Dynamic Solid Surface Tension Causes Droplet Pinning and Depinning.

M Van Gorcum,B Andreotti,J H Snoeijer,S Karpitschka

doi:10.1103/physrevlett.121.208003

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 localized 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.

Highlights

The contact line of a liquid drop on a solid exerts a nanometrically sharp surface traction
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
Our findings reveal a deep connection between stick-slip processes and newly identified dynamical surface effects

Summary

Introduction

The contact line of a liquid drop on a solid exerts a nanometrically sharp surface traction. 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. 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.

Results

Conclusion