Denis Dumont
CEA, SPEC (Paris-Saclay)

Dense Granular Flow: A Fluid of Hard Spheres ?

Granular materials such as sands, gravels, cereals, or pellets are involved in a wide range of applications such as industries, geophysical events, and even biology. Since the dynamic of a grain is fully dictated by Newtown mechanics and Coulomb friction laws, a naive first look at granular media could suggest that those systems are easy to explain and known for centuries. Nevertheless, an emergent and extreme complexity shows up when considering the collective behavior of a large assembly of grains. This leads to the observation of surprising and counter-intuitive behaviors such as the well-known Jenssen and Brazil nut effects. Despite decades of research, the theoretical understanding of the rheological properties remains largely elusive, especially at the grain scale. Besides, most theoretical models are semi-empirical and show strong shortcomings. Very recently, we tackled the case of dense granular layers flowing down inclined planes. Macroscopic and microscopic properties are obtained from Discrete Element Method simulations for both frictional and frictionless grains. We built a toy-model based on an analogy with the statistical mechanics of a thermal fluid of hard spheres. This theoretical approach properly rationalizes the observations and eventually provides insight into the microscopic foundation of a very popular semi-empirical model, the mu(I)-rheology.