Dynamic and rate-dependent yielding in model cohesive suspensions

Abstract

An experimental system has been found recently, a set of coagulated CaCO3 suspensions, which shows very variable yield behaviour depending upon how it is tested and, specifically, at what rate it is sheared. At Péclet 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 controlled stress scan routinely used to measure flow curves, it can show very erratic and irreproducible behaviour. All of these features appear to arise from 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 rate-dependence was a consequence of Péclet 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 inter-particle 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 σ=σ0g(γ̇)+σiso+kγ̇n (in simple shear). The proposition that rate-dependent yield could be general for cohesive suspensions is amenable to critical experimental testing by a range of means and along lines suggested.