Capillary Pressure and Contact Line Force on a Soft Solid

TL;DR

This paper demonstrates that soft solids experience capillary pressure similar to liquids, affecting their deformation and contact line forces, with experimental evidence showing liquid pulls inward at the contact line.

Contribution

It provides experimental evidence and quantitative analysis of capillary pressure in soft solids, revealing the direction of the liquid pull near the contact line.

Findings

Soft solids exhibit capillary pressure similar to liquids.

The effective surface tension of the solid matches liquid-vapor surface tension.

The liquid pulls on the solid towards the interior, not along the interface.

Abstract

The surface free energy, or surface tension, of a liquid interface gives rise to a pressure jump when the interface is curved. Here we show that a similar capillary pressure arises at the interface of soft solids. We present experimental evidence that immersion of a thin elastomeric wire into a liquid induces a substantial elastic compression due to the solid capillary pressure at the bottom. We quantitatively determine the effective surface tension from the elastic displacement field, and find a value comparable to the liquid-vapor surface tension. Most importantly, these results also reveal the way the liquid pulls on the solid close to the contact line: the capillary force is not oriented along the liquid-air interface, nor perpendicularly to the solid surface, as previously hypothesized, but towards the interior of the liquid.