The nonlinear elastic deformation of liquid inclusions embedded in elastomers

Elastomers filled with liquid inclusions — as opposed to conventional solid fillers — are a recent trend in the soft matter community because of their unique range of mechanical and physical properties. Such properties stem, in part, from the very large deformations that the underlying liquid inclusions are capable of undergoing. With the objective of advancing the understanding of the mechanics of this emerging class of materials, this paper presents a combined experimental/theoretical study of the nonlinear elastic deformation of initially spherical liquid inclusions embedded in elastomers that are subjected to quasistatic mechanical loads. The focus is on two fundamental problems, both within the limit regime when elasto-capillarity effects are negligible: (i) the problem of an isolated inclusion and (ii) that of a pair of closely interacting inclusions. Experimentally, specimens made of a polydimethylsiloxane (PDMS) elastomer filled with either isolated or pairs of initially spherical liquid glycerol inclusions are subjected to uniaxial tension. For the specimens with pairs of inclusions, three orientations of the two inclusions with respect to the direction of the applied macroscopic tensile load are considered, 0°, 45°, and 90°. The liquid glycerol is stained with a fluorescent dye that permits to measure the local deformation of the inclusions in situ via confocal laser scanning fluorescent microscopy. Theoretically, a recently developed framework — wherein the elastomer is considered to be a nonlinear elastic solid, the liquid comprising the inclusions is considered to be a nonlinear elastic fluid, and the interfaces separating the elastomer from the liquid inclusions can feature their own nonlinear elastic behavior (e.g., surface tension) — is utilized to carry out full-field simulations of the experiments. Inter alia, the results show that the deformation of liquid inclusions is significantly non-uniform and strongly influenced by the presence of other liquid inclusions around them. Interestingly, they also show that the large compressive stretches that localize at the poles of the inclusions may result in the development of creases.