. 7824


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ACS Nano roll-off angles of the mushroom array for a wide range of liquids were properties of the superomniphobic surface. Therefore, the mushroom array effectively promotes dewetting and maintains the liquid-free surfaces. The doubly re-entrant geometry ensured that the triple line was pinned at the edge even under external pressure. Although the external pressure deformed the liquid−vapor interface to be convex downward, the geometry prevented the procession of the triple line until the local contact angle became smaller than the geometric angle or the liquid−air interface contacted the bottom solid. We designed the micropillars to have a height of 44 μm, which was comparable to the interpillar distance of 40 μm, to avoid the liquid invasion by contact between the liquid− air interface and the bottom solid. To characterize the maximum external pressure that the array of mushroom-shaped structures can support, or the breakthrough pressure, a drop of 3.3 μL ethanol was deposited on the array and vaporized to reduce the size as denoted with red circles and insets in Figure 3a. The drop maintained a high apparent contact angle until the radius of the drops decreased to 195 μm. During the volume reduction, the Laplace pressure along the top vapor−liquid interface increased as denoted with blue triangles in Figure 3a, where the pressure, P(t), was calculated from the radius of curvature, R(t) P(t ) = 2γlv /R(t )

(4)

where γlv is a surface tension between air and ethanol, 22.39 mN m−1. As the radius was further reduced, the apparent contact angle abruptly dropped, indicating the transition from the Cassie−Baxter state to the Wenzel state. The radius of curvature at the moment of transition was 195 μm, from which the breakthrough pressure was estimated as 230 Pa. The breakthrough pressure was possibly underestimated as the drop was supported by only a few micropillars at the moment of transition and a drop took a dynamical contact angle smaller than its equilibrium angle due to pinning of the triple line. We further studied the influence of interpillar spacing on the breakthrough pressure. With four distinct arrays with interpillar spacings of 20, 44, 60, and 80 μm, the breakthrough pressures of ethanol droplets were measured, as shown in the left axis of Figure 3b; the diameter of the mushroom head was maintained as 17 μm. As the spacing increased, the breakthrough pressure decreased. This is because the curvature of liquid−air interface that can be accommodated by the pillar array without imbibition increases as the spacing decreases. Nevertheless, smaller spacing is not always better as apparent contact angle is smaller, as shown in the right axis of Figure 3b. Smaller spacing makes a larger value of f SL, thereby resulting in smaller apparent contact angle as expected from the Cassie−Baxter model. In addition, the larger value of f SL causes larger contact angle hysteresis because of the increase of surface heterogeneity. Therefore, the spacing should be properly optimized depending on the conditions required for target applications. The soft-molding, photofluidization, and surface fluorination are highly compatible with various substrates, including flexible polymeric film. We prepared the mushroom structures on the surface of polyethylene terephthalate (PET) film, as shown in the left panel of Figure 3c. Moreover, the flexible film could be bent and applied on the curved surface, as shown in the right panel of Figure 3c, where the film was attached to the surface of a glass rod with a radius of 6 mm with the aid of a commercial adhesive. The omniphobic property was retained even on the

Figure 3. (a) Radius and Laplace pressure of an ethanol droplet on the array of mushroom-like micropillars as a function of evaporation time. Insets show the ethanol drop during evaporation. (b) Breakthrough pressure and apparent contact angle of an ethanol droplet on the array of mushroom-shaped micropillars as a function of interpillar spacing. Solid line indicates contact angle estimated by Cassie−Baxter model. (c) Photographs showing the high flexibility of the omniphobic coating on polyethylene terephthalate (PET) film (left panel) and the slipping of the olive oil drop on the curved surface with a radius curvature of 6 mm (right panel). The logo is used with permission from KAIST.

curved surface. The drops of olive oil slipped over the curved surface, as shown in movie S1 of the Supporting Information. The superior omniphobic property of the mushroom-shaped structures originated from the doubly re-entrant edge nanogeometry. The geometric angle at the edge significantly influenced the omniphobic property. To systematically study the influence, we prepared four distinct edge geometries by adjusting the exposure time while maintaining the light intensity of 14.4 mW cm−2, as shown in Figure 4a. Without any exposure, the plain micropillars had a geometric angle, ψ, of 90°, as shown in the first panel, where the angle was measured from the horizontal line of the micropillar side. As the azopolymer radially proceeded from the top surface of the micropillar for an exposure time of 2 h, the geometric angle became negative yet small, −10°, as shown in the second panel. With an exposure time of 4 h, the azopolymer film was highly 7825


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Figure 4. (a) Series of SEM images showing the transformation of edge geometry for four different exposure times as denoted in the panels. The geometrical edge angle (ψ) measured from a horizontal line is denoted in each panel. (b) Geometrical edge angle (ψ) as a function of light fluence, where three different intensities of light are used. The geometry is solely dictated by fluence. (c) Schemetic illustration of the maximally deformable composite interface of liquid with equilibrium contact angle 0° that the re-entrant edge of three different angles can suspend. (d) Apparent contact angle (θ*) as a function of the equilibrium contact angle (θE) for the arrays of pillars with different values of ψ. The solid line indicates the Cassie−Baxter model, and the dashed line indicates the Wenzel model. Only the pillars with ψ = −85° provide the Cassie−Baxter state for all liquids.

folded downward, forming a large negative angle of −85°, as shown in the third panel. A longer exposure time of 6 h led to the collapse of the film into the body of the micropillars, forming a geometric angle of −25°, as shown in the last panel. The geometric angle was influenced by light intensity and exposure time, where the intensity determines the rate of deformation and the exposure time determines the period of deformation. In our experiment, laser beams with three different intensities, 7.2, 14.4, and 21.6 mW cm−2, produced the same geometry as long as the exposure times were adjusted to make the same fluences, 0, 104, 207, and 311 J cm−2, as shown in Figure 4b; the fluence is a product of intensity and time. The geometric angle was deterministically controlled by the light fluence, which demonstrated a high reproducibility and accuracy in terms of structural control. The bottom interface of a drop experiences the positive pressure exerted by Laplace pressure along the top interface and hydrostatic pressure of drop height. The pressure is as small as approximately 49.79 Pa for a drop of hexane, which is supported by Laplace pressure along the slightly curved bottom interface; an angle of the triple line measured from horizontal line can be roughly estimated as 2.19°. Therefore, all of the pillars with negative geometric angles can support the pressure and potentially provide a Cassie−Baxter state in the absence of external pressure. However, in practice, a relatively large external pressure is exerted on the bottom interface when a drop is deposited, which leads to transition from a Cassie to Wenzel state, thereby losing omniphobicity. The maximum allowable pressure that the bottom interface can endure is

determined by the magnitude of negative geometric angle, as sketched in Figure 4c. We deposited drops of glycerol, olive oil, and hexane on the pillar arrays with four different geometric angles respectively and measured the apparent contact angles, as shown in Figure 4d. Only the array with ψ = −85° was able to maintain the Cassie−Baxter state for all three liquids. The arrays with ψ = −10° and −25° allowed the invasion of hexane, although they maintained the Cassie−Baxter state for olive oil and glycerol. This indicated that simple re-entrant geometry, characterized by ψ = 0°, was insufficient to provide a robust omniphobic property for liquids with a small equilibrium contact angle, as external pressure was exerted in practical uses. The maximum allowable pressures and experimental wetting states for all the liquids and geometric angles are summarized in Table S1, from which the magnitude of the external pressure can be estimated to have the value between 571 and 1250 Pa. The surface with ψ = 90° maintained the Cassie−Baxter state only for glycerol, as expected from the geometric angle.

CONCLUSION In this work, we present a pragmatic method to create robust omniphobic surfaces. The mushroom-shaped micropillars were prepared by localized photofluidization of the azopolymers in a highly controllable and reproducible manner. The doubly reentrant nanostructures with a large negative geometric angle prohibited the invasion of various liquids, including a liquid whose equilibrium contact angle was almost zero, even in the presence of external pressure, thereby enabling the maintenance of a stable air mat. Moreover, a high areal fraction of 7826


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ACS Nano the air mat led to a high apparent contact angle and low roll-off angle, which rendered the surfaces superomniphobic. The omniphobic coating can be prepared on the various substrates, including flexible polymer films. Therefore, the coating can be transferred onto curved surfaces to be stain-free or corrosionfree. In addition, a strip of the coating can be potentially rolled up along a short dimension to make nonbiofouling tubing with an omniphobic inner surface.5 Moreover, the micropillars can be regioselectively reconfigured using a photomask because the photofluidization is driven by light. Therefore, advanced functions, such as molecular concentration at a predefined location for chemical analysis8 and encryption/decryption of graphical codes for anticounterfeiting,42 can be potentially achievable because the patterned omniphobic surfaces allow regioselective spreading of liquids. The fabrication procedures of our approach are simple, economical, scalable, and compatible with common materials and processes; as the method is highly reproducible with a fluence of laser beam, the area can be increased by simply employing larger spot of a laser beam using a beam expander. We believe that the superior omniphobic property as well as the easy processing and potential functions will provide prominent opportunities for various applications that require liquid-free surfaces.

To measure the equilibrium contact angle, the smooth films of PDO 3 were prepared by spin-coating 5 w/w % tetrahydrofuran solution of PDO 3 on a silicon wafer at 3000 rpm for 60 s.

EXPERIMENTAL METHODS

AUTHOR INFORMATION

Preparation of the Cylindrical Pillar Array with Azopolymers. A PDMS mold containing the array of cylindrical holes was replicated from a template of a silicon master containing the array of cylindrical posts. The surface of the silicon master was treated with tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane, on which a mixture of a prepolymer and a cross-linker (Sylgard 184, Dow Corning, Midland, MI) at a weight ratio of 20:1 was gently poured and cured at 70 °C for 2 h; the mixture was degassed by vacuum suction before use. The PDMS mold was then released from the silicon master. The azopolymer, PDO 3, with a molecular weight of 4700 g mol−1 and a polydispersity index of 1.74 was synthesized by solid-state step polymerization of 5.8 mmol disperse orange 3 (Sigma-Aldrich) and 5.8 mmol bisphenol A diglycidyl ether (Sigma-Aldrich), as previously described elsewhere.43 The synthesized PDO 3 was dissolved in cyclohexanone at 30 w/w% and a drop of the solution was spread on a glass substrate. The PDMS mold was then placed on the substrate. The holes in the PDMS mold were completely filled with the solution by capillary action. After the solvent was fully evaporated at 60 °C for 12 h in a vacuum, the PDMS mold was gently removed from the substrate, yielding the array of cylindrical pillars on the glass substrate. The same molding procedure was performed on PET substrates to yield a flexible omniphobic surface. Photofluidization and Surface Fluorination of the Micropillars. The 532 nm wavelength laser (diode pump solid-state, Melles Griot) with an output power of 20 mW was circularly polarized using a set of half- and quarter-wave plates, which was then spatially filtered and collimated. The intensity of the beam was controlled by neutral density filters. The array of cylindrical pillars was illuminated by the beam with a diameter of 1 cm for a controlled duration using an electronic shutter. The reconfigured micropillars were further subjected to surface fluorination through reactive ion etching with C4F8 under the conditions of 15 mTorr and 30 sccm for 500 s. Characterization. The azopolymer structures were observed with a scanning electron microscope (SM-701, TOPCON) at 10 keV, where the samples were rendered to be conductive prior to observation by a coating of 10 Å platinum. To analyze the detailed structure of the top surfaces, the micropillars on the glass substrate were cleaved to show their cross sections by cutting the substrate. The thickness of the CFx-deposited layer was characterized by atomic force microscopy (psia XEI-100 systems). The contact angles of the liquids were measured using a goniometer (DSA 10-Mk2, KRUSS) where 3.3 μL drops were deposited using a computer-controlled syringe pump.

Corresponding Authors

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01783. SEM image of a square array of cylindrical micropillars; details of soft-molding of micropillars with varying azopolymer concentration; UV−vis absorption spectrum of the azopolymer; SEM image of doubly re-entrant structures reconfigured by linearly polarized light; the transmission of the azopolymer films with various thickness; irradiation of azopolymer pillars with two opposite gravity direction; apparent contact angle as a function of the surface tension of liquids; equilibrium contact angles; contact angle hysteresis; the maximum allowable pressures and experimental wetting states (PDF) Movie showing a demonstration of the superomniphobic property on a curved surface (MPG)

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yeon Sik Jung: 0000-0002-7709-8347 Shin-Hyun Kim: 0000-0003-4095-5779 Hee-Tak Kim: 0000-0003-4578-5422 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the End-Run program (code no. N11160060) through the Korea Advanced Institute of Science and Technology (KAIST) and the Midcareer Researcher Program (2017R1A2A2A05001156) through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning. REFERENCES (1) Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Robust Omniphobic Surfaces. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 18200−18205. (2) Choi, W.; Tuteja, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H. Fabrics with Tunable Oleophobicity. Adv. Mater. 2009, 21, 2190−2195. (3) Vilčnik, A.; Jerman, I.; Šurca Vuk, A.; Koželj, M.; Orel, B.; Tomšič, B.; Simončič, B.; Kovač, J. Structural Properties and Antibacterial Effects of Hydrophobic and Oleophobic Sol−gel Coatings for Cotton Fabrics. Langmuir 2009, 25, 5869−5880. (4) Zhou, H.; Wang, H.; Niu, H.; Fang, J.; Zhao, Y.; Lin, T. Superstrong, Chemically Stable, Superamphiphobic Fabrics from Particle-free Polymer Coatings. Adv. Mater. Interfaces 2015, 2, 1400559. (5) Leslie, D. C.; Waterhouse, A.; Berthet, J. B.; Valentin, T. M.; Watters, A. L.; Jain, A.; Kim, P.; Hatton, B. D.; Nedder, A.; Donovan, K.; Super, E. H.; Howell, C.; Johnson, C. P.; Vu, T. L.; Bolgen, D. E.; Rifai, S.; Hansen, A. R.; Aizenberg, M.; Super, M.; Aizenberg, J.; Ingber, D. E. A Bioinspired Omniphobic Surface Coating on Medical 7827


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