Fingering in granular flows

The flow of granular materials on inclined planes is of interest within the contexts of both industrial processing of powders and geophysical instabilities such as landslides and avalanches1,2. These flows have been found to be complex, exhibiting several different flow regimes3–7 as well as particle segregation effects8,9 and instabilities10. Here we describe an instability that occurs when a front of granular material propagates down a rough inclined plane. The front, which is initially uniform in cross-section, rapidly breaks up into fingers. Although this is similar in appearance to the instability seen in viscous fluids flowing down a plane11–17, in these latter cases the instability is driven by surface tension, whereas granular materials have no surface tension. We show that the fingering instability in this case is instead induced by the size segregation that develops during the flow.

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