Physicists crack the ‘water-enhanced Brazil nut effect’

First there was the “Brazil nut effect”, then the“reverse Brazil nut effect” – and now physicists in the UK have cracked the “water-enhanced Brazil nut effect”.

For those of you not familiar with these phenomena, grab a can of mixed nuts and give it a good shake – but make sure the top of the can is always pointing upwards.

When you open the can, you’ll be amazed to find that most of the Brazil nuts have risen to the top – and the smaller peanuts and hazel nuts will be hiding at the bottom.

That’s great if, like me, Brazil nuts are your favourite.

If not, you might prefer the reverse Brazil nut effect, whereby the larger bodies sink to the bottom.

Physicists have been puzzled by this effect for at least 35 years and now they have one more variant to worry about thanks to Michael Swift and team at the UK’s University of Nottingham.


Computer simulation showing a time sequence of the water-enhanced effect

The researchers placed a steel “Brazil nut” (radius 3.5 mm) in a water-filled box and then topped it up with a thick layer of glass beads (radius 1 mm). The box is then vibrated vertically under the watchful eye of a high-speed camera. The experiment is then repeated in a dry box.

They found that the water makes the Brazil nut rise much faster that in the dry situation. To understand why, the team did a series of computer simulations.

The simulations suggest that when the beads are thrown upwards during the vibration cycle, the Brazil nut travels further because its motion is less affected by fluid drag. But when the beads fall back, the Brazil nut cannot drop to its former height because beads have filled the space beneath it.


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