Sedimentation and viscous flow of a weakly flocculated concentrated dispersion. A comarative study

Abstract

A comparison is made of the viscous-flow and sedimentation behaviour of weakly flocculated dispersions comprising polystyrene latex flocculated with sodium carboxymethylcellulose. The dispersions, which were shear-thinning in steady-shear flow, had zero-shear viscosities in the range 0.2–20 Pa s depending upon concentration. At volume fractions above 0.2 sedimentation was very slow under the influence of normal gravitational acceleration; the rate of sedimentation could, however, be much enhanced by means of a slow-speed centrifuge. The collective friction coefficient derived from measurements of the initial sedimentation rate over the range 1–20 g was found to show a dependence on acceleration rather similar to the dependence of the viscosity on shear stress. It was also found that the centrifuge curves could be mimicked using an ad hoc model which embodied the idea that the viscosity and friction coefficients had similar concentration dependences.