Dynamics of two-particle granular collisions on a surface

We experimentally examine the dynamics of two-particle collisions occurring on a surface. We find that in two-particle collisions a standard coefficie nt of restitution model may not capture crucial dynamics of the system. Ins tead, 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 typically larger by a factor of 5-6 than the losses due to particle inelasticity. In addition, momentum can be transferred to the substrate, so that the momentum of the two particles is not necessarily conserved. Finally, we measure the angular momenta of particles immediatel y following the collision, and find that angular momentum can be lost to th e substrate following the collision as well.