Sliding Behavior of Water Droplets on Smooth Hydrophobic Fluoroalkylsilane Coatings with Different Surface Coverage Ratio

Abstract

Smooth fluoroalkylsilane (FAS17) coatings (Ra<0.2 nm) with different surface coverage ratios were prepared on a Si substrate by changing the deposition time. Then the internal fluidity of water droplets sliding on the coatings was evaluated using particle image velocimetry (PIV). The water droplets slid downward with acceleration by a caterpillar-like rotation with slippage on the solid-liquid interface. The sliding acceleration of the advancing contact line decreased concomitantly with the decreasing surface coverage ratio. However, the slipping/rolling ratio to the overall sliding acceleration was almost constant against surface coverage ratio. This result suggests that physical roughness and chemical composition contributed differently to slipping and rolling motions of the droplets.