Granular Coarsening

The phenomenon of coarsening has been the topic of intensive study, chiefly in material science, where droplets coalesce1,2 or successive layers of material deposited on a substrate form complex morpholgies3,4. These problems are of technological interest because properties of composite materials such as polymer blends and alloys depend on the size and structure of included droplets5,6; likewise deposited layers are used in applications ranging from semiconductor design to catalysis7,8. In this letter we examine coarsening in a different context: granular flow. If a shallow bed of fine grains is shaken at moderate amplitude and in the presence of air, heaps develop that coarsen over time9,10 . We find experimentally that this coarsening scales as a power law over time with exponent 1/3, and we present a simplified model and direct analysis that scale with the same exponent.