Confirmation of Faraday's explanation of bunkering in vibrated granular beds

Abstract

Faraday, like Chladni earlier, saw a shallow layer of a fine powder collect in a circular heap (a ‘minibunker’) at an antinode of a vibrating plate. He saw ‘Faraday circulation’: powder motion in a shallow surface layer of the heap, downward from its center to its edge; inward motion centerward at the edge; by inference, motion upward toward the middle of the heap's interior. What he saw led him to postulate formation of a partial vacuum beneath the powder heap, creating (1) an external wind blowing the powder toward an antinode and (2) a flow of air inward from the heap's edge, driving powder toward its center. Although some experimentalists have recently advanced alternative explanations of vibrated-bed heaping (‘bunkering’), we are able to confirm the essentials of Faraday's thought. At suitable amplitude and frequency, vertical sinusoidal vibration of a fine-powder bed causes it, in each vibration cycle, to experience a free-flight interval during which pressure gradients in its interior drive powder centerward. A center-high bunker forms, displaying Faraday circulation. When bed-floor collision terminates flight, pressure gradients reverse direction; but passage of a compaction front has locked particles against further movement. Before a next flight interval, an increase in porosity will reverse the compaction that accompanied heap-floor collision. In particles 177-μm and larger, we see the compaction front cinematographically; in 88-μm particles, we infer it from floor-pressure data. Although, given sufficient time, a bed of large particles (e.g., 707 μm) will form a bunker, it displays the wall-friction-driven circulation elucidated by Muchowski, not the Faraday pattern.