For many years, for the sake of simplicity, physicists studying granular materials have limited themselves to spherical grains. However, in everyday life, granular materials can be highly anisotropic, with some even having a very elongated shape (such as basmati rice, logs of wood, certain medications…). In this seminar, I will present recent results on the shear behavior of granular materials composed of elongated particles (Figure a). For this purpose, a shear cell with imposed vertical pressure an lateral confinement is used (Figure b). These studies are part of a broader effort to investigate the specificities of granular materials composed of anisotropic particles (for example, grain orientation) and to determine which laws derived for spherical grains remain valid for anisotropic grains. In the first part, I will focus on the kinematics of the particles, particularly their rotation. I will show that the average orientation of the particles, which naturally has no equivalent for spherical particles, does not correspond to the main flow direction. A comparison will be made with Jeffery's model, which describes the orientation of an ellipse in a sheared fluid. The second part will be dedicated to the friction between the medium and the lateral walls of the shear cell. Using numerical simulations, we find that the mobilization of wall friction is well described by a scaling law based on a balance between the sliding and rolling movements of the grains (Figure c). This scaling law is very robust, with only a slight dependency on the elongation of the grains, and remarkably, it is also valid in another geometry where the flow is induced by gravity.
Accès Salle des séminaires FAST-LPTMS (Bât. 530, salle C.120, 1er)