Granular matter

Rice, sand, and others: we call granular matter any collection of solid particles with sizes larger than one micron. This has a precise meaning: submicronic particles have Brownian motion. Larger particles have none: granular matter is essentially at zero temperature. But this does not imply that the sample is in a unique ground state! A heap of grains can achieve many shapes: most of them being metastable. (For instance, on a flat support, the only stable states correspond to a monolayer—no stacking—where the gravitional energy is minimized).