Contact tribology also affects the slow flow behavior of granular emulsions

TL;DR

This study investigates how contact mechanics, especially friction, influence the slow flow behavior of granular emulsions, revealing that microscopic interactions significantly affect macroscopic rheology and flow stability.

Contribution

It introduces a systematic analysis of contact mechanics effects on dispersed media flow by synthesizing soft hydrogel particles with tunable friction coefficients.

Findings

Low friction particles exhibit emulsion-like rheology with unexpectedly high effective friction.

Increasing particle friction induces flow instability and alters flow fluctuations.

Microscopic contact properties critically influence the macroscopic flow behavior of granular emulsions.

Abstract

Recent work on suspension flows has shown that contact mechanics plays a role in suspension flow dynamics. The contact mechanics between particulate matter in dispersions should depend sensitively on the composition of the dispersed phase: evidently emulsion droplets interact differently with each other than angular sand particles. We therefore ask: what is the role of contact mechanics in dispersed media flow? We focus on slow flows, where contacts are long-lasting and hence contact mechanics effects should be most visible. To answer our question, we synthesize soft hydrogel particles with different friction coefficients. By making the particles soft, we can drive them at finite confining pressure at all driving rates. For particles with a low friction coefficient, we obtain a rheology similar to that of an emulsion, yet with an effective friction much larger than expected from their microscopic contact mechanics. Increasing the friction coefficient of the particles, we find a flow instability in the suspension. Particle level flow and fluctuations are also greatly affected by the microscopic friction coefficient of the suspended particles. The specific rheology of our "granular emulsions" provides further evidence that a better understanding of microscopic particle interactions is of broad relevance for dispersed media flows.