Liquid-solid phase transition in a confined granular suspension

We experimentally investigate the phase behaviour of a model two-dimensional granular system undergoing stationary sedimentation. Buoyant cylindrical particles are rotated in liquid-filled drum, thus confined in a harmonic centripetal potential with tunable curvature, which competes with gra- vity to produce various thermal-like states : the system can be driven from liquid-like to solid-like configurations as the rotation rate is increased. We study the statistics of the local hexagonal order across this transition, which exhibits large fluctuations and heterogeneities as the dense phase of the system develops spatial ordering. In particular, the spontaneous appearance of locally crystal- lized regions in the disordered phase points to the existence of an underlying critical point : size distribution displays scale invariance when approaching the transition, which in turn occurs when the packing fraction crosses the random close packing. This suggests that the ordering transition is only due to geometrical effects, in contrast with other experiments on liquid-solid phase transition of granular media. Finally, a simple force balance explains when the perfect crystalline state occurs yielding an upper bound for the critical frequency of the transition.