Randomly driven granular fluids: collisional statistics and short scale structure.

We present a molecular-dynamics and kinetic theory study of granular material, modeled by inelastic hard disks, fluidized by a random driving force. The focus is on collisional averages and short-distance correlations in the nonequilibrium steady state, in order to analyze in a quantitative manner the breakdown of molecular chaos, i.e.

Cahn-hilliard theory for unstable granular fluids.

A Cahn-Hilliard-type theory for hydrodynamic fluctuations is proposed that gives a quantitative description of the slowly evolving spatial correlations and structures in density and flow fields in the early stages of evolution of freely cooling granular fluids. Two mechanisms for pattern selection and structure formation are identified: unstable modes leading to density clustering (mechanismlike spinodal decomposition, or "uplifting" in structural geology), and selective noise reduction (mechanismlike peneplanation in structural geology) leading to vortex patterns. As time increases, the structure factor for the density field develops a maximum, which shifts to smaller wave numbers.