Physics of Drop Coalescence on a Surface with Graded Wettability

Abstract

Drops on a surface that has graded wettability move when the wettability gradient is greater than hysteresis. We experimentally demonstrate that coalescence of drops initiate the motion of resultant drop on a surface that has hysteresis greater than wettability gradient. Importantly, our experiments show that in addition to shift of center of mass towards high wetting region, drop coalescence on a surface having graded wettability also results in motion of merged drop. We also explain the mechanism by which drop coalescence on a surface having graded wettability results in motion of otherwise stationary drops. Based on the mechanism, we analytically show that on a surface that has wettability gradient larger than hysteresis coalescence leads to reduction in drop size at which drop starts to move. Our analytical results show that reduction of drop departure radius due to coalescence depends on the local wetting conditions of the surface with wettability gradient.