The non-monotonic shear-thinning flow of two strongly cohesive concentrated suspensions

Abstract

The behaviour in simple shear of two concentrated and strongly cohesive mineral suspensions showing highly non-monotonic flow curves is described. Two rheometric test modes were employed, controlled stress and controlled shear-rate. In controlled stress mode the materials showed runaway flow above a yield stress, which, for one of the suspensions, varied substantially in value and seemingly at random from one run to the next, such that the up flow-curve appeared to be quite irreproducible. The down-curve was not though, as neither was the curve obtained in controlled rate mode, which turned out to be triple-valued in the region where runaway flow was seen in controlled rising stress. For this first suspension, the total stress could be decomposed into three parts to a good approximation: a viscous component proportional to a plastic viscosity, a constant isostatic contribution, and a third shear-rate dependent contribution associated with the particulate network which decreased with increasing shear-rate raised to the -7/10th power. In the case of the second suspension, the stress could be decomposed along similar lines, although the strain-rate softening of the solid-phase stress was found to be logarithmic and the irreducible isostatic stress was small. The flow curves are discussed in the light of 2 recent simulations and they conform to a very simple but general rule for nonmonotonic behaviour in cohesive suspensions and emulsions, namely that it is caused by strain-rate softening of the solid phase stress.

Dynamic and rate-dependent yielding in model cohesive suspensions

“…CaCO3 [1] φ=0.4 Peak stress on flow start--up @ constant shear--rate. 1 0.5 -1 (rate--dep t ) Strain sweep or staircase.…”

“…0.56 0.26 -0.36 Extrapolation from flow curve. 0.13 0 In [1] it was suggested that modifying or extending the Herschel-Bulkley equation [3] thus could account for the flow curves obtained by controlled rate testing,…”

“…An earlier paper [1] described the shear flow of two strongly cohesive suspensions showing highly non-monotonic flow curves, one of which was a 40% v/v suspension of 4.5 µm CaCO 3 in water, coagulated by having the pH its 'natural' pH close to the iso-electric point. Here the rheology of the CaCO 3 system is examined in more detail with an emphasis on transient behaviour and how it controls the steady state.…”

“…The way in which the original 40%v/v CaCO 3 system presented itself as a yield stress liquid was found to depend upon how it was caused to flow [1], as is summarised in table 1 below. With regard to the table, please note that 'CR' denotes 'controlled rate', that 'CS' means 'controlled stress' and that Pe 0 is the so-called 'bare' Péclet number 6πa 3 µ !…”

Flow and evolution of ice-sucrose crystal mushes

“…At higher volume fractions (the subject of this paper), there is no universal theory of attractive particle rheology and flow properties depend on the details of the interparticle interactions. Strongly cohesive suspensions can display steep shear thinning and non-monotonic flow curves [37,38]. When making rheological measurements on such systems, pseudo-plasticity introduces problems in simple geometries.…”