Structural and dynamical behavior of a vibrated granular system of hard-cubes

Abstract

We study experimentally and by molecular dynamics simulations a quasi-two-dimensional granular system of cubic particles, confined in a squared box and kept in a dynamic steady state via vertical oscillations. First, we compared the dynamics of a single spherical bead and a cube. Remarkably, the trajectory of the cube shows characteristics of Brownian motion induced by the particle geometry, i.e. an erratic path generated by the collisions of the vertexes and edges of the cube with the vibrated plate; in contrast to the more predicted ballistic motion of a spherical bead. Upon the addition of more cubic particles to the system, the erratic motion is enhanced by multiple collisions, leading to a rich collective behavior. As the number of particles is increased, for instance, both the radial distribution function and the mean square displacement exhibit the qualitative features of the gas, liquid and crystalline-solid phases observed in thermally driven colloidal systems. The results obtained with experiments and simulations are in good agreement and, combined, suggest that a vibrated system of granular cubic particles provides a macroscopic ideal model of Brownian motion.