Dynamics of spherical particles on a surface: About collision induced   sliding and other effects
  We present a model for the motion of hard spherical particles on a two dimensional surface. The model includes both the interaction between the particles via collisions, as well as the interaction of the particles with the substrate. We analyze in details the effects of sliding and rolling friction, that are usually overlooked. It is found that the properties of this particulate system are influenced significantly by the substrate-particle interactions. In particular, sliding of the particles relative to the substrate after a collision leads to considerable energy loss for common experimental conditions. The presented results provide a basis, that can be used to realistically model the dynamical properties of the system, and provide further insight into density fluctuations and related phenomena of clustering, structure formations, and inelastic collapse. 
Ljubinko (Lou) Kondic