Anisotropic hysteresis on ratcheted superhydrophobic surfaces

TL;DR

This paper investigates how anisotropic surface patterns influence the behavior and hysteresis of liquid drops, revealing a new superhydrophobic state and explaining directional drop motion on butterfly wings.

Contribution

It introduces a novel partially suspended superhydrophobic state caused by anisotropic patterning and explains directional hysteresis in drop motion on such surfaces.

Findings

Directional contact angle hysteresis depends on surface patterning.

A new partially suspended superhydrophobic state is identified.

The results explain water drop motion on butterfly wings.

Abstract

We consider the equilibrium behaviour and dynamics of liquid drops on a superhydrophobic surface patterned with sawtooth ridges or posts. Due to the anisotropic geometry of the surface patterning, the contact line can preferentially depin from one side of the ratchets, leading to a novel, partially suspended, superhydrophobic state. In both this configuration, and the collapsed state, the drops show strong directional contact angle hysteresis as they are pushed across the surface. The easy direction is, however, different for the two states. This observation allows us to interpret recent experiments describing the motion of water drops on butterfly wings.