Flow mechanisms and rheology of dense aqueous suspensions of titanium(IV) oxide

Rheograms obtained from testing concentrated suspensions of titanium(IV) oxide in water indicated that, depending on temperature and concentration, Newtonian, shear thickening, shear thinning or mixed behaviour could occur. Tests at constant shear rate and increasing temperature indicated that different flow mechanisms were dominant depending on temperature, shear rate and composition. Phenomenologically, the temperature ramp results could be interpreted in terms of three flow types, namely, fluid (Newtonian) flow, plastic flow and an intermediate power law flow, a description which in many respects is similar to that used for deformation in crystalline materials. Power law and plastic flow could be combined into a single hybrid equation which, together with an expression for Newtonian flow, gave a reasonably good description of behaviour both from a qualitative and quantitative point of view. Structure parameters appearing in the model depended only on solids concentration and not on the externally controlled variables of temperature and strain rate, suggesting that perhaps much of the apparent thinning or thickening behaviour is an artefact of changes in flow mechanism.