Granular eruptions: Void collapse and jet formation


A steel ball dropped onto loose, very fine sand creates a jet exceeding the release height of the ball:

 

 

Upon impact, sand is blown away in all directions, forming a splash. The ball digs a cylindrical void in the sand and the jet is formed when this void collapses: The focused sand pressure pushes the jet straight up into the air. When the jet comes down again, it breaks up into fragments, i.e., granular clusters.

For sufficiently high impact velocity, air is entrained by the collapsing void, forming an air bubble in the sand. This bubble slowly rises to the surface, and upon reaching it causes a granular eruption. This looks like a boiling liquid, or even a volcano!

  • Read more about granular eruptions here.
  • Our 3-minute movie of the ball impact is one of the winning entry of the Gallery of Fluid Motion in 2002. It can be viewed here.
  • For detailed information, see our recent article in Physical Review Letters (2004)
  • The jet featured prominently on the poster of the workshop "Structures in Granular Matter", held at the Lorentz Center, Leiden, August 19-30, 2002.

 

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