Drop dynamics on hydrophobic and superhydrophobic surfaces

TL;DR

This study explores how micron-scale drops move on hydrophobic and superhydrophobic surfaces, revealing a transition from sliding to rolling motion and identifying key factors influencing their dynamics.

Contribution

It introduces a mesoscopic slip capillary number and explains the difference in drop motion on superhydrophobic surfaces compared to smooth ones.

Findings

Velocity profile transitions from quadratic to linear with height.

Rolling velocity increases with viscosity.

Drops move faster on superhydrophobic surfaces due to tank treading.

Abstract

We investigate the dynamics of micron-scale drops pushed across a hydrophobic or superhydrophobic surface. The velocity profile across the drop varies from quadratic to linear with increasing height, indicating a crossover from a sliding to a rolling motion. We identify a mesoscopic slip capillary number which depends only on the motion of the contact line and the shape of the drop, and show that the angular velocity of the rolling increases with increasing viscosity. For drops on superhydrophobic surfaces we argue that a tank treading advance from post to post replaces the diffusive relaxation that allows the contact line to move on smooth surfaces. Hence drops move on superhydrophobic surfaces more quickly than on smooth geometries.