Fabrication of spatially periodic double roughness structures by directional viscous fingering and spinodal dewetting for water-repellent surfaces.

Water-repellent and self-cleaning properties of lotus leaves are considered to be due to its double roughness structure, protrusion structure (approximately 20 microm) and hairy structure (0.2-1.0 microm). In this study, attempts to fabricate a spatially periodic double roughness structure by two far-from-equilibrium self-organization phenomena, a directional viscous fingering and a spinodal dewetting, were made. A mixture of an octylsilyl titanium dioxide particle having an average diameter of 35 nm suspended in volatile silicone, decamethyl cyclopentasiloxane, and octyl p-methoxycinnamate was spread on a glass plate by dragging an applicator across the top. Formation of a stripe pattern parallel to the direction of dragging, called directional viscous fingering, was sometimes observed. Influences of spreading conditions on the pattern formation were analyzed. In addition, attempts were made to apply the stripe pattern formation to the preparation of a water repellent surface. We have succeeded in preparing a highly water-repellent surface by immersing a glass plate, on which a spatially periodic stripe pattern having a characteristic wavelength of 200-700 microm was formed, in water, after the completion of evaporation of decamethyl cyclopentasiloxane. In this case, dewetting patterns having a characteristic wavelength at around 5 microm were formed at the bottom part of the stripe patterns. Neither the surface on which only the mesoscopic spatially periodic stripe pattern was formed nor the one on which only the microscopic dewetting pattern was formed showed high water-repellent properties, indicating that the coexistence of the two different scales of patterns increased the water-repellent properties of the hydrophobic surface.