Granular materials are collections of discrete macroscopic solid grains that can move independently. Despite this seeming simplicity, granular materials exhibit a series of fascinating effects, such as intruder clustering, due to dissipative nature of forces acting on interacting grains, such as inelastic collisions and friction. In addition to its importance in industries and its ubiquitous existence in our daily lives, in the past two decades, it has attracted a large amount of research attention, and is becoming one of the most active research fields.
At first, we have studied the relationship between the size of intruder and the intruder clustering by using event driven method. The simulation results demonstrate that strength of the interaction increases with the diameter of the intruder particle increasing for the intruders with the same diameter. But for the neutrally buoyant intruder particles with different diameters, we find one short-range attractive force which has a range of five small intruder diameters, and one repulsive force extending up to a separation of about ten small intruder diameters. The attractive and repulsive interactions between intruders are found to depend on not only the phase of the vibration cycle but also the relative height of the intruders within the bed. When the small intruder fluctuates below the large one, interaction can be changed from attractive to repulsive; however when the small intruder moves upwards above the large one, repulsion can be turned to attraction. The strength and the range of interaction can also be varied by altering the relative height of the two intruders within the bed.
Furthermore,we have studied the border effect of intruder clustering. The results of simulation demonstrate that the Influence of boundary on Intruder clustering is strong. Between single intruder and the boundary, a strong short-range attractive interaction and a weak long-range repulsive interaction can be found ,and the strength of these interactions grows as the diameter of intruder increases.
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