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Dual Wettability on Diarylethene Microcrystalline Surface Mimicking a Termite Wing

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Dual Wettability on Diarylethene Microcrystalline Surface Mimicking a Termite Wing ARTICLE Corrected: Publisher Correction https://doi.org/10.1038/s42004-019-0192-6 OPEN Dual wettability on diarylethene microcrystalline surface mimicking a termite wing Ryo Nishimura 1, Kengo Hyodo1, Hiroyuki Mayama 2, Satoshi Yokojima3,4, Shinichiro Nakamura 4 & Kingo Uchida 1,4 1234567890():,; The termite wing has a specific property of wetting in contact with a water droplet: it adsorbs water mist, whereas larger water droplets are bounced on the surface. This is owing to the survival strategy of termites. Here, we reproduce the termite wing’s dual wettability by a photoinduced crystal growth technique. Upon UV irradiation to a microcrystalline surface of a mixture of two diarylethenes, two types of needle-shaped crystals of distinctly different sizes are observed to grow. The surface shows behavior akin to the termite wing’s dual wettability. The bouncing ability of a water droplet is attributed to the smaller-sized needle crystals, whereas the adhesive property is owing to the larger-sized ones, explaining the micro- structures of the termite wing. Considering dissipation energy and adhesion energy, the bouncing ability and dual wettability can be explained theoretically. The surface could potentially be used in water harvesting applications. 1 Department of Materials Chemistry, Ryukoku University, Seta, Otsu, Shiga 520-2194, Japan. 2 Department of Chemistry, Asahikawa Medical University, 2-1- 1-1 Midorigaoka–higashi, Asahikawa, Hokkaido 078-8510, Japan. 3 School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan. 4 RIKEN Cluster for Science, Technology and Innovation Hub, Nakamura Laboratory, 2-1 Hirosawa, Wako, Saitama 351- 0198, Japan. Correspondence and requests for materials should be addressed to H.M. (email: [email protected]) or to K.U. (email: [email protected]) COMMUNICATIONS CHEMISTRY | (2019) 2:90 | https://doi.org/10.1038/s42004-019-0192-6 | www.nature.com/commschem 1 ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-019-0192-6 n nature, many plants and insects have specifically structured their flight. Interestingly, the termites that fly during rainy period Isurfaces that show specific surface properties. The unique have hair structures on their wings, but other termites have no micro- and nanostructures on such surfaces can be seen in the hair shaped structures on their wings (Supplementary Figs. 2–4). self-cleaning effect of lotus leaves1, the superhydrophobic forces Owing to these functions, the termite can minimize interaction exerted by a water strider’s leg2, the attachment mechanism of with micro-sized water droplets while flying in rain or geckos3, the structure colors of the peacock4, or morpho even storm. butterfly5,6, and many other natural phenomena7–12. These Consequently, termite wings have a structure that forms the structures have received much attention, and artificial systems foundation of their survival strategy. Increasingly, sustainability is have been fabricated by mimicking such natural surfaces. Water also important in human society and the management of water, repellence supports the performance of self-cleaning materials, particularly regarding the important issues of the environment and and it has been studied in both natural and artificial systems12. energy. We believe that better understanding the structure of termite Historically, the most important finding is the “lotus effect” of wings would provide great hints for material design as an essential Barthlott and Neinhuis13. The superwater-repellent and self- component of humidity management in human society. And to cleaning effects of the lotus leaf have been attributed to the build it is to understand. Therefore, this research attempts to fab- double-roughness structure of surfaces with micro- and nanos- ricate functional material replicating termite wing from diarylethene. tructures (trichomes, cuticular folds, and wax crystals), as well as Diarylethene derivatives are well-known compounds whose to the hydrophobic properties of the epicuticular wax14. photochromic properties, such as the thermal stability of both Recently, it was reported that many terrestrial insects use non- isomers, and high photoreactivity even in the crystalline states, wetting surfaces to reduce the risks of living in environments with are excellent among the photochromes. These unique properties much rain and other water surfaces that the insect may are indispensable to the current research, and we applied them to encounter15–20. Watson et al.18 reported that the complex the design of photoinduced topographical control systems21–27. structure of the termite’s wing surface shows specific wettability In previous papers, we showed that surface topography could be (Supplementary Fig. 1)19. The termite is one of the social insects, structured by using a crystal growth technique (CGT)27 of pho- and it is known that high humidity is important for their togenerated closed-ring isomer 1c of thermally stable photo- breeding. One of the termites Nasutitermes sp. and Micro- chromic diarylethene 1 (Fig. 1)21. Upon UV irradiation to the cerotermes sp. fly during the rain period to avoid attacks by microcrystalline film of 1o (Fig. 1), the needle-shaped crystals of predators, whereas water is essential for building nests and soil 1c grew on the surface. Then, a superhydrophobic property was tunnels, i.e., moist soil is necessary for burrowing18,19. Accord- observed. Moreover, we can switch this property by melting these ingly, they have delicate structures, or “dual wettability”, on their crystals by irradiation with visible light, thus causing the super- wings, and these specific structures work to minimize interaction hydrophobicity to disappear. In the next step, we mimicked the with water bodies at various length scales to effectively reduce the double-roughness structured surface of the lotus leaf and repro- body mass18,19. The termite’s wing surface is covered with two duced it on the microcrystalline surface of 1o. Consequently, we types of projections of different sizes (Supplementary Fig. 1). One succeeded in demonstrating the water droplet-bouncing phe- is a hair-like projection (macrotrichia) that is ~ 50 μm long and 1 nomena as observed for the lotus leaf in nature25. The photo- μm wide, whereas the other is a star-shaped projection (micra- induced topographical changes of 2o were similar to those of 1o, sters) of 5–6 μm in height and width. These hairs protect the wing whereas the size of the needle-shaped crystal of 2c was much membrane from the contact with large water droplets, whereas smaller than that of 1c, when they grew at 30 °C23,24. We took the small projections work to keep the small droplets on the advantage of this difference in the current study. surface of the wing18,19. The small droplets gather to build up Here we prepare a photo-responsive superhydrophobic surface large ones and are finally removed via the hair arrays. Further- using a mixture of diarylethenes 1o and 2o. The mixture of two more, the approximation of body weight after wetting suggested types of diarylethenes may produce the surface with the sum of the importance of collection ability by micraster. The estimated two different characteristics or completely different character- additional body weight relation to the total body weight of istics. The obtained photogenerated rough surface consisting of Nasutitermes sp. and Microcerotermes sp. are several %. In fact, the two different sized needle-shaped crystals 1c and 2c shows larger droplets were repelled by hair-like structures, whereas specific bouncing ability mimicking a termite wing as we have small droplets were collected by star-shaped structures18. Watson intended. Then, we analyze the surface structures and clarified the et al.18 argued the Nasutitermes sp. and Microcerotermes sp. have relation between the wettability by monitoring the different sizes water-collecting ability, and owing to the ability, they can control of water droplet on the surfaces. F F F F F F F F ν F F h F F OMe OMe Me hν′ Me Si Si Me Me S S Me Si S MeO S Si Me MeO Me Me Me Me Me Me 1o 1c F F F F F F F F F F F F hν Me Me Me Me S Me S Si ν′ Me S S Si Me h S Me S Me S S Me Si Si Me Me Me Me Me Me 2o 2c Fig. 1 Molecular structures of diarylethenes. Molecular structures and photoisomerization of diarylethenes 1 and 2 2 COMMUNICATIONS CHEMISTRY | (2019) 2:90 | https://doi.org/10.1038/s42004-019-0192-6 | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-019-0192-6 ARTICLE CA : 122.8 ± 2.0° CA : 163.7 ± 0.9° CA : 120.1 ± 2.3° abVis. c >480 nm UV 1 h 313 nm 5 min UV 313 nm 5 min def Fig. 2 Wettability and reversible crystal growths on the surface. Images of a water droplet (Volume: 1.5 μL) on rough surfaces and SEM images a–f of topographical changes of microcrystalline films made from a mixture of 1o and 2o (molecular ratio, 1o:2o = 1:1) by alternating irradiation with UV and visible light. a Before UV irradiation; b surface UV (313 nm) light irradiation for 5 min followed by storage at 30 °C in the dark for 9 days; c visible (λ > 480 nm) light irradiation for 1 h while kept at 80 °C. Scale bars: 5.00 μm for a, b, c; 20.0 μm for d; 1.00 μm for e, 3.33 μm for f Results with good reproducibility. Upon visible (λ > 480 nm) light Preparation of crystalline films mimicking the termite wing.To irradiation for 1 h at 80 °C, the needle-shaped crystals were examine whether the surface of the mixture of 1o and 2o melted (Fig. 2c). The formed structures were very similar, in possess the properties of termite wing as we have expected, we spite of the difference in the ratio, indicating that the crystal first investigated its properties.
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