Unsteady flow and particle migration in dense, non-Brownian suspensions

TL;DR

This study investigates unsteady flow behaviors and particle migration in dense non-Brownian suspensions, revealing unsteady shear bands in the discontinuous shear thickening regime and supporting stress-dependent friction theories.

Contribution

It provides experimental evidence of unsteady flow and particle migration in dense suspensions, challenging steady-state shear band assumptions in DST regimes.

Findings

Shear thickening involves unsteady flow in DST regime.

Steady-state shear bands are ruled out due to pressure and stress balance constraints.

Long-lived transient shear bands can exist in dense suspensions.

Abstract

We present experimental results on dense corn-starch suspensions as examples of non-Brownian, nearly-hard particles that undergo continuous and discontinuous shear thickening (CST and DST) at intermediate and high densities respectively. Our results offer strong support for recent theories involving a stress-dependent effective contact friction among particles. We show however that in the DST regime, where theory might lead one to expect steady-state shear bands oriented layerwise along the vorticity axis, the real flow is unsteady. To explain this, we argue that steady-state banding is generically ruled out by the requirement that, for hard non-Brownian particles, the solvent pressure and the normal-normal component of the particle stress must balance separately across the interface between bands. (Otherwise there is an unbalanced migration flux.) However, long-lived transient shear-bands remain possible.