Rheology of dense granular suspensions: Particle shape matters

Failure and flow of granular materials are involved in many natural disasters like avalanches, landslides, magma flows, debris flow, etc, and also many industrial processes. Several peculiar phenomena such as shear rate dependence behaviour, yield stress ratio onset, normal stress difference, etc., distinguish them dramatically from classical Newtonian flows. These kinds of phenomena seem to be strongly ruled by the microscopic nature of inter-particle interaction forces and particle coupling with interstitial fluid. While hydrodynamic interactions or lubrication forces between the particles are important in the dilute regime, they become of lesser significance when the concentration is increased, and direct particle contacts become dominant in the rheological response. Despite that these materials  have been studied extensively, none of which is clearly understood so far.

Depending on boundary condition and stress distribution, dense granular flows can shows two different regimes : (i) rate-independent quasi-static flow, for moderate shear rate deformation, partially well described by continuous viscous-plastic models, inspired by soils mechanics, and (ii) rate-dependent behaviour, in which shear a normal stresses shows a power law function of the shear rate deformation in volume imposed rheology. In this frame of work, jamming transition is presented in a classical viscous approach as a shear viscosity divergence. In order to unifies these two regimes, a non-classical rheology is suggested, wherein normal pressure, P, is imposed on the sample instead of the volume. Thus, a non-classical-frictional approach is suggested, in which the effective friction coefficient, defined by the ratio of macroscopic shear and normal stresses, and the flowable global solid-volume fraction are mainly controlled by an empirical power-law function of the ratio of macroscopic shear rate deformation and the normal pressure (perpendicular to the gradient direction).

In the present seminar we will present a general scope of main concepts concerning granular flows, we will also present some experimental observations and the main impact on the pursuit of a physical model for heterogeneous flows.