The effect of premature wall yield on creep testing of strongly flocculated suspensions

Abstract

Measuring yielding in cohesive suspensions is often hampered by slip at measurement surfaces. This paper presents creep data for strongly-flocculated suspensions obtained using vane-in-cup tools with differing cup-to-vane diameter ratios. The three suspensions were titania and alumina aggregated at their isoelectric points and polymer-flocculated alumina.The aim was to find the diameter ratio where slip or premature yielding at the cup wall had no effect on the transient behaviour. The large diameter ratio results showed readily understandable material behaviour comprising linear viscoelasticity at low stresses, strainsoftening close to yielding, time-dependent yield across a range of stresses and then viscous flow. Tests in small ratio geometries however showed more complex responses. Effects attributed to the cup wall included delayed softening, slip, multiple yielding and stick-slip events, and unsteady flow. The conclusion was that cups have to be relatively large to eliminate wall artefacts. A diameter ratio of three was sufficient in practice, although the minimum ratio must be material dependent.

On basic equations and kinematic-wave theory of separation processes in suspensions with gravity, centrifugal and Coriolis forces

“…A quantitative description of the motion of a sediment at a wall is quite difficult [71][72][73][74][75][76], and there are still many open questions. The following problems may be of relevance, but are beyond the scope of the present investigation: resuspension of particles [77,78] and related processes [79]; shear-induced migration of particles in concentrated suspensions [73][74][75]; not a sharp interface [80] and shear-induced corrugation of interfaces [81]; wall slip (depending on surface properties and particle material) [82][83][84][85][86][87][88][89]; stick-slip events [90]. For the present investigation, it is only of importance how the motion of the sediment affects the bulk flow, as this is what controls the settling time and the concentration in the suspension during the settling process.…”

Determining the significance of flotation variables on froth rheology using a central composite rotatable design

“…The authors have reported that two different equations for converting vane speed to shear rate need to be used depending on the existing scenarios [13]. Stickland and co-workers at the University of Melbourne [24] concluded that a diameter ratio of three of the tube to the vane was sufficient in practice to eliminate the wall effect.…”

Filtration Process Optimization: Rheological Behavior of a Filter Cake at Different Moisture Contents

“…All measurements were carried out at 25°C by varying the shear rate from 1 s -1 to 200 s -1 logarithmically during 150 s. The usual recommendation for a vane-in-cup ratio to neglect wall effect is 3:1 [16]. The vane-in-cup used in the experiments had a ratio of 1.3:1.…”