Plastic flow of foams and emulsions in a channel

TL;DR

This study investigates the flow behavior of confined foams and emulsions, revealing correlations between plastic event localization, velocity profiles, and shear strain, supported by experiments and lattice-Boltzmann simulations.

Contribution

It provides new experimental and simulation data on plastic rearrangements in confined foams, highlighting their relation to flow profiles and shear strain, extending understanding of soft-glassy material behavior.

Findings

Plastic event localization correlates with velocity profile localization.

Distribution of plastic event orientations relates to local shear strain.

Experimental and simulation results support cooperativity mechanisms in foams.

Abstract

In order to understand the flow profiles of complex fluids, a crucial issue concerns the emergence of spatial correlations among plastic rearrangements exhibiting cooperativity flow behaviour at the macroscopic level. In this paper, the rate of plastic events in a Poiseuille flow is experimentally measured on a confined foam in a Hele-Shaw geometry. The correlation with independently measured velocity profiles is quantified. To go beyond a limitation of the experiments, namely the presence of wall friction which complicates the relation between shear stress and shear rate, we compare the experiments with simulations of emulsion droplets based on the lattice-Boltzmann method, which are performed both with, and without, wall friction. Our results indicate a correlation between the localisation length of the velocity profiles and the localisation length of the number of plastic events. Finally, unprecedented results on the distribution of the orientation of plastic events show that there is a non-trivial correlation with the underlying local shear strain. These features, not previously reported for a confined foam, lend further support to the idea that cooperativity mechanisms, originally invoked for concentrated emulsions (Goyon et al. 2008), have parallels in the behaviour of other soft-glassy materials.