Studies of cluster formation due to collisions in granular material

We present experiments on the role of inelastic collisions in a two-dimensional model granular medium. Steel spheres roll on a smooth flat surface excited by a periodic impulsive motion of one of the enclosing walls. The particles are observed to be randomly distributed inside the enclosure at low densities. However, strong clustering near the side opposite the driving wall is observed as the particle density is increased. The phenomenon is studied statistically by measuring the center of mass location, and probability distributions for particle density as a function of particle number, rate of energy input, and distance from energy source. The role of an external field on the formation of clusters is studied by tilting the plane of the surface. Clustered domains persist but migrate downward. Velocities of individual particles are also measured. The clustered domains result in a narrowed velocity distribution, and the collision rate is enhanced within the clusters. A simulation in one-dimension also shows the migration of clusters as gravity is introduced.