Droplets move over viscoelastic substrates by surfing a ridge.

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

doi:10.1038/ncomms8891

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.

Highlights

Liquid drops on soft solids generate strong deformations below the contact line, resulting from a balance of capillary and elastic forces
A dissipation singularity arises at the moving contact line[14] and its regularization at the nanoscopic scale can result from various processes[13,15]
Pioneering experiments have shown that the softness drastically slows down the wetting dynamics[24,25] in comparison to rigid solids

Summary

Introduction

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. 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. The theoretical description of moving contact lines over soft solids is so far limited to global dissipation arguments[17], which, at least for wetting of rigid solids, are known not to capture the entire physics behind the process

Objectives

Methods

Conclusion