ABSTRACT
Two types of superhydrophobic surfaces which show lotus and petal effects were induced on photochromic diarylethene microcrystalline surfaces by UV and visible light irradiation and temperature control. On the surfaces showing the lotus effect, a low-adhesion superhydrophobic property is attributed to the surface structure being covered with densely standing needle-shaped crystals of the closed-ring isomer. On surfaces showing the petal effect, a high-adhesion superhydrophobic surface consists of fine needle-shaped crystals with high density together with a few rod-shaped crystals, where an invasion phenomenon occurs between these rod-shaped crystals. Furthermore, the different superhydrophobic properties of the surfaces are theoretically explained using multipillar surface models.
Subject(s)
Light , Hydrophobic and Hydrophilic Interactions , Surface Properties , WettabilityABSTRACT
We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase IIUV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase IIItemp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase IIUV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase IIItemp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase IIUV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 µm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of µm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.