Topological rearrangements and stress fluctuations in quasi-two-dimensional hopper flow of emulsions

TL;DR

This study investigates how microscopic droplet rearrangements in a quasi-2D emulsion flow influence stress fluctuations and flow dynamics, revealing local structural changes' role in stress redistribution.

Contribution

It provides experimental evidence linking T1 rearrangement events to stress fluctuations and demonstrates the local impact of these events on inter-droplet forces.

Findings

T1 events correlate with significant stress drops.

Stress decreases propagate up to 3 droplet diameters from rearrangements.

Microscopic rearrangements influence macroscopic stress fluctuations.

Abstract

We experimentally study the shear flow of oil-in-water emulsion droplets in a thin sample chamber with a hopper shape. In this thin chamber, the droplets are quasi-2D in shape. The sample is at an area fraction above jamming and forced to flow with a constant flux rate. Stresses applied to a droplet from its neighbors deform the droplet outline, and this deformation is quantified to provide an ad hoc measure of the stress. As the sample flows through the hopper we see large fluctuations of the stress, similar in character to what has been seen in other flows of complex fluids. Periods of time with large decreases in stress are correlated with bursts of elementary rearrangement events ("T1 events" where four droplets rearrange). More specifically, we see a local relationship between these observations: a T1 event decreases the inter-droplet forces up to 3 droplet diameters away from the event. This directly connects microscopic structural changes to macroscopic fluctuations, and confirms theoretical pictures of local rearrangements influencing nearby regions. These local rearrangements are an important means of reducing and redistributing stresses within a flowing material.