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

The rheology of suspensions of Brownian, or colloidal, particles (diameter d≲1 μm) differs markedly from that of larger grains (d≳50 μm). Each of these two regimes has been separately studied, but the flow of suspensions with intermediate particle sizes (1 μm≲d≲50 μm), 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.