Interplay between wall slip and shear banding in a thixotropic yield stress fluid

TL;DR

This study reveals a novel shear-banding behavior in thixotropic yield stress fluids, showing wall slip dependence on shear rate and proposing a differential lever rule model to describe the flow profile and wall slip phenomena.

Contribution

It introduces an original shear-banding scenario where flow profile is governed by shear rate rather than critical shear rate, incorporating wall slip as a key factor.

Findings

Flow profile set by applied shear rate, not critical shear rate

Wall slip velocity depends non-trivially on shear rate

Differential lever rule effectively models observed behavior

Abstract

We study the local dynamics of a thixotropic yield stress fluid that shows a pronounced non-monotonic flow curve. This mechanically unstable behavior is generally not observable from standard rheometry tests, resulting in a stress plateau that stems from the coexistence of a flowing band with an unyielded region below a critical shear rate $\dot \gamma_c$. Combining ultrasound velocimetry with standard rheometry, we discover an original shear-banding scenario in the decreasing branch of the flow curve of model paraffin gels, in which the flow profile of the flowing band is set by the applied shear rate $\gd$ instead of $\dot \gamma_c$. As a consequence, the material slips at the walls with a velocity that shows a non-trivial dependence on the applied shear rate. To capture our observations, we propose a differential version of the so-called lever rule, describing the extent of the flowing band and the evolution of wall slip with shear rate. This phenomenological model holds down to very low shear rates, at which the dimension of the flowing band becomes comparable to the size of the wax particles that constitute the gel microstructure, leading to cooperative effects. Our approach provides a framework where constraints imposed in the classical shear-banding scenario can be relaxed, with wall slip acting as an additional degree of freedom.