Sliding of water droplets on microstructured hydrophobic surfaces

Sliding behaviors of liquid droplets on solid surfaces are among the fundamental results of wettability. To remedy the lack of quantitative correlation between sliding angle and roughness of the surface, which is known to be effective at enhancing wettability, we report in this paper the observation that the onset of water droplets sliding under gravity on inclined micropillar-structured hydrophobic surfaces always starts with detachment of the rear contact lines of the droplets from the pillar tops. We also establish an explicit analytical model, based on the observed mechanism, by which the sliding angle is fully determined by the fraction of water-solid interface area, droplet volume, and Young's contact angle. This model gives predictions of sliding angles that agree well with experimental measurements.