Editor's Summary
25 June 2009
Caught on camera
A free-falling stream of liquid quickly breaks into droplets because of surface tension, a result of the attraction between molecules. Surprisingly, a similar effect can occur in a falling stream of a granular material such as sand. Surface tension is not an obvious presence in such flows, and until now the clustering mechanism involved had not been fully resolved. A study of the break up and rupture of falling streams of glass and copper grains, using a high-speed camera falling alongside the granular stream, now shows that tiny cohesive forces between the individual grains are responsible, corresponding to a granular surface tension some 100,000 times weaker than that of ordinary liquids. While the droplet shapes resemble those predicted for nanoscale liquid jets, current theoretical frameworks cannot adequately explain the results.
News and Views: Granular media: Structures in sand streams
An ingenious experiment that involves dropping a costly, high-speed video camera from a height of several metres reveals how free-falling streams of granular matter, such as sand, break up into grain clusters.
Detlef Lohse & Devaraj van der Meer
doi:10.1038/4591064a
Letter: High-speed tracking of rupture and clustering in freely falling granular streams
John R. Royer, Daniel J. Evans, Loreto Oyarte, Qiti Guo, Eliot Kapit, Matthias E. Möbius, Scott R. Waitukaitis & Heinrich M. Jaeger
doi:10.1038/nature08115
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