Making soft solids flow: microscopic bursts and conga lines in jammed emulsions

TL;DR

This study combines experiments and simulations to reveal how microscopic droplet rearrangements in jammed emulsions depend on shear rate, showing coordinated clusters at high rates and large bursts at low rates, thus linking microscopic dynamics to macroscopic flow.

Contribution

It provides a microscopic framework explaining how droplet rearrangements govern flow in jammed soft solids, influenced by shear rate, with detailed 3D structural insights.

Findings

Fast shear induces conga line droplet clusters

Slow shear causes large correlated rearrangement bursts

Microscopic structures are shaped by shear rate

Abstract

It is well known that jammed soft materials will flow if sheared above their yield stress - think mayonnaise spread on bread - but a complete microscopic description of this seemingly sim- ple process has yet to emerge. What remains elusive is a microscopic framework that explains the macroscopic flow, derived from a 3-D spatially resolved analysis of the dynamics of the droplets or particles that compose the soft material. By combining confocal-rheology experiments on compressed emulsions and numerical simulations, we unravel that the primary microscopic mechanisms for flow are strongly influenced by the rate of the imposed deformation. When shearing fast, small coordinated clusters of droplets move collectively as in a conga line, while at low rates the flow emerges from bursts of droplet rearrangements, correlated over large domains. These regions exhibit complex spatio-temporal correlation patterns that reflect the long range elasticity embedded in the jammed material. These results identify the three-dimensional structure of microscopic rearrangements within sheared soft solids, revealing that the characteristic shape and dynamics of these structures are strongly determined by the rate of the external shear.