The paradox of contact angle selection on stretched soft solids

TL;DR

This paper develops a theoretical framework for understanding how contact angles of liquids on soft elastic solids are influenced by elasticity and surface energy effects, validated through experiments on silicone gels.

Contribution

It introduces a variational approach to derive contact angle selection equations on stretched soft solids, linking elasticity, pinning, hysteresis, and the Shuttleworth effect.

Findings

Stretching the substrate increases contact line mobility.

The theory accurately predicts contact angles on silicone gels.

Elasticity influences contact angle hysteresis and pinning behavior.

Abstract

The interfacial mechanics of soft elastic networks play a central role in biological and technological contexts. Yet, effects of solid capillarity have remained controversial, primarily due to the strain-dependent surface energy. Here we derive the equations that govern the selection of contact angles of liquid drops on elastic surfaces from variational principles. It is found that the substrate's elasticity imposes a nontrivial condition that relates pinning, hysteresis and contact line mobility to the so-called Shuttleworth effect. We experimentally validate our theory for droplets on a silicone gel, revealing an enhanced contact line mobility when stretching the substrate.