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Blog

Physicists crack the ‘water-enhanced Brazil nut effect’

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

By Hamish Johnston

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.

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.

paste.jpg

You can read all about it in this paper in Europhysics Letters.

That brings me to another food-related effect that I first spotted years ago – the “anomalous curry paste effect”.

Take a jar of Patak’s curry paste (other brands are available), scoop out a few spoonfuls, replace the lid tightly and then place in a cupboard at room temperature for a few weeks. You will find that oil from the paste somehow escapes through the lid, and flows down the sides of the jar to make a messy red ring on the shelf.

Repeat the experiment with the paste in the refrigerator and the oil stays in the jar.

If you can explain why, there’s an IgNobel prize waiting for you at Harvard!

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5 comments

  1. Martin

    To make things more complicated, I observed the Patak’s effect inside my fridge the other day.

  2. Anna

    This happens with any and all pastes as well. Tomato, garlic etc. The oil invariably comes out no matter how well sealed.

  3. Hamish,
    When made for maximum flavour the spices that make up the curry are ground in the oil, which dissolves the volatile aromas and flavours to keep them longer for our enjoyment. The oily solution thereby acquires a higher vapour pressure up to the rim above the paste.
    When it condenses, the oil, pushed by whatever vapour pressure is still available, is therefore able to fall over the rim and do what oil does best, i.e., seep between inner rim of lid and outer glass of jar.
    The curled-in rims of curry paste lids also collect oil.
    The vapour pressure of the oil in the refrigerator is much less.
    Alfred Bhulai
    Edward B. Beharry & Co. Ltd. [Manufacturers of Curry]
    4057 Area Y, Mandela Avenue,
    Ruimveldt Industrial Site,
    Georgetown, Guyana.

  4. dingodel

    I observed some Patak-like effect with some half filled box (thick) soup in the fridge last week; my wife as well; no further comment; except : great thanks for pointing the universality of the effect

  5. garybau

    the ‘brazil nut effect’??..well known to farmers with rocks in their paddocks for hundreds of years…also well explained many(many) years ago!
    French farmers still experience WW1 artifacts ‘coming to the surface’ after 80 years of farming
    Anyone with produce also knows the large apples are at the top..etc..
    Truck drivers have the large aggregate in the top layers after transporting their load any distance.
    simple physics myriads of applications..isn’t physics wonderful( or at least the explanations of commonly observed, and less well explained phenomena).
    Jearl Walker covered the same in early 70s ( SciAm amateur scientist) and then in Flying Circus of Physics( with answers!) recently reprinted with URL links and YouTube examples.

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