We often think of fluids in the continuum limit, while knowing full well that they are really made of molecules. This immediately raises questions like: "In what ways do sand and other granular substances flow like liquids?" Now Stephen L Conway of Rutgers, the State University of New Jersey, and colleagues have shown that if you put granular material between two concentric cylinders and rotate them sufficiently quickly, the Taylor bands, which have been known in fluids since 1923, appear.
While this characteristic instability occurs in the sand, much as it does in liquids, it turns out to be accompanied by new mixing-segregation transitions that have no direct counterparts in normal fluid mechanics. The work has potential applications in understanding countless geological problems as well as in industry, where mixing of powders is a common task.
Further reading
S L Conway, T Shinbrot and B J Glasser 2004 Nature 431 433.
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Compiled by Steve Reucroft and John Swain, Northeastern University