Surface tension and the mechanics of liquid inclusions in compliant solids

TL;DR

This paper extends Eshelby's inclusion theory to soft materials by incorporating surface tension effects, revealing how liquid inclusions influence the mechanical properties of compliant solids depending on size and surface stress.

Contribution

It introduces a model that accounts for surface tension in liquid inclusions within soft solids, showing how it alters shape, stiffness, and stress fields, which was not considered in traditional Eshelby theory.

Findings

Surface tension significantly affects inclusion shape and stress fields.

Liquid inclusions can stiffen a soft solid when smaller than 1.5 times the elastocapillary length.

Surface tension can cloak the far-field signature of inclusions at a specific size.

Abstract

Eshelby's theory of inclusions has wide-reaching implications across the mechanics of materials and structures including the theories of composites, fracture, and plasticity. However, it does not include the effects of surface stress, which has recently been shown to control many processes in soft materials such as gels, elastomers and biological tissue. To extend Eshelby's theory of inclusions to soft materials, we consider liquid inclusions within an isotropic, compressible, linear-elastic solid. We solve for the displacement and stress fields around individual stretched inclusions, accounting for the bulk elasticity of the solid and the surface tension (\textit{i.e.} isotropic strain-independent surface stress) of the solid-liquid interface. Surface tension significantly alters the inclusion's shape and stiffness as well as its near- and far-field stress fields. These phenomenon depend strongly on the ratio of inclusion radius, $R$, to an elastocapillary length, $L$. Surface tension is significant whenever inclusions are smaller than $100L$. While Eshelby theory predicts that liquid inclusions generically reduce the stiffness of an elastic solid, our results show that liquid inclusions can actually stiffen a solid when $R<3L/2$. Intriguingly, surface tension cloaks the far-field signature of liquid inclusions when $R=3L/2$. These results are have far-reaching applications from measuring local stresses in biological tissue, to determining the failure strength of soft composites.