Phenomenological approach of the effective viscosity of hard. sphere suspensions in shear-thinning media

In this work we investigate the rheological behaviour of macroscopic buoyant hard spheres dispersed in a shear-thinning suspending fluid. We focus on the phenomenological study of the influence of the shear-thinning behaviour of the suspending medium on the effective apparent suspension viscosity at different volume fractions. In the oil industry, the effective viscosity concept is widely used and very useful to quickly characterize a change of viscosity due to an increase of the solid content. Viscosity measurements are compared to the effective viscosity of a suspension of hard spheres in an Ostwald fluid. The power law index of the suspending fluid is shown, both experimentally and theoretically, to influence strongly the volume fraction dependence of the suspension effective viscosity. All experimental results are shown to be quite correctly plotted on a master curve, with only one adjustable parameter, the maximum packing fraction phi(m). The best fit is obtained for phi(m) = 0.57, corresponding to the theoretical maximum random packing volume fraction.