Compaction force in a confined granular column.

Experiments to determine the force required to push a granular column confined within a cylinder were performed. The experimental apparatus was mounted on a material testing system machine in order to obtain force and displacement measurements simultaneously. Experiments were performed for two different sphere diameters, two different cylinder diameters and for a range of piston displacement velocities. The force necessary to displace the column increases rapidly with the column height, in accordance with Janssen's theory. More importantly, we found that this force also increases with the displacement velocity. This unexpected behavior is an indication of the transition to rate-dependent behavior in dense granular flows.

Particle segregation in vibrofluidized beds due to buoyant forces.

We utilize two-dimensional discrete element computer simulations to investigate the equilibrium position of an impurity in a vibrofluidized bed of otherwise homogeneous particles. The steady state equilibrium height of the impurity increases with increasing vibration velocity amplitude and decreases with the impurity to the surrounding particle density ratio. A simple model whereby the impurity weight is balanced by a "buoyant" force due to the surrounding particle impacts makes a good prediction of the impurity position.