Sliding of Water Droplets on Smooth Hydrophobic Silane Coatings with Regular Triangle Hydrophilic Regions

The effect of the triangular pinning region on the sliding of water droplets on the smooth hydrophobic surface was investigated. Smooth hydrophobic silane coatings with various regular triangle hydrophilic regions were prepared using photolithography and octadecyltrimethoxysilane (ODS). The hydrophilic area in the surfaces was aligned hexagonally with a constant area fraction. Thereby water contact angles of the coatings were almost equivalent. The water droplet sliding velocity increased continuously with increasing pattern size. Anisotropic sliding velocity was observed on the surface, suggesting different pinning effects. The sliding motion of water droplets on the gradient surface with changing hydrophilic region size deflects against the downward direction. The deflection length depends on the direction of triangle hydrophilic regions and the initial sliding position. These results demonstrate that control of the sliding velocity while sustaining the static contact angle is feasible by designing the shape and alignment of chemical heterogeneity.