  The terminology granular matter refers to systems with a large number of hard
objects (grains) of mesoscopic size ranging from millimeters to meters.
Geological examples include desert sand and the rocks of a landslide. But the
scope of such systems is much broader, including powders and snow, edible
products such a seeds and salt, medical products like pills, and
extraterrestrial systems such as the surface regolith of Mars and the rings of
Saturn. The importance of a fundamental understanding for granular matter
properties can hardly be overestimated. Practical issues of current concern
range from disaster mitigation of avalanches and explosions of grain silos to
immense economic consequences within the pharmaceutical industry. In addition,
they are of academic and conceptual importance as well as examples of systems
far from equilibrium. Under many conditions of interest, granular matter flows
like a normal fluid. In the latter case such flows are accurately described by
the equations of hydrodynamics. Attention is focused here on the possibility
for a corresponding hydrodynamic description of granular flows. The tools of
nonequilibrium statistical mechanics, developed over the past fifty years for
fluids composed of atoms and molecules, are applied here to a system of grains
for a fundamental approach to both qualitative questions and practical
quantitative predictions. The nonlinear Navier-Stokes equations and expressions
for the associated transport coefficients are obtained.
