The Brazil Nut Effect

Ever wondered why on packets of cereal it sometimes says something to the effect of “Please turn packet on its side to redistribute nuts evenly”? I’d always thought that this was because any of the larger particles would have sunk to the bottom. It turned out that this was in fact not the case.
Yesterday was the day of the MRSC08 conference for postgraduate maths students in Manchster. One of the talks explained the so called “Brazil nut effect”, the phenomenon of larger nuts rising to the top of packets of mixed nuts during transport. The following video (taken from here) shows the effect working on a brass cylinder among polystyrene foam beads.

It turns out that although gravity pulls the cylinder down relative to the polystyrene, there are granular flow effects that also push the cylinder up which are far greater than expected. I can’t claim to be an expert, but the talk was incredibly interesting. (See Prof. Nico Gray’s website for research about granular flows in Manchester).

Here’s a slow motion version of the above video to maybe give some intuitio for what’s going on. I have to admit, I still don’t quite understand from where the upward force comes.

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