Non-equilibrium Steady-State of Granular Fluids*

We experimentally investigate the non-equilibrium steady-state (NESS) structure of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. For the first time we experimentally show that the structure (as measured by radial distribution function, bond order parameter, and shape factor) of a NESS behaves identically to that of it's equilibrium counterpart. In our driven dissipative system there is a constant energy flow through the system making it far from equilibrium, yet it behaves as though it were in equilibrium with a maximization principle like entropy. It appears that homogeneity and stationarity are more important than energy conservation for producing a thermodynamic state. The existence of a thermodynamics for NESS has profound consequences for granular systems and more broadly for any systems, which are out of equilibrium due to energy flux, and may lead to a fundamental justification for a theory of gradients in a near-NESS analogous to Navier-Stokes equations for near-equilibrium systems.