If the system is free from external forces, granular gases exhibit clustering instabilities. However, granular gases flow under driving forces, where the external supply of energy is balanced with the energy dissipation and the system reaches a non-equilibrium steady state. Then, a new class of hydrodynamic instabilitites can be observed.
If the system is moderately dense, but is still below the jamming point, the granular material behaves like a liquid, where complex or glassy dynamics of the constituent particles cause the anomalous rheology of granular materials.
Anomalous mechanical responses of jammed granular materials originate from complicated restructuring of force-chain networks. Because such the microscopic changes are stochastic rather than deterministic, a master equation for the probability distribution functions of contact forces well describes the macroscopic behavior.
The macroscopic behavior of granular materials strongly depends on the microscopic contact forces, where an atomic-scale study is needed to understand the origin of contact dynamics, e.g. inelastic collisions.