Photoinduced Formation of Superhydrophobic Surface on Which Contact Angle of a Water Droplet Exceeds 170° by Reversible Topographical Changes on a Diarylethene Microcrystalline Surface

Abstract

A superhydrophobic surface on which the contact angle of a water droplet exceeds 170° was reversibly produced by alternate irradiation with UV and visible light. Superhydrophobicity is due to the formation of densely generated submicrometer sized needle-shaped crystals (less than 0.2-0.3 μm diameter and 2.2-2.5 μm long) at 30 °C, which is much lower than the eutectic temperature of either isomers of the diarylethene. Below the eutectic temperature, the generated crystals were much smaller than those generated above the eutectic temperature. These smaller crystals more effectively enhanced the superhydrophobicity.

Photochromic Crystalline Systems Mimicking Bio‐Functions

“…Diarylethene 3 o is known to form submicron‐scale needle‐shaped crystals of 3 c after UV irradiation to the crystalline film of 3 o followed by storage at 30 °C for 9 days in the dark . The diameters and lengths of the needle‐shaped crystals of 3 o are around 0.20–0.35 μm and 2.2–2.5 μm, respectively (Figure h).…”

“…Small needle-shapedc rystals of 3c covered the surface, whereas CA reached 1738 and SA was less than 18,i ndicating very excellent super water-repellency. [12,13] These resultsi ndicate that the crystal growth proceeds above the compound's T g . The importance of T g was already reported by Ts ujioka et al for metal vapor deposition on diarylethene crystalline surfaces.…”

“…Diarylethene 3o is known to form submicron-scale needleshaped crystalso f3c after UV irradiation to the crystalline film of 3o followed by storagea t3 0 8Cf or 9d ays in the dark. [12] The diameters and lengths of the needle-shaped crystals of 3o are around 0.20-0.35 mma nd 2.2-2.5 mm, respectively (Figure 5h). The activation energy for formation of 3c was only 58 kJ mol À1 ,w hich is much smaller than that of 1c (143 kJ mol À1 ).…”

“…Another surface of 3 o after UV irradiation was stored for nine days at 30 °C, which is above the glass transition temperature ( T g ) of 3 o , was observed. Small needle‐shaped crystals of 3 c covered the surface, whereas CA reached 173° and SA was less than 1°, indicating very excellent super water‐repellency . These results indicate that the crystal growth proceeds above the compound's T g .…”

Dual wettability on diarylethene microcrystalline surface mimicking a termite wing

“…Diarylethene derivatives are well-known compounds whose photochromic properties, such as the thermal stability of both isomers, and high photoreactivity even in the crystalline states, are excellent among the photochromes. These unique properties are indispensable to the current research, and we applied them to the design of photoinduced topographical control systems [21][22][23][24][25][26][27] . In previous papers, we showed that surface topography could be structured by using a crystal growth technique (CGT) 27 of photogenerated closed-ring isomer 1c of thermally stable photochromic diarylethene 1 (Fig.…”