The air-driven brazil nut effect
Abstract: It is well known that a large, heavy intruder in a bed of smaller particles will rise under vertical tapping or vibration, the "Brazil nut" effect. It has also been shown that the time taken for the intruder to rise to the top of a bed can be influenced by interstitial air. Here we describe a strongly air-driven Brazil nut effect for fine granular beds. Experiments using a box with a base that is porous to air demonstrate that the behaviour of the intruder can be altered significantly if the flow of air through the bed is changed. The mechanism can be traced to the differential influence of the air on the bed and on the intruder. We perform soft-sphere molecular dynamics simulations which include an approximate treatment of the air flow. These simulations confirm the proposed air-driven mechanism and cast interesting light on the influence of parameters such as the bed friction.