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.
We present different experiments on dense granular assemblies with the aim of clarifying
the notion of ‘jamming transition’ for these assemblies of non-Brownian particles. The
experimental set-ups differ in the way in which external perturbations are applied in order
to unjam the systems. The first experiment monitors the response to a locally
applied deformation of a model packing at rest. The two other experiments study
local and collective dynamics in a granular assembly weakly excited by vibration.
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