Towards a Unified Description of the Rheology of Hard-Particle Suspensions

The rheology of suspensions of Brownian, or colloidal, particles (diameter d less than or similar to 1 mu m) differs markedly from that of larger grains (d greater than or similar to 50 mu m). Each of these two regimes has been separately studied, but the flow of suspensions with intermediate particle sizes (1 mu m less than or similar to d less than or similar to 50 mu m), which occur ubiquitously in applications, remains poorly understood. By measuring the rheology of suspensions of hard spheres with a wide range of sizes, we show experimentally that shear thickening drives the transition from colloidal to granular flow across the intermediate size regime. This insight makes possible a unified description of the (noninertial) rheology of hard spheres over the full size spectrum. Moreover, we are able to test a new theory of friction-induced shear thickening, showing that our data can be well fitted using expressions derived from it.