Shear banding and flow-concentration coupling in colloidal glasses

TL;DR

This paper uncovers shear banding in colloidal glasses caused by shear-concentration coupling, showing localized flows and velocity profile non-linearity at low shear rates, supported by experimental observations and a simple theoretical model.

Contribution

It demonstrates shear banding in colloidal glasses due to shear-concentration coupling, a mechanism previously considered insignificant in such materials.

Findings

Shear banding observed below a critical shear rate

Localized flow correlates with concentration gradients

Model explains flow fluctuations and shear rate dependence

Abstract

We report experiments on hard sphere colloidal glasses that reveal a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability arising from shear-concentration coupling, a mechanism previously thought unimportant in this class of materials. Below a characteristic shear rate $\dot\gamma_c$ we observe increasingly non-linear velocity profiles and strongly localized flows. We attribute this trend to very slight concentration gradients (likely to evade direct detection) arising in the unstable flow regime. A simple model accounts for both the observed increase of $\dot\gamma_c$ with concentration, and the fluctuations observed in the flow.