Transient Shear Banding in a Simple Yield Stress Fluid

TL;DR

This study demonstrates that simple, non-aging yield stress fluids exhibit transient shear banding phenomena before reaching a homogeneous flow, with the duration of banding decreasing as a power law with shear rate.

Contribution

It provides experimental evidence of transient shear banding in simple yield stress fluids without aging or thixotropy, highlighting a power-law relationship with shear rate.

Findings

Transient shear banding occurs before steady homogeneous flow.

Duration of shear banding decreases as a power law with shear rate.

Heterogeneous flows can persist for up to 10^5 seconds at low shear rates.

Abstract

We report a large set of experimental data which demonstrates that a simple yield stress fluid, i.e. which does not present aging or thixotropy, exhibits transient shear banding before reaching a steady state characterized by a homogeneous, linear velocity profile. The duration of the transient regime decreases as a power law with the applied shear rate $\dot\gamma$. This power law behavior, observed here in carbopol dispersions, does not depend on the gap width and on the boundary conditions for a given sample preparation. For $\dot\gamma\lesssim 0.1$ s$^{-1}$, heterogeneous flows could be observed for as long as 10$^5$ s. These local dynamics account for the ultraslow stress relaxation observed at low shear rates.