The Breakdown of Kinetic Theory in Granular Shear Flows
  We examine two basic assumptions of kinetic theory-- binary collisions and molecular chaos-- using numerical simulations of sheared granular materials. We investigate a wide range of densities and restitution coefficients and demonstrate that kinetic theory breaks down at large density and small restitution coefficients. In the regimes where kinetic theory fails, there is an associated emergence of clusters of spatially correlated grains. 
Gregg Lois, Anael Lemaitre, Jean M. Carlson