Ordering of a granular layer of cubes under strain-induced shear and vibration

We experimentally studied the ordering process of a slightly tilted two-dimensional system of millimeter-sized cubes, by annealing the system using vibration and mechanical deformations. When the system is only under vibration, at a fixed amplitude and frequency, a transition from a disordered particle configuration to a more ordered one with square clusters aligned with the walls of the container is observed; however, the formation of some grains differently oriented prevents a complete system ordering. This configuration is very stable and no further increase of order occurs. When the system is submitted additionally to cyclic shearing by deforming the boundaries (a rhomboidal frame), a transition towards a complete square crystal configuration rapidly occurs. The larger the strain, the shorter the annealing time, although with lower stability. We characterize the structural features of the system through the radial distribution function and fourth and sixth-bond-orientational order parameters. Our results reveal an efficient and simple mechanism to optimize the packing of particles of cubic geometry, with potential applications in material storage, transport, and in industrial processes.