Effect of size polydispersity on granular materials

We present particle dynamics simulations on fluidization of vertically vibrated granular beds where solidlike and fluidlike domains appear spontaneously. The existence of polydispersity in granular size significantly a alters where the boundary between solidlike and fluidlike regions appears. A threshold value of polydispersity sigma(th) exists above which the system becomes insensitive to the change in the. degree of polydispersity. The value of sigma(th) shows good agreement with the critical polydispersity reported on the solid-fluid phase transition in thermodynamic hard-core systems. The present result implies that the existence of polydispersity is crucial in some granular systems.