Triggers and signatures of shear banding in steady and time-dependent flows

TL;DR

This paper provides a practical guide to identifying shear banding in complex fluids across various flow protocols, emphasizing the importance of time-dependent effects and cautioning against assuming homogeneous flow in experiments.

Contribution

It systematically summarizes signatures of shear banding in different flow protocols and highlights the prevalence of banding in time-dependent flows even without steady-state banding.

Findings

Shear banding signatures vary across flow protocols.

Time-dependent flows can induce banding in fluids that don't shear band steadily.

Caution is advised when interpreting experimental data assuming homogeneous flow.

Abstract

This precis is aimed as a practical field-guide to situations in which shear banding might be expected in complex fluids subject to an applied shear flow. Separately for several of the most common flow protocols, it summarises the characteristic signatures in the measured bulk rheological signals that suggest the presence of banding in the underlying flow field. It does so both for a steady applied shear flow, and for the time-dependent protocols of shear startup, step stress, finite strain ramp, and large amplitude oscillatory shear. An important message is that banding might arise rather widely in flows with a strong enough time-dependence, even in fluids that do not support banding in a steadily applied shear flow. This suggests caution in comparing experimental data with theoretical calculations that assume a homogeneous shear flow. In a brief postlude, we also summarise criteria in similar spirit for the onset of necking in extensional filament stretching.