Impact in dense suspensions: Key role of pore-pressure

Abstract eng:
We investigate the impact of a freely-falling rigid sphere onto a granular pile immersed in a Newtonian liquid. Two very different behaviors are observed depending on the initial packing fraction of the pile. For initially loose packing, the ball readily sinks in the suspension as in a liquid whereas, for densely packed beds, the ball stops immediately as it hits the suspension. By combining local measurements of the interstitial fluid pressure and a simple impact model, we show that this liquid–solid transition is controlled by the coupling between the dilatancy of the pile and the pore–pressure generated by the impact. These results unify the description of impacts in various particulate media including cornstarch suspensions. They show that complex responses can arise from transient fluid–particle coupling rather than from the intrinsic rheology of the material.

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