Cohesion and aggregates in unsaturated wet granular flows down a rough incline

Multi-phase flows, encountered in nature or in industry, exhibit non-trivial rheological properties, which we attempt to better understand thanks to model materials and appropriate rheometers. Unsaturated wet granular flows down a rough inclined plane turn out to be steady and uniform for a wide range of parameters, despite the cohesion and the grain aggregates. The cohesive Mohr-Coulomb yield criterion extended to inertial granular flows, with a cohesion stress dependent on the liquid content and an internal friction coefficient dependent on the inertial number, allows for predictions in good agreement with our experimental measurements, when one introduces a grain aggregate size, which defines the appropriate length and relaxation time scales in the inertial number. We found that the grain aggregate size depends not monotonically on the liquid content and does not scale with the cohesion length induced by the cohesion stress, due to the non-trivial distribution of the liquid within the granular material.