Wet granular flows over a rough incline: frictional and cohesive rheology

Multi-phase flows encountered in nature or industry (such as landslides, mudflows, powder mixtures, ...) exhibit non trivial rheological properties, that can be understood better thanks to model materials and appropriate rheometers. Here, we use unsaturated wet granular materials: assemblies of frictional spherical particles bonded by a small quantity of a wetting liquid, over a rough inclined plane. This later is relevant for free-surface flows of particulate solids, because of applications and  theoretical concerns about constitutive modelling, as regimes of steady uniform flows lead to effective friction controlled experiments.

Our results show steady uniform flows for a wide range of parameters (the inclination angle and the mass flow-rate).  

A theoretical model, based on the Mohr-Coulomb yield criterion, is extended to inertial flows: τ = τ_c + μ(I) P,  in which τ_c and μ(I) are the cohesion stress and the internal friction coefficient respectively. The cohesion stress induces the emergence of a height threshold for shear and leads to localization of shear in the bottom layer with a non-sheared top layer (plug flow). 

Predictions are in quantitative agreement with experimental measurements only when one considers that dry and wet samples have straightforwardly different internal friction coefficients commonly described by the so-called μ(I)-rheology.

The liquid bridges bounding grains not only induce cohesion, but modify the internal friction of the wet assemblies, by enhancing it.