Matt Harrington, Joost H. Weijs, Wolfgang Losert
Mixtures of granular materials tend to segregate under various disturbances, such as shear and gravity. While convective flows have been implicated in many segregation processes, the particle-scale rearrangements associated with segregation are not well understood. We have performed experiments on a three-dimensional bidisperse mixture in a split-bottom geometry, under both steady and cyclic shear. The pile segregates under steady shear, but the cyclically driven system either remains mixed or segregates slowly, depending on shear amplitude. The motion of individual grains, imaged in three dimensions using a Refractive Index Matched Scanning technique, shows no signs of particle-scale segregation dynamics that precede bulk segregation. Instead we find that the transition from non-segregating to segregating flow is accompanied by a transition to significantly less reversible particle trajectories (even for monodisperse systems), and the emergence of a collective convective flow field.
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http://arxiv.org/abs/1302.3788
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