Dynamics of two-particle granular collisions on a surface
  We experimentally examine the dynamics of two-particle collisions occuring on a surface. We find that in two-particle collisions a standard coefficient of restitution model may not capture crucial dynamics of this system. Instead, for a typical collision, the particles involved slide relative to the substrate for a substantial time following the collision; during this time they experience very high frictional forces. The frictional forces lead to energy losses that are larger than the losses due to particle inelasticity. In addition, momentum can be transfered to the substrate, so that the momentum of the two particles is not necessarily conserved. Finally, we measure the angular momenta of particles immediately following the collision, and find that angular momentum can be lost to the substrate following the collision as well. 
Benjamin Painter and R. P. Behringer