Granular convection (Brazil nut effect)
Why, when a container of mixed different-sized granular particles (like nuts or cereal) is shaken, do the largest particles reliably rise to the top rather than settling based on density?
History and references
Partly settled. Some of it is understood, some is still argued about.
The Brazil nut effect is a everyday kitchen-cupboard observation that puzzled physicists studying granular materials, which behave neither quite like solids nor liquids. Percolation of small particles into voids beneath large ones, combined with shaking-induced convection currents that carry large particles up the middle and small ones down the sides, together explain most cases, as shown in simulations and experiments since the late 1980s. However, a fully general, unified theory covering all particle-size, shape, and vibration regimes (including the reverse Brazil-nut effect seen under some conditions) is still being refined.
A phenomenon anyone can reproduce by shaking a jar of mixed nuts turns out to require genuine statistical-mechanics theory of driven granular matter to explain.
Explanations that circulated
- That it is simply density segregation like a liquid (it is not; even a large low-density particle rises above smaller denser ones)
References
- Why the Brazil nuts are on top: size segregation of particulate matter by shaking, A. Rosato, K. J. Strandburg, F. Prinz, R. H. Swendsen, Physical Review Letters, 1987
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