Competition and cooperation: aspects of dynamics in sandpiles

In this paper, we review some of our approaches to granular dynamics, now well known (Mehta 1994 Granular Matter: an Interdisciplinary Approach ed A Mehta (New York: Springer)) 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; we review their geometrical characteristics and compare theoretical results with those of independent simulations. Cooperative excitations due to local density fluctuations are also responsible for relaxation at the angle of repose; the competition between these fluctuations and external driving forces can, on the other hand, result in a (rare) collapse of the sandpile to the horizontal. Both these features are present in a theory reviewed here. An arena where the effects of cooperation versus competition are felt most keenly is granular compaction; we review here a random graph model, where three-spin interactions are used to model compaction under tapping. The compaction curve shows distinct regions where ‘fast’ and ‘slow’ dynamics apply, separated by what we have called the single-particle relaxation threshold. In the final section of this paper, we explore the effect of shape—jagged versus 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.