Granular Casimir effect and depletion induced phenomena in mixtures of rods and spheres

A novel phenomenon, analogous to the quantum and critical Casimir effects, is observed in a quasi-2D vertically shaken granular gas of rods and spheres. This granular Casimir attraction increases as the shaking intensity. Velocity distributions of spheres inside and outside the Casimir-like configurations are measured, showing a clear suppression of the momentum acquired by the spheres in direction perpendicular to the rods. Excluded volume effects are measured through liberated area as a function of time. The apparition of stable Casimir-like configurations reveals a stronger attraction than the one purely induced by depletion forces. Furthermore, evidence of layering of spheres close to the boundaries (resembling the ordering of water on a surface) is reported.