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Marijn Sandtke, Devaraj van der Meer, Michel Versluis, and Detlef Lohse, University of Twente When a 6-mm layer of fine sand with an average grain size of 40 µm is poured into a cylindrical container and shaken vertically, thin jets are seen to emerge from an airy cloud of grains, almost like protuberances from the corona of the sun. A quasi two-dimensional setup reveals the jet-formation process: The sand accumulates in ripples, and below each of them air is entrained by the convective motion of the grains. Air bubbles form at the bottom and slowly rise through the sand. Every time a bubble reaches the surface, it creates a void, which thereupon collapses and focuses into a jet. See http://stilton.tnw.utwente.nl/granular/sand_jets.mpg for the full video. |
