Cohesion and aggregation in unsaturated wet granular flows over a rough inclined plane

Multi-phase flows encountered in nature or in industry, exhibit non trivial rheological properties, that can be understood better thanks to model materials and appropriate rheometers. Here, we use model unsaturated granular materials: frictional spherical particles bonded by a small quantityof a wetting liquid, flowing over a rough inclined plane. Our results show steady uniform flows for a wide range of parameters (the liquid content, the inclination angle and the mass flow rate), despite the existence of grain aggregates. We analyse our experimental results in the framework of 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. Such a rheology gives predictions in quantitative agreement with our experimental measurements, when one introduces a grain aggregate size, which defines the appropriate length scale in the inertial number. Interestingly we found that the grain aggregate size is a function of the liquid content, and this variation is not monotonic, in agreement with our predictions, basedon the force balance and using experimental measurements of the liquid distribution from the literature. It is important to note that this aggregate size does not scale with the cohesion length induced by the cohesion stress, due to the non trivial distribution of the liquid. This raises theopen question of the size of aggregates in other systems, in case of cohesion of another nature than capillary