Brazil nut effect: why larger mixed nuts "float" to surface

Video link. While I'm thinking about particle physics, here's something that might interest those of you who puzzle over why the dried fruit often ends up on the top of your cereal, or why cashews and Brazil nuts often end up pouring out on top in a can of mixed nuts. It's because of a phenomenon called granular convection, also known as the muesli effect or the Brazil nut effect. Shaking the contents of a container effectively liquidizes them, causing the bigger ones to float and cluster together. The phenomenon has been of interest to scientists since the 1930s.

• [ROSSINDETROIT]

  In an agitated container, material circulates by rising at the sides and falling at the middle. This is due to friction against the sides, where it’s easier to push something up than down.

• [Anonymous]

  What would explain why after digging in your garden getting rid of any sizable rocks, the next year more have moved their way up to the surface?

• [Anonymous]

  I’ve noticed this effect here at the beach in Oregon, and tried to explain it to people who dig to find the big agates. Mostly, if there are big ones washed up, they’ll be right on top or in the first couple inches of gravel. Below that, it’s all pretty uniformly sand.

• [Anonymous]

  I just don’t want to Troll. But this is a rather complex problem, with multiple factors contributing to the effect.
  No full model has yet been deviced explaining the phenomena in full. Though approximations have been made.
  I’m brave enough to say I don’t fully understand the effect.
  (Little side note: I do have PhD in theoratical physics)

  • [Anonymous]

    I 100% agree. I am currently DOING a PhD into size and density segregation of granular materials.

    We know very little about the mechanisms which drive these kinds of segregation, but what we do know seems to indicate that there is a different reason for each of the examples shown.

• [Ugly Canuck]

  Want to sort your nuts?
  Then shake shake shake senora:

  http://www.youtube.com/watch?v=Bk3sLHZzZRI

  That’ll sort them out!

• [CastanhasDoPara]

  Huh, interesting… now I know why I always seem to end up on top after being agitated.

• [Aenieron]

  An interesting way to test this would to try it in a zero G environment.

• [ROSSINDETROIT]

  An easier way to see this is that smaller particles have more density for the volume that they occupy because there’s less air between them. Therefore when agitated the large particles ‘float’ on them. Worked that one out a long time ago.
  Speaking of particle physics, I’m 90% of the way through Lisa Randall’s book Warped Passages, about strings, branes, extra dimensions & related difficult things. It’s been educational, but the next time Gareth Branwyn tells me to read something I’ll scan before I buy.

• [howaboutthisdangit]

  It is easier for small particles to fall through the gaps between large particles than the other way around. It is natures own sieve sort.

• [Aknaton]

  I like to take this as a comprehensible example of decrease of entropy in a non-closed system (energy is being added to agitate).

  Now use it against creationists who insist that life couldn’t have evolved, because entropy would have overtaken any evolutionary advances. (The Earth is not a closed system.)

  • [Aenieron]

    Why is there a patern? from the other experiment shown, the larger objects (e.g. Washers) clustered together, but in his last experiment, they spread into a uniform partern, is this because of the horizontal motion, with the earths gravity forcing the breakup of the energy in different places? (non phd, just curious XD)

    Now to use again evolutionists, just because you have all the parts of a watch in a box and start shaking it, doesn’t mean you have a watch.

• [scottunder]

  I remember reading about this many years ago in Scientific American. As #2 suggests, the explanation that stayed with me is that when gaps open up during vibration, small particles can fit in any size gaps, but big particles cannot, so small things end up under big things.

  Another place this effect shows up is in rocky agricultural land, where farmers still remove boulders in fields that have been plowed for centuries.

• [ROSSINDETROIT]

  In an agitated heterogeneous material the particles form ‘cells’ of rising and falling material. All material rises with equal ease but the largest particles find it more difficult to circulate back down because they have a large bottom surface area, is another way of looking at it.

• [Anonymous]

  Granular convection is a phenomenon where granular material subjected to shaking or vibration will exhibit circulation patterns similar to types of fluid convection.

  Under experimental conditions, granular convection of variously-sized particles has been observed forming convection cells similar to fluid motion. The convection of granular flows is becoming a well-understood phenomenon.

  In geology, this phenomenon is one of the causes of inverse grading which can be observed in many situations including soil liquefaction during earthquakes or mudslides.

  Brian Raymond Callahan
  Orlando, Florida

• [xzzy]

  Any kid with a bucket of lego understands this intuitively.

  My parents stored my pieces in those clear plastic containers that fit under beds.. I spent a lot of time holding them above my head trying to find specific tiny pieces.

  These days I use the behavior a bit differently.. I put all my pieces in a ziploc bag, seal it, shake it around a bunch, let the stuff settle naturally. Then I scoop off the top layer of big pieces, makes pawing through the remaining tiny pieces a lot easier.

• [Anonymous]

  God is a fan of big nuts… so they are summoned to the top to keep relevant.

• [Lobster]

  You can see this at work in modern culture, too. Shake things up and the biggest nuts come out on top. ;)

• [Anonymous]

  Cat owners also know this. Shake the litter pan and the clumped bits come to the surface. Much easier for cleaning that way.