Colloidal suspensions down an incline plane: at the crossroad between granular and glassy rheology.

Suspensions composed of large, respectively small, enough particles flow according to granular, respectively glassy, rheology. In practice, many colloidal suspensions sit right at the crossover between these two regimes, where a comprehensive description of the suspension rheology is still lacking.

 

Performing confocal observations of frictionless colloidal suspensions flowing down an incline plane, we obtain very different rheological behaviors across a small range of particle sizes. Combining our observations with existing numerical results, we show that the rheology of such suspensions is well described by an additive model for the stresses. An important output of this model is the non-trivial dependance of the critical effective friction coefficient on the confining pressure. There is a critical confining pressure, above which It jumps discontinuously from zero to a small but finite value, before increasing towards a limiting value different from the one expected for granular suspensions. Our results point at a size dependent crossover in shear rate, above which the quasi-static flows are governed by the granular rheology and below which the flow arrest is dictated by the ratio of the thermal yield stress to the static pressure at the glass transition.