Nonfluorinated Superomniphobic Surfaces through Shape-Tunable

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Surfaces, Interfaces, and Applications

Nonfluorinated Superomniphobic Surfaces through Shapetunable Mushroom-like Polymeric Micropillar Arrays Hyunchul Kim, Heetak Han, Sanggeun Lee, Janghoon Woo, Jungmok Seo, and Taeyoon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17181 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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ACS Applied Materials & Interfaces

Nonfluorinated Superomniphobic Surfaces through Shape-tunable Mushroom-like Polymeric Micropillar Arrays Hyunchul Kim,† Heetak Han,† Sanggeun Lee,† Janghoon Woo,† Jungmok Seo‡, #, * and Taeyoon Lee†, * †School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, Republic of

Korea ‡

Center for Biomaterials, Biomedical Research Institute, Korea Institute of Science and

Technology (KIST), Seoul 02792, Republic of Korea #

Division of Bio-Medical Science & Technology, KIST School, Korea University of Science

and Technology (UST), Seoul 02792, Republic of Korea

*Corresponding author: [email protected], [email protected]

Keywords: superomniphobic, nonfluorinated surface, doubly re-entrant structure, micropillar, springtail 1 ACS Paragon Plus Environment

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Abstract Superomniphobic surfaces showing extremely liquid-repellent properties have received a great amount of attention as they can be used in various industrial and biomedical applications. However, so far, the fabrication processes of these materials mostly have involved the coating of perfluorocarbons onto micro and nanohierarchical structures of these surfaces, which inevitably causes environmental pollution, leading to health concerns. Herein, we have developed a facile method to obtain flexible superomniphobic surfaces without perfluorocarbon coatings that have shape-tunable mushroom-like micropillars (MPs). Inspired by the unique structures on the skin of springtails, we fabricated mushroom-like structures with downward facing edges (i.e., a doubly-re-entrant structure) on a surface. The flexible MP structures were fabricated using a conventional micromolding technique and the shapes of the mushroom caps were made highly tunable via the deposition of a thin aluminum (Al) layer. Due to the compressive residual stress of the Al, the mushroom caps were observed to bend toward the polymer upon forming doubly-re-entrant–MP structures. The obtained surface was found to repel most low-surface-tension liquids such as oils, alcohols, and even fluorinated solvents. The developed flexible superomniphobic surface showed liquid repellency even upon mechanical stretching and after surface energy modification. We envision that the developed superomniphobic surface with high flexibility and wetting resistance after surface energy modification will be used in a wide range of applications such as self-cleaning clothes and gloves.

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Introduction Superomniphobic surfaces that can repel various types of liquids have received an extensive amount of attention because they can be applied in various applications such as efficient heat transfer,1 drag reduction,2,3 antibiofouling,4,5 liquid transportation,6–8 and biological assays.9,10 Conventionally, superomniphobic surfaces have been fabricated through a combination of micro and nanohierarchical structures and subsequent chemical treatments. To make micro and nanohierarchical structures, diverse methods such as stacking micro and nanospheres,11,12 texturing fibers,13 spray coating,10,14–16 and photolithography17,18 have been used. Subsequent chemical treatments have been conducted by the dip-coating10,12,13,15 and vapor deposition of perfluorocarbon materials onto hierarchical structured surfaces.14,17,18 The functionality of superomniphobic surfaces is considered to be maintained when both their structural and chemical properties are preserved. In addition, lubricant-infused superomniphobic surfaces have been developed inspired by Nepenthes pitcher plant.19–23 Despite their stable characteristic against the pressure and physical damage, these surfaces also require subsequent chemical treatments to stabilize the perfluorocarbon lubricating film on the surfaces. Commonly, perfluorocarbon materials are vulnerable to chemical and physical damage when subjected to different treatments,24,25 leading to the liquid-repellent properties of the surfaces being steadily degraded or lost, which limits the use of the surfaces in practical applications. Moreover, the use of these materials has led to environmental and health concerns.26 Recently, the unique structures found on the skin of springtails have attracted a lot of attention because of their extraordinary liquid-repellent properties. This characteristic is due to unique and sophisticated microstructures called doubly-re-entrant structures on their skin.27,28 Doubly-re-entrant structures are mushroom-like structures with downward facing edges. Only a few research groups have reported the fabrication of structures that mimic this sophisticated 3 ACS Paragon Plus Environment


structure. To mimic the unique structures on the skin of springtails, Liu et al. made micropillar structures on a silicon wafer that can repel almost all types of liquid. To implement doubly reentrant micropillar (D–MP) structures, they used multiple and repeated steps of thermal silicon oxidation and etching by reactive-ion etching.29 Also, Choi et al. made doubly re-entrant structures through photopolymerization of micropillar structures. A perfluorocarbon material was sequentially coated on the surfaces to reduce the solid surface energy.30 Recently, Liu et al. and Dong et al. made those sophisticated structures by using the 3D direct laser writing method.31,32 Although these studies have shown ways of mimicking the superomniphobicity of the skin of springtails, the use of inflexible substrates, extra chemical treatments or 3D direct laser writing method which takes a long time for fabrication still hinders the practical use of these surfaces in a diverse range of applications. In this regard, the flexible, nonfluorinated superomniphobic surfaces are highly necessary. In this study, we have developed a facile method to fabricate a flexible superomniphobic surface that can repel various liquids, including extremely low surface tension solvents. To make the D–MP structures, we first used a conventional micromolding technique to produce flat mushroom-like MP structures made of polydimethylsiloxane (PDMS) elastomer. Subsequently, metal thermal evaporation was conducted on the structures. Due to the compressive residual stress of aluminum (Al) on the PDMS-based structures, the cap edges of the MPs became bent after the Al deposition, which led to the formation of the D–MP structures on the surface. Generally, the superomniphobic surfaces are coated with perfluorocarbon materials to repel the low surface tension liquids. However, despite the high surface energy of the Al layer and without the use of extra chemical treatments, the developed surfaces showed extreme liquid repellency, with only the use of a microscale structure. The developed superomniphobic surface with D–MP structures not only exhibited extreme liquid 4 ACS Paragon Plus Environment

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repellency with a high liquid contact angle (CA > 150°) and low sliding angle (SA) against a diverse range of liquid droplets but also maintained its super-repellency under a high strain (~50%), repeated cyclic stretching (>1,000 cycles) and even after oxygen plasma treatment. In addition, viscous liquids, such as blood, could be repelled on the superomniphobic surface, even in a highly curved state. We believe that the developed superomniphobic surfaces can be used in practical applications such as self-cleaning clothes and gloves.

Materials and Method Fabrication of the MP and F–MP structures. A glass slide substrate (2 × 2 cm2) was cleaned with acetone, isopropyl alcohol, and deionized water. A photoresist (AZ P4620, Microchemicals GmbH) was spin-coated on the cleaned glass slide at 1,000 rpm for 30 s, followed by 5 min of soft baking on a hot plate at 100 °C. The spin coating and soft baking processes were conducted twice to create a 30 μm diameter dot-patterned mask-thick photoresistant film. To create the holes of the micropillar structures in the photoresistant mold, UV exposure with a 10 μm diameter dot-patterned mask was performed using a mask aligner (MDA-400M, Midas System Co., Ltd.). Then, additional UV exposure was performed on the underside of the substrate without any mask so that only a thin layer at the bottom was exposed. This exposure produced a thin cap structure on the micropillar structure. The precise development of the photoresist was performed in NaOH solution (0.5 wt%) to produce the mushroom-like structures in the mold. Subsequently, PDMS (Sylgard 184, Dow Chemicals) mixed with a curing agent in a weight ratio of 5:1 was gently poured on the photoresistant mold and degassed for 15 min in a vacuum chamber. We used the 5:1 weight ratio of the PDMS elastomer (Young’s modulus ≈3.5 MPa) rather than 10:1 weight ratio of the PDMS elastomer (Young’s modulus ≈2.5 MPa) to sustain the cap structure more stably. The PDMS substrate 5 ACS Paragon Plus Environment


was cured in an oven at 80 °C for 2 h. After the curing of PDMS, the photoresistant mold was cleanly removed using ethanol. The flat MP was then easily peeled off the glass substrate without any physical damage to the mushroom structure. To make the F–MP structure, oxygen plasma treatment was performed on the MP patterned surface at 100W for 20 s (CUTE-MPR, Femto Science). Then, a perfluorocarbon material (1H,1H,2H,2H-perfluorooctyltrichlorosilane, Sigma-Aldrich) was coated using a vapor deposition method39 for 12 h in a vacuum chamber, resulting in an F–MP structure. Fabrication of a flexible and nonfluorinated D–MP patterned surface. The MP structure (cap diameter: 23 μm, pillar height: 30 μm) was coated with a 50 nm thick Al layer via metal evaporation (60 Å/min) in a vacuum of 10−6 Torr. Surface chemical modification by oxygen plasma treatment. To make PDMSbased substrates in a highly wetted state, the Al-flat, MP and D–MP patterned surface were treated with oxygen plasma at 100 W for 10 s (CUTE-MPR, Femto Science). The wettability of the oxygen plasma treated surfaces was fully changed from hydrophobic to superhydrophilic. Blood repellency on the D–MP patterned surface attached to a latex glove. To check the feasibility that the surface can be used for a wide range of diverse applications, the developed D–MP patterned surface (1 × 1 cm2) was attached to the finger of a latex glove using double-sided adhesive tape. 3 μL of blood droplets (horse blood citrated, MB cells) were dropped onto the curved surface attached to the latex glove to confirm the blood repellency of the curved D–MP patterned surface. Characterization. The surface morphologies were observed using a JEOL JSM7001F field emission scanning electron microscope (FE-SEM) and an optical microscope. The bending angles of the mushroom cap were calculated from the SEM images. The static CAs and SAs of the liquids were measured using a CA measurement device with a dynamic image 6 ACS Paragon Plus Environment

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capture camera (SmartDrop Standard, FEMTOBIOMED. Inc.). To measure all of the CAs and SAs, ~2 μL of liquid droplets were dispensed onto the surface. In this work, the SAs were defined as the roll-off angle as a droplet started to move on the surface and the tilting angle was controlled by the device. Numerical simulations were performed using a commercial numerical solver (COMSOL Multiphysics 5.1). The following parameter values were used: cap diameter, 23 μm; cap thickness, 2.2 μm; Al overcoat thickness, 50 nm. X-ray photon spectroscopy (XPS) was examined using PHI 5000 VersaProbe (ULVAC PHI, Japan)

Results and Discussion Fabrication of mushroom-like structures on a flexible PDMS substrate. Figure 1a-(i) shows an image of a springtail and the corresponding scanning electron microscopy (SEM) images show the regular texture on its skin.27,33 The SEM images clearly show the arrayed pillar structures with cavities, which are microscale in diameter and cover the whole body of the insect. Figure 1a-(ii) shows a schematic profile image of the skin of the springtail. Notably, the pillar structures on their skin feature mushroom-like doubly re-entrant structures in the cross-sectional view. These structures can suspend diverse liquids because the liquids become pinned under the downward edges of the cap, creating an air pocket between the surface and the liquid. Accordingly, springtails can breathe and survive in watery or oily environments. Figure 1b shows the fabrication process of the D–MP structures on the PDMS substrate, which mimic the special structures on the skin of springtails. First, ultraviolet (UV) exposure with a photomask was conducted on the top of a glass substrate covered with a photoresist to make the cavities of the micropillar structure. Subsequently, additional UV exposure without a photomask was conducted on the downside of the glass substrate. Since UV light can pass through glass substrates, only the thin bottom layer was exposed to UV light. 7 ACS Paragon Plus Environment


During the development process, the UV exposed photoresist was removed from the top of the micropillars. When the developer reached the bottom layer, passing but limitedly through the micropillars, the thin bottom layer exposed to UV light was gradually removed, forming cavities like those observed in a mushroom cap structure. Through exact time control of the development process, the size of the bottom cavities could be adjusted. After pouring and curing of the PDMS elastomer onto the developed photoresist mold, the entire photoresist was removed with ethanol and the MP structures were obtained without any physical damage. Subsequently, Al was deposited on the MP structures to bend the cap edges of the MPs. Since the Al overcoat on the PDMS-based MP structures produces compressive residual stress, the cap edges of the MP structures permanently bent toward the polymer, forming D–MP structures. We chose the PDMS elastomer due to its adequate Young’s modulus (≈3.5 MPa) for elastic deformation of the substrate itself and its ability to create D–MP structures after only nanometer-scale Al deposition onto the elastomer. The straightforward fabrication method is advantageous for the facile fabrication of D–MP patterned flexible superomniphobic surfaces that have high structural fidelity and uniformity, as shown in Figure 1c and Figure 1d. Engineering of diverse parameters related to the D–MP structures. Engineering the parameters related to the D–MP structures (e.g. the pillar diameter, pillar height, cap diameter, cap thickness, inter-pillar spacing and bending angle of the cap edges) is crucial for implementing superomniphobic surfaces.18,29 Figure 2a shows the SEM images of the MP structures before the Al deposition. Using a 10 μm diameter dot-patterned photomask and 30 μm thick photoresist, we obtained a highly uniform and stable micropillar array from the photoresistant mold. To make stable cap structures on the top of the micropillar array like in real mushrooms, the cap diameter and cap thickness values were optimized through the development time and the bottom UV dose control, respectively. Through the fabrication of 8 ACS Paragon Plus Environment

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cap structures with different development times and bottom UV doses, we observed that the cap diameter increased upon an increase in the development time and the cap thickness increased upon an increase in the bottom UV dose (Figure S1). Interestingly, when the cap diameter was over 30 μm or the cap thickness was less than 2 μm, all the cap structures were exceedingly unstable and easily collapsed (Figure S2). When the cap diameter was less than 20 μm or the cap thickness was over 3 μm, the cap edges rarely bent after the Al deposition. Considering these results, we aimed for cap diameter and cap thickness values of about 23 μm and 2.2 μm, values which made it possible to sustain the shape of the structures without any considerable collapsing and bending of the cap edges. In addition to considering the cap diameter and cap thickness, the inter-pillar spacing between the MP structures is another important factor in implementing superomniphobic surfaces. To optimize the inter-pillar spacing between the MP structures, we considered the Cassie–Baxter equation, which assumes the existence of an air pocket between the liquid and the surface.34 The equation is expressed as: cos θ* = fs(cos θi +1) – 1

(1)

where fs is the solid fraction (i.e. the ratio of the solid-liquid contact area to the entire projected area) at the mushroom cap, θi is the intrinsic CA and θ* is the apparent CA. Figure 2b shows the relationship between θ* and fs with different θi values in the Cassie–Baxter model. According to the Equation (1), the fs value should be less than ~6.7% to enable low-surfacetension liquids (i.e. θi ≈ 0°) to be suspended in a super-repellent state (i.e. θ* > 150°) on the surface (see the supporting information for more details). Since we optimized the cap diameter value to 23 μm, inter-pillar spacing should be larger than 78 μm to make fs value of 6.7%. Given that too large spacing makes the surface vulnerable to liquid invasion, we could set the inter-pillar spacing value to 80 μm, leading to an fs value of ~6.5%. However, in reality, it is 9 ACS Paragon Plus Environment


hard to suspend low-surface-tension liquids with such a small fs value because the pressures exerted by the liquids (e.g. hydrostatic or Laplace pressures) enable the liquids to be easily pushed into cavities and spontaneously spread.29 Thus, stable structures such as the D–MP structure are necessary to withstand the pressures exerted by liquids and to suspend lowsurface-tension liquids in a super-repellent state.27,29,33 To implement the D–MP structure, we conducted the thermal evaporation of Al on a flat MP structure. After a few nm of deposition, the cap of the MP structure began to bend downwards. This bending effect can be attributed to the compressive residual stress produced by the Al.35,36 The bending angle of the cap edge is strongly related to the thickness of the Al overcoat. For a 5 nm deposition of Al on the PDMS, the bending angle of the cap structure was ~28°. As the deposition thickness increased, the bending angle became larger. When the deposition thickness was over 50 nm, the bending angle became saturated at ~84°, which is in good agreement with data produced from simulations (Figure 2c-2d). Superomniphobicity of the D–MP structure. To evaluate the wetting properties of the obtained MP and D–MP structures, we first compared the liquid repellency of the MP structure with that of the D–MP structure. The only structural difference between the MP structure and the D–MP structure is the presence or absence of downward facing cap edges. Figure 3a shows optical images of deionized (DI) water (surface tension: 72 mN/m) and 50% ethanol solution (surface tension: ~28 mN/m) on the MP and D–MP structures. Even though both structures maintained super-repellency against DI water, the MP structure was fully wetted by the ethanol solution, but the D–MP structure still maintained super-repellency against the ethanol solution. This difference can be explained by the wetting mechanisms of both structures. Figure 3b shows the different wetting mechanisms of the MP and the D–MP structures. Since the MP structure is made using a PDMS elastomer with a low surface energy 10 ACS Paragon Plus Environment

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(25 mN/m), DI water can form a larger CA (θi) than the cap edge angle (θcap) under the cap structure. Accordingly, a contact line between the DI water and the MP structure can be formed under the cap structure and the MP structure can suspend DI water in a super-repellent state. However, it cannot form a contact line between the ethanol solution and the MP structure under the cap structure due to the low surface tension of the liquid (θi < θcap). Thus, the ethanol solution pushes into the cavities and fully wets the MP structure. Different from the MP structure, the D–MP structure also exhibits super-repellency against the ethanol solution. This is because the downward facing cap edges of the D–MP structure form a negative cap edge angle (θcap < 0°) where the liquid moves upward to wet the whole structure. Therefore, a contact line between the ethanol solution and the D–MP structure is formed under the downward cap edges despite the low surface tension of the liquid. In this context, even extremely low-surfacetension liquids (θi < 10°) can form a contact line under the downward cap edges of the D–MP structure. Figure 3c shows the super-repellency behavior against a diverse range of liquids such as DI water, vegetable oil (surface tension: 32 mN/m) and ethanol (surface tension: 21.8 mN/m) on the D–MP-structure-patterned surface. To quantify the liquid repellency of the MP and D–MP structures, we measured the CAs and SAs of a diverse range of liquids (Figure 3d). Although the MP structure could not repel liquids with surface tensions below ~32 mN/m, the D–MP structure repelled all liquids, even including fluorinated solvents (Krytox 100; surface tension: 17 mN/m, FC-770; surface tension: 14.8 mN/m), maintaining large CAs (>150°) without any chemical treatments (Figure S3-4). Given that even the perfluorocarbon-materialcoated–MP (F–MP) structure is easily wetted by liquids with surface tensions below ~25 mN/m, the D–MP structure has extremely high resistance against liquids, without requiring any chemical treatment. In addition to quantifying the CAs of liquids on the structure-patterned surfaces, we also measured the CAs of liquids on non-patterned flat surfaces to compare the 11 ACS Paragon Plus Environment


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measured θi values with the θ* values. Figure S5 and Table S1 show the CAs of the liquids (DI water, ethanol and FC-770) on non-patterned flat surfaces (flat PDMS, perfluorocarbonmaterial-coated flat PDMS and Al deposited flat (Al-flat) PDMS surfaces). Notably, FC-770, a fluorinated solvent, formed an extremely low CA on the Al-flat PDMS surface (θi ≈ 0°), while FC-770 existed in a super-repellent state (θ* = 152.2°) on the D–MP structure. This difference can be explained by the fact that Al has a much higher surface energy than that of FC-770, which leads to complete wetting of the Al-flat surface. However, in the case of the D–MP patterned surface, its structure prevents the invasion of liquid into the cavities regardless of the intrinsically high surface energy of the Al and extremely low surface tension of the liquid. Generally, the size of the liquid droplets influences their wetting state on a superrepellent surface because smaller liquid droplets exert a higher Laplace pressure than larger droplets.29,30 To evaluate the resistance of the D–MP structure against the pressure exerted by the liquid, we dispensed a ~2 μL ethanol droplet, as an example of one of the volatile liquids, and observed the evaporation of the droplet on the surface (see movie S2). During the evaporation process, we calculated the exerted pressure using the Young–Laplace equation, which is given by: Δp = 2γ/R

(2)

where Δp is the pressure difference between the liquid and the air, γ is the surface tension of the liquid and R is the droplet radius. Figure S6 shows the relationship between the time and the droplet diameter (and the corresponding pressure difference calculated using Equation (2)). As the R further reduced to ~490 μm by the evaporation (with a corresponding Δp of ~85.7 Pa), the ethanol perfectly wetted on the surface because of the high increase in the pressure exerted by the small droplet. This result shows that the D–MP structure can resist the pressure


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exerted by the droplet and maintain super-repellency when there is a reduction in the size of the droplet. Flexibility of the superomniphobic surfaces. The flexibility of superomniphobic surfaces is of great significance for practical applications. Flexible polymer-based surfaces enable not only the bending of the cap edges after Al deposition, but also deformation of the surface itself. We stretched the developed D–MP superomniphobic surface by ~50% (in the xdirection) and observed the change in the inter-pillar spacing. When the surface was stretched in the x-direction, it automatically compressed in the orthogonal y-direction due to the Poisson’s ratio of the surface, deforming gradually from a square array into a rectangular array (Figure 4a). Based on the Cassie–Baxter model in Equation (1), the apparent CA of the liquid droplet on the stretched surface, θ*str, can be calculated using the following equation: cos θ*str =

πDC2

(cos θi +1) – 1

4SXSY

(3)

where DC is the cap diameter, SX is the inter-pillar spacing in the stretched direction and SY is the inter-pillar spacing in the compressed direction. Since we used a rectangular array D–MP structure, the solid fraction of the stretched surface, fs_str, can be defined as fs_str = πDC2/4SXSY. As the strain varies from 0% to 50%, the corresponding fs_str can be calculated as ~6.5% to ~5.5% (Table S2 for more details). After stretching the surface, we measured the CAs of a 70% ethanol solution (surface tension: ~25mN/m) on the surface and compared them with the CAs calculated using Equation (3). As shown in Figure 4b and 4c, the measured CAs were very similar to the calculated CAs and the surface maintained its superomniphobicity under high strain (~50%) and repeated stretching. Moreover, the surface could repel diverse liquids under different uniaxial strains (Figure S7). In addition, the developed superomniphobic surface still maintained wetting resistance even after surface energy modification. Typically, UV and oxygen plasma treatments can decompose perfluorocarbon materials, leading to a degradation 13 ACS Paragon Plus Environment


in the super-repellency of conventional super-repellent surfaces coated with perfluorocarbon materials.10,37,38 Figure 4d shows the CAs of DI water before and after oxygen plasma treatment on the different surfaces. Notably, DI water was in a highly wetted state (θi < 10°) on the Al-flat PDMS and MP structures after the plasma treatment. In contrast, the D–MP structure still maintained a super-repellent state (θ* > 150°) after the plasma treatment. Given that XPS spectra shows the significant increase in intensity of O 1s peak after the oxygen plasma treatment (Figure S8), the developed superomniphobic surfaces can sustain wetting resistance after surface energy modification and that super-repellency can only be maintained in the D–MP structure, regardless of the solid surface energy. To apply superomniphobic surfaces in practical applications, developed surfaces should maintain their superomniphobicity after being subjected to a diverse range of deformation processes, such as stretching, bending, and twisting. After diverse and repeated deformations, the surfaces were observed to maintain their superomniphobicity against diverse liquids (Figure 4e and Figure S9-10). Due to the flexible and wetting-durable characteristics of the developed superomniphobic surfaces, the developed surfaces can be applied to clothes to prevent liquid wetting. Figure 4f shows a viscous liquid, a blood droplet, dropped onto a highly curved surface attached to a latex glove. The blood droplet was observed to easily roll off the highly curved surface, leaving no stain. This implies that the developed superomniphobic surfaces can be used in a variety of practical applications where a diverse range of deformation processes needed.

Conclusions In summary, we have presented a facile method to make nonfluorinated, flexible superomniphobic surface inspired by the structural features of the skin of springtails. Different 14 ACS Paragon Plus Environment

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from conventional superomniphobic surfaces, which require micro- and nanohierarchical structures and extra chemical treatments, the developed superomniphobic surface only required a microscale structure. The theoretical and experimental results showed that a bending of the cap edges induced by Al deposition led to the extraordinary superomniphobicity and blocked the infiltration of a diverse range of liquids into the structure, including fluorinated solvents, the intrinsic CAs of which are almost zero. In addition, the developed superomniphobic surface maintained its super-repellency under high strain (~50%), repeated cyclic stretching, bending, and twisting (>1,000 cycles), and even after oxygen plasma treatment. Furthermore, the developed superomniphobic surface can repel viscous liquids such as blood, even in a highly curved state. We believe that nonfluorinated, flexible and easily producible superomniphobic surfaces could be used in practical applications such as self-cleaning clothes and gloves.

Acknowledgements This work was supported by the National Research Foundation (NRF) of Korea (NRF2017M3A7B4049466), R&D program of MOTIE/KEIT [10064081, Development of fiberbased flexible multimodal pressure sensor and algorithm for gesture/posture-recognizable wearable devices] and KIST institutional program (2E27930, 2V06510).



FIGURES

Figure 1. Fabrication of a D–MP structure as a flexible and nonfluorinated superomniphobic surface. a-i) Photograph of a springtail and SEM images of the regular texture in the skin. a-ii) A profile image of liquid suspension on the skin of the springtail. b) Schematic diagram of the fabrication process of the D–MP structure. The inset shows a profile image of liquid suspension on the D–MP structure. c) A photograph of the flexible and nonfluorinated D–MP patterned surface. d) Low and high magnification SEM images of a D–MP array (cap diameter: 23 μm 16 ACS Paragon Plus Environment

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and Al overcoat: 50 nm).

Figure 2. Optimization of the MP and D–MP specifications. a) SEM images of MP structures with optimized parameters (inter-pillar spacing: 80 μm, cap diameter: ~23 μm, cap thickness: ~2.2 μm). b) Relationship between the θ* and fs values with different θi values (from 0° to 120°) in the case of ideal Cassie–Baxter state droplets. c) The bending angle of the D–MP structure as a function of the Al overcoat thickness. The inset shows the bending angle of the mushroom cap defined as the tangential angle at the tip under the mushroom cap. d) SEM images and simulation data of the D–MP cap edges with different Al overcoat thicknesses (5, 20, 50 nm).



Figure 3. Superomniphobicity of the D–MP structure. a) Optical images of the DI water and ethanol solution droplets with a volume of 2 μL on the MP and D–MP structures. b) Schematic images of the different wetting mechanisms of the MP and D–MP structures. Δp is the pressure difference between the liquid and the air. c) Photograph of DI water (red), vegetable oil (sky blue) and ethanol (transparent) droplets on the D–MP patterned surface. d) The CAs and SAs of various liquid droplets on the MP, F–MP and D–MP structures (the patterned box indicates that no liquids were available below 10 mN/m).


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Figure 4. Flexibility of the superomniphobic surface. a) Photographic and optical images of the relaxing and stretching of the surface under 0% and 50% strains. b-c) CAs and SAs of the ethanol solution (70 wt%) on the D–MP patterned surface under different uniaxial strains and numbers of stretching cycles. d) CAs of the DI water droplets before and after oxygen plasma treatment on the Al-flat, MP patterned, and D–MP patterned surfaces. e) Photographic images of the stretching, bending, and twisting of the surface. The insets show the ethanol solution (70 wt%) droplet on the surface after each deformation. f) Sequential photographs of a ~3 μL blood droplet rolling off the highly curved surface attached to a glove.



ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publication website at DOI:xx.xxxx/acsami.xxxxxxx. Figures S1-S7 (PDF) Movie S1: FC-770 advancing and receding on the superomniphobic surface (AVI) Movie S2: Ethanol evaporation on the superomniphobic surface (AVI)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ORCID Jungmok Seo: 0000-0002-8898-044X Taeyoon Lee: 0000-0002-8269-0257 Author Contributions H.K. contributed to the main idea, the overall experiments, measurements, analysis and writing the manuscript. Dr. H.H. provided the assistance on the experiments and analysis. S.L. performed the overall simulation. J.W. supported the measurements and analysis. Prof. J.S. designed the study, and revised the manuscript. Prof. T.L. and Prof. J. S. supervised the project. Notes The authors declare no competing financial interests.


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