Role of Air in Granular Jet Formation


A steel ball impacting on a bed of very loose, fine sand results in a surprisingly vigorous jet which shoots up from the surface of the sand [D. Lohse et al., Phys. Rev. Lett. 93, 198003 (2004)]. When the ambient pressure p is reduced, the jet is found to be less vigorous [R. Royer et al., Nature Phys. 1, 164 (2005)]. In this Letter we show that p also affects the rate of penetration of the ball: Higher pressure increases the rate of penetration, which makes the cavity created by the ball close deeper into the sand bed, where the hydrostatic pressure is stronger, thereby producing a more energetic collapse and jetting. The origin of the deeper penetration under normal ambient pressure is found to lie in the extra sand fluidization caused by the air flow induced by the falling ball.





Copyright 2007 Physical Review Letters. Recommended citation: Caballero, Gabriel, Raymond Bergmann, Devaraj van der Meer, Andrea Prosperetti, and Detlef Lohse. "Role of air in granular jet formation." Physical review letters 99, no. 1 (2007): 018001. DOI: 10.1103/PhysRevLett.99.018001 URL: Reproduced in accordance with the original publisher’s licensing terms and with permission from the author(s).