Stiffening solids with liquid inclusions

TL;DR

This paper demonstrates that small liquid inclusions can significantly stiffen soft solids, contradicting traditional theories, by incorporating surface tension effects into Eshelby's inclusion model.

Contribution

The study extends Eshelby's theory to include surface tension, explaining size-dependent stiffening effects of liquid droplets in soft solids.

Findings

Small liquid droplets increase the stiffness of soft solids.

Surface tension effects dominate when droplet size is below the elastocapillary length.

Experimental and theoretical results show size-dependent deformation behavior.

Abstract

From bone and wood to concrete and carbon fibre, composites are ubiquitous natural and engineering materials. Eshelby's inclusion theory describes how macroscopic stress fields couple to isolated microscopic inclusions, allowing prediction of a composite's bulk mechanical properties from a knowledge of its microstructure. It has been extended to describe a wide variety of phenomena from solid fracture to cell adhesion. Here, we show experimentally and theoretically that Eshelby's theory breaks down for small liquid inclusions in a soft solid. In this limit, an isolated droplet's deformation is strongly size-dependent with the smallest droplets mimicking the behaviour of solid inclusions. Furthermore, in opposition to the predictions of conventional composite theory, we find that finite concentrations of small liquid inclusions enhance the stiffness of soft solids. A straight-forward extension of Eshelby's theory, accounting for the surface tension of the solid-liquid interface, explains our experimental observations. The counterintuitive effect of liquid-stiffening of solids is expected whenever droplet radii are smaller than an elastocapillary length, given by the ratio of the surface tension to Young's modulus of the solid matrix.