A flowing granular material can behave like a collection of individual interacting grains or like a continuum fluid, depending in large part on the energy imparted to the grains. As yet, however, we have no general understanding of how or under what conditions the fluid limit is reached. Marston, Li & Thoroddsen (J. Fluid Mech., this issue, vol. 704, 2012, pp. 5–36) use high-speed imaging to investigate the ejection of grains from a granular bed due to the impact of a spherical projectile. Their high temporal resolution allows them to study the very fast processes that take place immediately following the impact. They demonstrate that for very fine grains and high impact energies, the dynamics of the ejecta is both qualitatively and quantitatively similar to what is seen in analogous experiments with fluid targets.
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