Granular materials (sand, powders, etc.,) behave quite differently from traditional continuous medium such as elastic solids and simple liquids. This is since granular materials are by nature out of equilibrium systems because of the inelastic collisions and nonlinear friction between the particles. Despite the great need to have an accurate theoretical description of granular materials for engineering and geotechnical applications, there exist so far only empirical macroscopic descriptions of these materials. Therefore it is of fundamental importance to develop a theory for granular materials starting from its microscopic dynamics and structure.

In the present talk, I will first talk about the recent work about microscale relaxation dynamics of hard granular ellipsoids subject to an oscillatory shear, which demonstrate that the relaxation dynamics of such "frictional" systems is qualitatively different from that of "thermal" systems and that granular materials are "marginal solids", i.e. stable if unperturbed but fluid under the slightest external perturbation.