On rheophysics of high-concentrated suspensions

Experiments with high-concentrated suspensions of solid particles show that that their viscosity increases by several times or even orders of magnitude, when the shear rate in the suspensions exceeds some critical value, which depends on the concentration and shape of particles and the physicochemical composition of carrier liquids. It is experimentally found that, within a certain range of shear rates, which is also predetermined by the composition of the suspensions, a steady flow becomes unstable, and regular or random oscillations arise in the flow with a period of several seconds. A theory is proposed for these phenomena, which is based on the fact that the increase in the viscosity with the shear rate is explained by the contact friction between particles, while the flow oscillations arise when the differential viscosity of a suspension acquires negative values.