A Study of the Phenomenon of Rheological Dilatancy in an Aqueous Pigment Suspension

Abstract

The objective of this study was to gain a better understanding of the mechanisms responsible for rheological dilatancy in an aqueous pigment suspension. Of particular interest was the relationship between the extent of dilatancy and colloidal stability. This relationship was studied using iron oxide suspensions which were dispersed by addition of sodium polymethacrylate having a molecular weight of 4000. The level of colloidal stability was varied by changing the degree of dissociation of the carboxyl groups on the polymer. Measurements of the extent of adsorption of the dispersant were made and the adsorbed charge was found to correlate very well with low-shear viscosity measurements. Additional experiments were made to determine the effect of pigment volume concentration on dilatancy over the range of 40–47%. A cup-and-bob viscometer was used to measure the shear stress over a shear rate range of 100–4000 sec−1. The extent of rheological dilatancy was found to increase with (a) decreasing colloidal stability, and (b) increasing pigment concentration. Based on these findings, it was postulated that rheological dilatancy results from a progressive increase in flocculation due to shear. The fact that a suspension can exhibit both pseudoplasticity and dilatancy was explained by considering floc formation and floc disruption as competitive rate processes.