Control of water droplet motion by alteration of roughness gradient on silicon wafer by laser surface treatment

Surface wettability gradient from super-hydrophobicity to hydrophobicity was produced by creating surface roughness gradient on polished silicon wafer with excimer laser. Water droplets move spontaneously towards a designed route on this gradient surface. In addition, an irreversible moving behavior with good resistance to external disturbances was observed, which makes the surface suitable for liquid directional transfers. It was also found that the wettability gradient does not always agree with the roughness gradient, which depends on the wetting mode of the droplets. A wetting mode transition from Cassie-Baxter to Wenzel was demonstrated necessary for generating self-motion of water drops. Control of water droplet motion can be achieved by employing these roughness gradient surfaces.