SHEAR HISTORY DEPENDENCE OF THE VISCOSITY OF AGGREGATED COLLOIDAL DISPERSIONS

The shear history dependence of the viscosity of a depletion flocculat ed dispersion of colloidal spheres was studied using two different rhe ometrical geometries. The observed rheological behavior is found to de pend on the geometry, due to effects of thixotropy and sedimentation. By comparing the results of a cone-plate and a Couette geometry, we we re able to obtain reliable data. The shear history dependence is explo red by measuring a flow curve before and after subjecting the aggregat ed dispersion to a constant shear rate for one hour.: The viscosity va lues of the flow curve after this hour turned out to be considerably l ower than the initial flow curve. The results were interpreted with a microrheological model for fractal aggregation in shear flow. The drop in viscosity is attributed to a shear induced compaction of the aggre gates. Combination of this model and the concept of compaction results in a satisfactory description of the experimental results. (C) 1996 S ociety of Rheology.