In this talk, I will review some approaches to granular dynamics, now well known to consist of both fast and slow relaxational processes. In the first case, grains typically compete with each other, while in the second, they cooperate. A typical result of cooperation is the formation of stable bridges, signatures of spatiotemporal inhomogeneities; their geometrical characteristics are explored and some theoretical results compared with those of independent simulations and experiments.

An arena where the effects of cooperation versus competition are felt most keenly is granular compaction; I will discuss a random graph model, where three-spin interactions are used to model compaction under tapping. The compaction curve shows distinct regions where slow’ dynamics apply, separated by what we have called the single-particle relaxation threshold. Finally, I will discuss the effect of shape – jagged vs.~regular – on the compaction of packings near their jamming limit. One of our major results is an entropic landscape that, while microscopically rough, manifests Edwards’ flatness at a macroscopic level. Another major result is that of surface intermittency under low-intensity shaking, which has been observed experimentally.

Bose National Centre for Basic Sciences