Dynamic and rate-dependent yielding in model cohesive suspensions

TL;DR

This paper investigates how the yield behavior of a model cohesive suspension varies with shear rate, revealing rate-dependent yield stress and non-monotonic flow curves, and proposes a generalized Herschel-Bulkley model to account for this dependence.

Contribution

It demonstrates that rate-dependent yield stress is significant in cohesive suspensions and suggests a generalized Herschel-Bulkley model to incorporate this effect.

Findings

Yield stress depends on shear rate and Péclet number.

Flow curves show hysteresis, shear banding, and irreproducibility.

Rate-dependent yield may be common in cohesive suspensions.

Abstract

An experimental system has been found recently, a coagulated CaCO3 suspension system, which shows very variable yield behaviour depending upon how it is tested and, specifically, at what rate it is sheared. At P\'eclet numbers Pe > 1 it behaves as a simple Herschel Bulkley liquid, whereas at Pe < 1 highly non-monotonic flow curves are seen. In controlled stress testing it shows hysteresis and shear banding and in the usual type of stress scan, used to measure flow curves in controlled stress mode routinely, it can show very erratic and irreproducible behaviour. All of these features will be attributed here to a dependence of the solid phase, or, yield stress, on the prevailing rate of shear at the yield point. Stress growth curves obtained from step strain-rate testing showed that this rate-dependence was a consequence of P\'eclet number dependent strain softening. At very low Pe, yield was cooperative and the yield strain was order-one, whereas as Pe approached unity, the yield strain reduced to that needed to break interparticle bonds, causing the yield stress to be greatly reduced. It is suspected that rate-dependent yield could well be the rule rather than the exception for cohesive suspensions more generally. If so, then the Herschel-Bulkley equation can usefully be generalized to read (in simple shear). The proposition that rate-dependent yield might be general for cohesive suspensions is amenable to critical experimental testing by a range of means and along lines suggested.