Rheological behavior of concentrated calcium halophosphate suspensions

Abstract

Suspensions of electrostatically stabilized calcium halophosphate particles of four unimodal sizes were studied under steady shear Couette flow conditions. The particle size ranged from 8.4 to 16.8 μm. The suspensions were prepared in an aqueous polyethylene oxide solution containing an ammonium neutralized polyacrylic acid dispersant. Shear thinning, followed by shear thickening at higher shear rates, was observed for suspensions containing either the individual unimodal fractions or their blends. The polyelectrolyte dispersant is a necessary component for shear thickening and also for reduction in viscosities below the high shear limit. At a constant total volume fraction of 0.38, the dependence of viscosity on shear rate changed with blend composition. In the shear thinning region at low shear rates, the viscosity increased linearly with decreasing median particle diameter, with no observable polydispersity effect over the range of size distribution investigated by blending the halophosphate fractions. At shear rates within the shear thickening transition, a broader size distribution significantly reduces the viscosity and delays the onset of shear thickening, even for blends containing size ratios as low as 1.64:1. A scaling analysis of the relative importance of forces acting on the particles in the shear thinning and shear thickening regimes is presented.