Granular flows give rise to a variety of phenomena that have both fluid and solid aspects. Numerical and theoretical studies on particles that are allowed to "cool" (lose kinetic energy) due to inelastic collisions have predicted striking phenomena of clustering and "inelastic collapse" (infinite numbers of collisions in finite time). We have studied the role of inelasitic collisions experimentally in a two dimensional system of spherical steel particles rolling on a smooth surface and driven by a moving wall.
The number of particles can be varied; at low densities (see Fig. (a))the particles are distributed uniformly; as the particle density is increased (Fig. (b)); pronounced clustering occurs. The yellow dots correspond to the particles, and the wall providing energy is at the bottom of the photo. The particle diameter is 3.2 mm.
Velocity profiles are also extracted by taking images closely spaced in time and measuring the particle displacements. As the particle density is increased, the mean velocity decreases, and the distribution of velocities becomes narrow.
The clusters migrate downwards in the presence of a slight tilt (0.22^o) but are not dispersed. A parametric instability is observed and arches are formed at high particle density.
Java Applet of inelastic collisions and collapse
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