Dynamic solid surface tension causes droplet pinning and depinning

TL;DR

This study reveals that dynamic solid surface tension variations cause droplet pinning and depinning, linking stick-slip instability to surface effects rather than bulk rheology, through direct visualization of wetting ridge behavior.

Contribution

The paper uncovers the role of dynamical surface tension changes in droplet depinning, providing new insights into contact line dynamics on soft solids.

Findings

Wetting ridge opening angle increases with spreading speed.

Depinning criterion based on dynamic surface tension is experimentally confirmed.

Surface tension dynamics are key to understanding stick-slip instability.

Abstract

The contact line of a liquid drop on a solid exerts a nanometrically sharp surface traction. This provides an unprecedented tool to study highly localised and dynamic surface deformations of soft polymer networks. One of the outstanding problems in this context is the stick-slip instability, observed above a critical velocity, during which the contact line periodically depins from its own wetting ridge. Time-resolved measurements of the solid deformation are challenging, and the mechanism of dynamical depinning has remained elusive. Here we present direct visualisations of the dynamic wetting ridge formed by water spreading on a PDMS gel. Unexpectedly, it is found that the opening angle of the wetting ridge increases with speed, which cannot be attributed to bulk rheology, but points to a dynamical increase of the solid's surface tensions. From this we derive the criterion for depinning that is confirmed experimentally. Our findings reveal a deep connection between stick-slip processes and newly identified dynamical surface effects.