Universal scaling law in frictional non-Brownian suspensions

We compare the rheology of two kinds of non-Brownian suspensions. One is made of spherical monodisperse polystyrene particles (diameter 80 μm) and the other is made of faceted sugar particles (100μm), both suspended in a Newtonian silicon oil. We perform shear reversal experiments on both suspensions for several particle volume fractions, Φ, and several shear stresses, τ. The two suspensions behave in a quite different fashion. For the faceted particle suspensions (FPS), a large shear-thinning is observed while it is much more moderate for the spherical polystyrene particle suspensions (SPS). Another striking difference lies in the value of the jamming packing fraction, Φm that is much lower for FPS than for SPS. Despite these differences, we will show that shear reversal experiments make it possible to obtain a universal scaling that holds for both FPS and SPS. In this scaling, the difference between the steady viscosity and the viscosity at the minimum that follows the shear reversal, normalized by the steady viscosity is shown to depend only on the ratio Φ/Φm(τ). The collapse of all the data suggests that concentrated non-Brownian suspensions behave in a universal way regardless of the mechanisms responsible for flow-hindering (rotation frustration or sliding friction).