Why big nuts rise to the top of the bowl

Why it's so hard to find pistachios in that nut mix

If you put your hand into a bag of mixed nuts or cereal, you might notice the big bits are at the top. Why does this happen?

A team from the University of Manchester, led by materials engineer Parmesh Gajjar, used time-resolved 3D imaging to show exactly how the big nuts rise to the top - and it is all to do with shape.

They found horizontal Brazil nuts did not "unmix" until they had rotated vertically, at which point they slowly rose through the mixture to the top.

Some of the smaller nuts, like peanuts, moved downward, but also trapped some of the Brazil nuts at the bottom by preventing them from turning vertically.

"Critically, the orientation of the Brazil nut is key to its upward movement," says Gajjar.

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"We have found that the Brazil nuts initially start horizontal but do not start to rise until they have first rotated sufficiently towards the vertical axis. Upon reaching the surface, they then return to a flat orientation.

"Our study highlights the important role of particle shape and orientation in segregation."

This type of particle separation can sometimes pose a problem for industrial mixtures that rely on an even mix - for example, active ingredients in medicinal tablets, or mining and constructing materials - because as larger, ovular particles move upwards, the overall quality of the product can be compromised.

Size segregation of irregular granular materials captured by time-resolved 3D imaging was published in Scientific Reports.

  • This article is published in partnership with Cosmos Magazine. Cosmos is produced by The Royal Institution of Australia.