Preparation of highly water-repellent surface by spontaneous formation of double scale roughness pattern

Hydrophobic organic-inorganic hybrid composite suspensions were prepared by mixing hydrophobic octylsilyl titanium dioxide particles having average diameter of 35 nm with drying oil or moisture cure room temperature vulcanization silicone gum in volatile silicone. They were spread on a glass plate by using a linear motor coater and an applicator. Spatially periodic stripe patterns parallel to the direction of dragging the applicator were usually generated. The phenomenon is called directional viscous fingering, i.e. spontaneous pattern formation by the growth of fluctuation in morphology of mobile interface during the dragging coat. The pattern spontaneously formed on the surface became double scale when stored samples were coated. In this case, the large scale spatially periodic pattern was formed by the directional viscous fingering and the small ragged random pattern may be due to the giant molecules formed by cross-linking of silicone gum. Double scale roughness patterns were also generated by double dragging coat. The large and small scale pattern was formed by the first and second dragging coat, respectively. The formation of double scale roughness enhanced the water-repellent property of the hydrophobic surface. In some cases, water contact angle increased by 20 degrees to realize super water-repellent surface with a value exceeding 150 degrees.