Droplets move over viscoelastic substrates by surfing a ridge

TL;DR

This paper reveals that liquid drops move over soft viscoelastic substrates by surfing a deforming ridge, with the ridge's shape and dynamics explained by substrate rheology, impacting understanding of wetting and soft matter physics.

Contribution

It introduces a new mechanism of drop motion over soft substrates via ridge surfing, supported by experimental measurements and a rheology-based theoretical model.

Findings

The contact line velocity determines the ridge's orientation angle.

The theory accurately predicts the dynamic contact angle.

It explains stick-slip motion through ridge depinning and reformation.

Abstract

Liquid drops on soft solids generate strong deformations below the contact line, resulting from a balance of capillary and elastic forces. The movement of these drops may cause strong, potentially singular dissipation in the soft solid. Here we show that a drop on a soft substrate moves by surfing a ridge: the initially flat solid surface is deformed into a sharp ridge whose orientation angle depends on the contact line velocity. We measure this angle for water on a silicone gel and develop a theory based on the substrate rheology. We quantitatively recover the dynamic contact angle and provide a mechanism for stick-slip motion when a drop is forced strongly: the contact line depins and slides down the wetting ridge, forming a new one after a transient. We anticipate that our theory will have implications in problems such as self-organization of cell tissues or the design of capillarity-based microrheometers.