Unlike two liquids which reach a well-mixed state when stirred, binary powder mixtures may tend to segregate when they are shaken or in a state of flow. The raisins are always at the bottom of your box of cereal! This tendency depends among other characteristics on the particles relative size, shape and roughness.
A mixture of large glass beads and small sand grains, partially filling a horizontal tube, segregates into bands of different composition when rotated slowly. The pattern of bands evolves in time: some bands shrink and disappear, while others grow at their expense. We study the characteristics of this evolution.
Segregation
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This is a tube filled with sand and beads. When it is spun stripes appear. The stripes coarsen, at an exponentialy decaying rate. |
References:
"Rotationally Induced Segregation of Granular Materials", O. Zik, D. Levine, S. G. Lipson, S. Shtrikman and J. Stavans, Phys. Rev. Lett. 73: 644 (1994).
"Avalanche Mediated Transport in a Rotated Granular Mixture", V. Frette and J. Stavans, Phys. Rev. E 56: 6981 (1997).
Mobility of a Sphere in Vibrated Granular Media,O. Zik, J. Stavans and Y. Rabin, Europhys. Lett. 17, 315 (1992).
"Self-Diffusion in Granular Flows", O. Zik and J. Stavans, Europhys. Lett. 16, 255 (1991).