Durable and Flexible Superhydrophobic Materials: Abrasion

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Durable and Flexible Superhydrophobic Materials: Abrasion/ Scratching/Slicing/Droplet Impacting/Bending/Twisting-Tolerant Composite with Porcupinefish-Like Structure Yoshihiro Yamauchi* International Center for Young Scientists, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

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Mizuki Tenjimbayashi, Sadaki Samitsu, and Masanobu Naito* Data-driven Polymer Design Group, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: Superhydrophobic materials with micro/nanotextured surface have attracted tremendous attention owing to their potential applications such as selfcleaning, antifouling, anti-icing, and corrosion prevention. Such a micro/nanotextured surface is a key for high water repellency. However, such a texture is fragile and readily damaged when the material is deformed, scratched, or sliced off. Thus, it is challenging to develop superhydrophobic materials that can sustain high water repellency after experiencing such a mechanical deformation and damage. Here we report abrasion/ scratching/slicing/droplet impacting/bending/twisting-tolerant superhydrophobic flexible materials with porcupinefish-like structure by using a composite of micrometer-scale tetrapod-shaped ZnO and poly(dimethylsiloxane). Owing to the geometry of the tetrapod and elasticity of poly(dimethylsiloxane), the composite material exhibits stable water repellency after 1000 abrasion and 1000 bending cycles, or even after their surfaces were sliced off many times. The material maintains superhydrophobicity even under a mechanically deformed state such as bending and twisting. The materials can be painted on a variety of substrates and molded into desired shapes and used in a myriad of applications that require superhydrophobicity. KEYWORDS: superhydrophobic, bioinspired, porous framework, robust, elastic

1. INTRODUCTION Micro-/nanotextured hydrophobic surfaces exhibit excellent water-repellency.1−6 Nature often adopts such a texture to create a superhydrophobic surface,7 as seen in lotus and rice leaves,8,9 water striders’ legs,10 and moth eyes.11 A surface with a water contact angle larger than 150° is defined as a superhydrophobic surface,12 which basically exhibits a positive Laplace pressure from air on a nano- or micrometer-scale texture, known as a Cassie−Baxter state.13−15 By mimicking the natural surface structure, a variety of applications, such as antifouling, anti-icing, corrosion prevention, prevention of bacterial growth, and reduction of flow resistance, have been demonstrated.16,17 However, the highly sophisticated acicular structures at the nano- and micrometer scale are easily damaged by mechanical impact or deformation. To address this fatal drawback, researchers have attempted to develop superhydrophobic materials with mechanical durability.18−24 One of several strategies to afford mechanically durable superhydrophobicity is to adopt porous structure such as a sponge by using polymer,21 metal,22 and soot.23 It enables exposure of the embedded acicular surface even after abrasion. However, in a mechanically deformed state, superhydrophobicity cannot be maintained only with this strategy, because the © XXXX American Chemical Society

acicular surface loses micro/nanotexture through extension of the distance between the patterns. Despite the development of robust superhydrophobic materials as mentioned above,18−24 superhydrophobic materials that possess robustness and flexibility at the same time has been rarely reported.25,26 Furthermore, it is still challenging to develop a superhydrophobic material that is tolerant against multimechanical deformation (bending,27,28 twisting29,30) and multimechanical damages31(abrasion,32−36 scratching,37−39 slicing,40 droplet impacting41), simultaneously. Here we developed porcupinefish-like structured materials with abrasion/scratching/slicing/droplet impacting/bending/ twisting-tolerant superhydrophobicity. A porcupinefish is known to protect itself with hard scales, which protrude through its elastic skin (Figure 1a(i)). The structure of the hard scales is a tetrapod (Figure 1a(ii,iii)). We downsized the tetrapod-shaped scales of porcupinefish to micrometer ranges to form a reasonable roughness for superhydrophobicity. In particular, the elastic acicular framework adopted a micrometer Received: June 4, 2019 Accepted: August 6, 2019

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DOI: 10.1021/acsami.9b09524 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Porcupinefish-like structured elastic acicular framework with flexibility and mechanical durability. (a) Structure of the porcupinefish and its skeleton. Photograph of the porcupinefish (i), CT scan images of the porcupinefish (ii), and its magnification (iii). (b) Schematic representation of our material. Acute angles (= 2φ) of 5.1−7.9° for the tip of the spine, and lengths of 4−10 μm of the spine were obtained by analyzing 20 independent ZnO-tetrapods by SEM. (c) SEM images of a single ZnO-tetrapod (i) and an entanglement of two and six ZnO-tetrapods (ii, iii). (d) SEM image of elastic acicular frameworks (weight fraction of ZnO-tetrapod, r = 0.5). Photographs of the materials showing superhydrophobicity with (e) slicing resistance, (f) bending resistance, and (g) twisting resistance. 2.2. Preparation of Superhydrophobic Elastic Acicular Frameworks. 2.2.1. Surface Coating. Typical procedures when r = 0.5, in which r = WZnO/(WZnO + WPDMS) (g/g), are as follows: PDMS (RTV silicone, 1.5 g) was dissolved in ethyl acetate (60 mL) by stirring for 1 min at room temperature (hexane is also available for this purpose). To the solution was added ZnO-tetrapod (1.5 g). The suspension was stirred for 10 min and added into a sprayer. The suspension was spray-casted onto a target substrate. The amount of suspension sprayed was 0.3 mL cm−2. The distance between the sprayer and substrate was 150 mm. 2.2.2. Monolith. Typical procedures when r = 0.5 are as follows: PDMS (RTV silicone, 1.0 g) was dissolved in ethyl acetate (6 mL) by stirring for 1 min at room temperature (hexane is also available for this purpose). To the solution was added ZnO-tetrapod (1.0 g). The suspension was stirred for 10 min and poured onto a Teflon-coated paper. The suspension was gradually evaporated to dryness at room temperature over 1 week. The evaporation speed was 36 μL h−1. 2.3. General. Material surfaces were analyzed with a laser scanning confocal microscope, Keyence Corporation (Japan) model VK-X260. Direct observation of air layer on the coated surface in water was conducted with a laser scanning confocal microscope, Olympus Corporation, Shimadzu Corporation (Japan) model OLS4100. Scanning electron microscope image was obtained with Hitachi High-Technologies Corporation (Japan) model Miniscope TM3000

scale inorganic tetrapod in an elastic polymeric resin (Figure 1b), which were prepared by conventional fabrication techniques, such as spray-coating, molding, and die-cutting. Our elastic acicular material sustained superhydrophobicity even after serious rubbing, scratching, and dicing (Figure 1e). Furthermore, our elastic acicular materials maintained superhydrophobicity under bent (Figure 1f) and twisted (Figure 1g) states. Such high-level multirobustness and a scalable 2D/3D fabrication process contribute to make superhydrophobic materials practically available.

2. MATERIALS AND METHODS 2.1. Materials. Reagents were used as received from Toray Industries, Inc. (Japan) [polydimethylsiloxane (PDMS), roomtemperature-vulcanizing (RTV) silicone (HC2100, solid component, 96.7%)], Panasonic [ZnO-tetrapod with average 10 μm length leg (pana-tetra Z-0501), apparent density: ca. 0.1 g cm−3], Wako Pure Chemical Industries (Japan) [ethyl acetate (EtOAc, 99.5%), hexane (n-C6H14, 96.0%)], Tokyo Chemical Industry (Japan) [phenolphthalein (98.0%) and ammonia solution (28% in water)], and SigmaAldrich Co. LLC (St. Louis, MO) [Nile Blue A, (C20H20N3O)2·SO4]. B


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5.6 g cm−3.44 This led to the formation of a porous framework with an acicular surface in an elastic PDMS, which was characterized by SEM (Figure 1d, Figure 2a−d). Energydispersive X-ray spectrometry indicated that most of the surface of ZnO tetrapods was covered with PDMS (Figure 2e).

or S-4800. Pore volumes and pore diameters were measured by a mercury porosimeter, Micromeritics-Shimadzu Corporation (Japan) model AutoPore IV 9520. 2.4. Measurement of Contact Angle. Contact angle of water/ air droplet was measured at room temperature on a contact angle meter, Kyowa Interface Science model DMs-401. Distilled water (8 μL, with a surface tension of 72.0−73.0 mN m−1) was used as the probe liquid. Advancing and receding contact angles were measured by increasing and decreasing a volume of water on the test surface, respectively. Dynamic behavior of water droplet or jet was recorded with a high-speed camera, Nikon model D5600, and analyzed with motion-capture software, Library model Move-tr/2D. 2.5. Abrasion Test. The abrasion test was conducted with a friction and wear testing device, Shinto Scientific model TYPE:40. A stainless steel (SUS) ball with 6 mm diameter under a 50 g load was mounted onto the test surface. One thousand abrasion cycles at a speed of 10 mm s−1 were applied onto the test surface. 2.6. Scratching Test. The scratching test of test surface was conducted with a pencil hardness tester, Allgood model BEVS1301, according to Japanese industrial standards (JIS K5600-5-4). It is designed to apply a load of 750 g ± 10 g to the tip of the pencil, where the angle of pencil against to the surface is fixed at 45° ± 1° (Figure S4, Supporting Information). Because the test surface was not damaged by the pencil hardness test from 6B to 6H, it was manually and thoroughly scratched with a SUS edge with 0.5 mm thickness. 2.7. Bending Test. The bending test was conducted to evaluate superhydrophobicity of the test surface in a bent state and maintain superhydrophobicity after 1000 bending cycles between curvature of 0.043 mm−1 and 0.095 mm−1. The curvature is defined as reciprocal of the circle radius, and it was calculated by assuming an approximate circle along the material surface. Due to difficulty of contact angle measurement of the water droplet in a deformed state, a water jet was applied to evaluate water repellency of the test surface. The size of the materials was 40 mm × 10 mm × 3 mm. 2.8. Twisting Test. The twisting test was conducted to evaluate superhydrophobicity of the test surface in a twisted state and maintain superhydrophobicity after 1000 twisting cycles between curvature of 0 mm−1 and 0.39 mm−1. Due to difficulty of contact angle measurement of water droplet in a deformed state, a water jet was applied to evaluate water repellency of the test surface. The size of the materials was 50 mm × 8 mm × 3 mm. 2.9. Temperature Dependence of Flexibility. Temperature dependence of the flexibility of the test sample was evaluated by DSC measurement (Figure S5, Supporting Information). DSC of the monolith (r = 0.5) was measured with a Shimadzu Corporation (Japan) model DSC-60 Plus. The monolith (9.4 mg) was enclosed in a PerkinElmer model Al crimp pan. As the reference, a-Al2O3 powder (18.8 mg) for D.T.A standard material (Shimadzu) was enclosed in the other Al crimp pan. Temperature in the range of −150 to 110 °C at a scan rate of 10 °C min−1 was applied to the material.

Figure 2. SEM and EDX images of the elastic acicular framework. (a− c) SEM images of the materials at different scales. (d) An EDX image of the surface of the material, where blue and pink colors indicate Zn and Si, respectively. (e) Elemental analysis of superhydrophobic frameworks showing peaks, C, O, Zn, and Si, which are derived from the component of ZnO-tetrapod and PDMS. Weight fraction of ZnOtetrapod (r) in the material is 0.5.

3.2. Strategy for Superhydrophobicity: Advantage of Tetrapod Geometry. A superhydrophobic surface basically exhibits a positive Laplace pressure from air present in a nanoor micrometer-scale texture. Rational design of our composite material is explained by the tetrapod geometry. In the ZnOtetrapod, four spines radiate from its center of gravity with an interior angle of arccos(−1/3) (ca. 109.5°). If the tetrapod is placed on a plane surface, one spine is vertical to the plane surface (tilt angle (α) = 0°). Considering an interior angle of ca. 109.5°, at minimum, a single spine falls in the range of the tilt angle (α) from −54.8° to +54.8° normal to the plane surface. Furthermore, as shown in Figure 3a, the Laplace pressure Pz is denoted as eq 1 (Figure S1, Supporting Information):45,46

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of Elastic Acicular Framework. Our elastic acicular superhydrophobic material was prepared from a one-pot solution process. Typically, our material was fabricated on the substrate by the spray coating of an ethyl acetate suspension (60 mL) containing a composite of ZnO-tetrapod (1.5 g)42 and a cross-linkable polydimethylsiloxane (i.e., room temperature vulcanizing silicone, hereafter abbreviated as PDMS, 1.5 g). The four spines of each ZnO-tetrapod had a relatively uniform shape with an acute angle of 2φ in the range of 5.1−7.9° for the tips of the spines and lengths of 4−10 μm (Figure 1b), where 20 independent ZnO-tetrapods were analyzed by scanning electron microscopy (SEM). Owing to geometrical hindrance, the tetrapods (Figure 1c(i)) were loosely packed (Figure 1c(ii,iii)) with an apparent density of ca. 0.1 g cm−3,43 about 56 times smaller than the true density of ZnO powder of

π i y Pz = γ π Ωf sin φ cos α sinjjjθ0 − − φzzz 2 k { ( −54.8° < α < +54.8°)

(1)

in which Ω is the density of the spine, f is the adhesion fraction of the solid−liquid interface, φ is the acute angle, θ0 is the static contact angle, and α is the tilt angle of the spine. When the value of α varies from −54.8° to +54.8° and φ varies from 0° to 20°, Pz always shows a positive value (Figure 2b). The ZnO-tetrapod with PDMS satisfies the requirements (0 < θ0 − C


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Figure 3. Advantage of the tetrapod geometry on the Laplace pressure for superhydrophobicity. (a) A schematic diagram of a liquid droplet on a spine-like structure on superhydrophobic frameworks, in which Pz denotes the Laplace pressure (i), φ denotes half of the acute angle of the spine, θ0 denotes the contact angle of the water droplet on the spine, and α denotes the tilt angle of the spine from the vertical axis (ii). (b) Relative Laplace pressure (Pz) at various α. When φ < 20°, Pz shows positive values in the range of α from −54.8° to +54.8°.

Figure 4. Superhydrophobic surface with acicular frameworks. (a) Laser microscopy images of elastic acicular frameworks at various weight fraction of ZnO-tetrapod, r = WZnO/(WZnO + WPDMS). (b) Contact angle of an 8 μL water droplet and an 8 μL air bubble under water on a surface of the superhydrophobic frameworks at various r. (c) Cross-sectional SEM image of the elastic acicular frameworks (r = 0.5) coated on a polyethylene terephthalate substrate.

π/2 − φ < π), where 2φ = 5.1−7.9° (Figure 1b) and θ0 = 112° ± 1 (Figure 4b) and thus possesses an advantage for forming a superhydrophobic surface. 3.3. Superhydrophobicity of the Elastic Acicular Framework. Our composite material had a micrometerscale textured surface that exhibited superhydrophobicity at an appropriate weight fraction of ZnO-tetrapods in the material (ZnO and PDMS), in which r = WZnO/(WZnO + WPDMS) (g/g) (Figure 4a). The contact angle of the water droplet on the surface of our material increased in a sigmoidal manner, and our composite material exhibited superhydrophobicity with a contact angle larger than 150° when r exceeded 0.5 (Figure

4b). In contrast, the contact angle of an air bubble in aqueous conditions inversely decreased in the critical region of r = 0.5. It indicates the presence of an air layer on the acicular surface, which was clearly observed by laser microscopy (Figure S2, Supporting Information). Water repellency of our material was almost similar when r > 0.5 (Figure 4b) while flexibility of the material decreases with r. Therefore, r = 0.5 is an appropriate ratio for preparation of the material with both high water repellency and high flexibility. The acicular surface could be prepared by spray coating the 1:1 suspension of the ZnOtetrapod and cross-linkable PDMS onto any substrate, such as stainless steel, aluminum, glass, paper, rubber, and cotton D


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Figure 5. (a) Photograph of blue-colored water droplets on various substrates coated by the frameworks (r = 0.5). (b) Time-resolved bouncing of a water droplet on surface of glass coated by the elastic acicular framework (r = 0.5). (i) We = 1.57 and w/h = 2.28. (ii) We = 14.5 and w/h = 4.87. (iii) We = 29.8 and w/h = 8.03. See Table S1 and movie S1.

Figure 6. Porosity of the elastic acicular framework. (a) Photograph of monoliths (r = 0.5) showing moldability. (b) Photograph of a monolith (r = 0.5) showing continuous porosity. An aqueous droplet of phenolphthalein was mounted on the monolith on a perforated lid on a vial containing aqueous ammonia. The color of the droplet changed from colorless to pink within 1 min. (c) Porosity and pore diameter of the monoliths at various r, obtained by a mercury porosimeter. (d) Distribution of pore size of the monolith at various r. (e) Total pore volume of the monolith at various r.

(Figure 5a). Water droplets on these coated surfaces showed a contact angle greater than 150°, indicating the formation of

superhydrophobic surface regardless of the type of substrates. Cross-sectional SEM analysis indicated that elastic acicular E


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Figure 7. Superhydrophobicity with abrasion resistance of the elastic acicular frameworks. (a) Water droplets on slashed surfaces of the monolith (r = 0.5). The newly exposed surface also showed superhydrophobicity. (b) The water jet was repelled even on the scratched surface of the monolith (r = 0.5). (c) The abrasion test of the monolith. (i) Experimental setup of the test. (ii) 1000 abrasion cycles were applied with a SUS ball (φ = 6 mm) under a 50 g load. The friction resistance was almost the same over 1000 abrasion cycles, indicating that the monolith did not incur serious damage. (iii−v) Advancing and receding contact angle of water droplet (a) before and (b) after the abrasion test.

we show in Figure 5(b) the high-speed camera images of a droplet impacting the surface, which indicates that the surface repelled the water impact with a high droplet deformation ratio (Figure 5b(iii)). Our composite material could take desired shapes by a simple molding or die-cutting method (Figure 6a). The freshly cut surface of the monolith demonstrated superhydrophobicity (Figure 7a) unlike general superhydrophobic materials having only an acicular surface layer.24 Loosely packed ZnO-tetrapods provided not only an “2D acicular surface” but also a “3D porous framework”, leading to superhydrophobicity even on a slashed surface. The porous framework was characterized with a mercury porosimeter (Figure 6c−e), generally utilized for the analysis of pores on a micrometer scale. When the r value was less than 0.4, the porosity (%) was negligibly small. However, the porosity increased in a sigmoidal manner when r = 0.4− 0.8. In addition, the pore size at r = 0.5 was estimated to be ca.

frameworks could be coated on the substrate with about 100 μm thickness (Figure 4c). The coated surface demonstrated high water penetration resistance, as it repelled water without adhesion with a Weber number of 1.57−29.8 and a droplet deformation ratio of 2.28−8.03 (Figure 5b, Table S1 and movie S1, Supporting Information), where the Weber number is denoted as We = ρ DU2/γ, in which ρ is droplet density, D is the diameter of the droplet, U is the collision speed vertically to the surface, γ is the surface tension of the droplet; w/h is the droplet deformation ratio, in which w is the maximum droplet width during impact to the surface, and h is the minimum droplet height during the impact. Because We can also be increased with a diameter of the impacting droplet, high We by large droplet diameter does not explain the high impact pressure in that this leads to the dispersion of impacting pressure47 toward superhydrophobic coatings. Thus, to accurately explain the high-pressure resistance of our coatings, F


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Figure 8. Mechanical deformation resistance of the elastic acicular framework quantified by bending and twisting. (a) A proposed mechanism of sustainable superhydrophobicity under mechanical deformation. ZnO-tetrapods were exposed in the stretched area because of the thinning of the elastic PDMS along the axis of deformation. (b) The water droplet was not absorbed on the surface after 1000 bending cycles, where the contact angle of the water droplet was larger than 150°. (c) 1000 bending cycles (i, ii) between a curvature of 0.043 mm−1 and 0.14 mm−1 were applied to the superhydrophobic framework (r = 0.5); remarkable mechanical damage was not observed. (iii) The water jet was highly repelled on the surface in a bent form, and water droplets did not adhere on the surface after 1000 bending cycles. (d) Twisting cycles (i, ii) between a curvature of 0 mm−1 and 0.39 mm−1 were applied to the material (r = 0.5); remarkable mechanical damage was not observed. (iii) The water jet was highly repelled on the surface in a twisted form. (e) Photograph and SEM images of the curved surface at various magnifications (i−iii), in which the arrows indicate the direction of the bending. Through the bending, the micrometer-scale texture was likely to be elongated along the deformation. However, the embedded micrometer-scale texture was newly exposed through the thinner PDMS film.

4.1 μm, in fair agreement with the diameter observed by SEM (Figures 1d, 2b). Here the continuous pore structure was confirmed with a simple gas/liquid permeation test (Figure 6b).30 Gaseous ammonia permeated the continuous pores in the framework, whereas the water droplet placed on the monolith with a superhydrophobic acicular surface could not pass through. 3.4. Mechanical Durability and Elasticity of the Elastic Acicular Framework. Our composite material showed an excellent abrasion-resistant superhydrophobicity. The abrasion resistance was confirmed as follows. First, a monolith with a plate shape (r = 0.5) was fabricated from the 1:1 suspension of ZnO-tetrapod and PDMS, and then a 50 g weight (SUS ball with φ = 6 mm) was applied and repeated 1000 cycles with a speed of 10 mm/s (Figure 7c(i)). During the 1000 abrasion cycles, the friction force was constant (Figure 7c(ii)), indicating that the acicular surface did not incur serious damage. We further confirmed the sustainability against this test by five independent measurements of the advancing and receding water contact angles before and after the abrasion test (Figure 7c(iii)). The hysteresis of the advancing and receding contact angles was kept constant less than 5° through the abrasion test (Figure 7c(iv,v)) which

means the water adhesion force was small as explained by classical Furmidge’s law.48 The superhydrophobicity was sustained even after the surface was subjected to intentional damage by scratching with a sharp edge (Figure 7b and movie S2, Supporting Information). The surface after scratching was observed by SEM; the acicular surface was retained on the damaged surface without any treatment after scratching (Figure 7d and Figure S3, Supporting Information). The elasticity and porosity of our composite material probably worked as a cushion and protected the acicular surface from the abrasion and scratching damage. In addition, our composite material (r = 0.5) sustained its superhydrophobicity after mechanical deformation, such as bending (Figure 8c and movies S3, S4, and S5, Supporting Information) and twisting (Figure 8d and movie S6, Supporting Information). Generally, it is difficult to repel water droplets on an an acicular (nano/microtextured) surface in a mechanically deformed state, because the distance between the spines in the acicular texture is elongated along with the mechanical deformation, resulting in a decrease in the Laplace pressure. FE-SEM observations clearly revealed how the acicular structure was obtained under mechanical deformation (Figure 8e). The SEM images of the stretched G


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ACS Applied Materials & Interfaces region are shown in Figure 8e(ii,iii). The ZnO-tetrapod was exposed in the stretched area, whereas the elastic PDMS became thinner along the axis of deformation. To our surprise, the superhydrophobicity of our composite material was sustained even after 1000 bending cycles (Figure 8b). This material can be regarded as a hybrid framework composed of an inorganic hard portion and an elastic polymer resin. The flexible polymer resin deformed when a force was imparted on this material, whereas the inorganic framework remained unchanged. As a result, the embedded spines were exposed to the surface, leading to the formation of a nascent acicular texture (Figure 8a). Furthermore, DSC profile of our material (r = 0.5) suggested flexibility over a wide temperature range from −30 °C to 110 °C (Figure S5, Supporting Information).

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AUTHOR INFORMATION

Corresponding Authors

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

Yoshihiro Yamauchi: 0000-0003-3611-5638 Sadaki Samitsu: 0000-0002-4139-1656 Notes

The authors declare no competing financial interest.

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4. CONCLUSION We fabricated multimechanical deformation (bending, twisting)- and multimechanical damage (abrasion, scratching, slicing, droplet impacting)-resistant superhydrophobic materials with porcupinefish-like structure. The material was designed by a “geometry-and-flexibility” concerted strategy of the ZnO-tetrapod and the PDMS elastomer, respectively. The geometry of the ZnO-tetrapod contributes to form not only 2D acicular surface but also 3D porous structure. The flexibility and porosity of the material enables cushioning against the mechanical impacts and deformations. From the viewpoint of processing, the materials were moldable as thin films, plates, and monoliths with a low-cost and all-wet process at ambient temperature. Although the body surface of porcupinefish does not exhibit superhydrophobicity, our material with porcupinefish-like structure exhibited superhydrophobicity by extracting an essence of the creature having a unique structure composed of a hard scale and elastic skin. The material gives insight into the design of superhydrophobic materials, that it is important not only to mimic natural superhydrophobic structure but also to learn from natural nonsuperhydrophobic structure.

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Showing elasticity of the material (r = 0.5) and demonstration of deformation-resistant superhydrophobicity of the elastic acicular framework (MP4)

ACKNOWLEDGMENTS This work was financially supported by Acquisition, Technology & Logistics Agency. We thank Mamoru Shimada and Naoki Yamamori from Nippon Paint and Chiharu Kawakita from National Maritime Research Institute for discussion toward practical applications. We thank Japan Multi-Industrial Company for kindly providing photographs and CT scan images of a porcupinefish.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b09524. Theoretical calculation of Laplace pressure (Figure S1); direct observation of air layer (Figure S2); scratching test (Figure S3); pencil hardness test (Figure S4); DSC of the elastic acicular framework (Figure S5); superhydrophobicity of spray-coated surface (Table S1); captions for supporting movies (PDF) Time-resolved bouncing of a water droplet on surface of glass coated by elastic acicular framework (r = 0.5) (MP4) Demonstration of scratch-resistant superhydrophobicity of the elastic acicular framework (r = 0.5) (MP4) Showing flexibility of the material (r = 0.5) and demonstration of deformation-resistant superhydrophobicity of the material (MP4) Demonstration of deformation-resistant superhydrophobicity of elastic acicular framework (r = 0.5) (MP4) Time-resolved bouncing of a water droplet on surface of elastic acicular framework (r = 0.5) at a bent state (MP4) H


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Durable and Flexible Superhydrophobic Materials: Abrasion

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