Non-Newtonian Behavior of ZnO-Water Suspensions

Abstract

The non-Newtonian behavior of ZnO-water suspensions stabilized with various amounts of sodium hexamethaphosphate was studied in a coaxial cylinder viscometer under conditions of constant shear rate and cyclic shearing. In both cases, the samples were sheared at a high shear rate for 5 minutes and then allowed to rest for 5 minutes prior to measurements. This was necessary because the transient behavior of these materials is heavily influenced by the previous history of shear rate and rest periods. The ζ-potential was also measured in order to discuss the rheological behavior on the basis of the theory of colloidal stability.
In the constant shear rate tests, rheopectic behavior was observed; the apparent viscosity in-creased with time and reached an equilibrium value. In the cyclic shearing tests, an irreversible hysteresis was observed. The up-curve showed first Newtonian and then pseudo-plastic flow with increasing shear rate; however, above a certain shear rate, dilatant behavior was observed. The down-curve showed pseudo-plastic flow, giving shear stresses higher than those of the up-curve.
The mechanisms of the rheopexy and dilatancy observed in this study were attributed to aggregation of the particles by shear, causing them to pass over a potential energy barrier into a primary minimum in the potential energy curve.