The Brazil Nut Effect: Explained!

[billy_boy_999]

why do the big brazil nuts always end up at the top of the bag? or when you put a rubber ball at the bottom of a bowl of rice and shake the bowl from side to side why does the rubber ball always come to the top? what is the explanation? I'm going crazy!

 

[HallsofIvy]

Density. Denser materials will go to the bottom, lighter materials will go to the top.

The big brazil nuts contain more air and are lighter. The rubber ball is lighter than the rice.

 

[Bystander]

You are seeing what used to be called "percolation" --- stones coming to the surface in plowed fields, shells from WW I in Flanders, big flakes to the top in cereal boxes and crumbs on bottom. Haven't found the right combination of key words to give you any links, sorry. Density, particle size ratios, surface to mass ratios, shapes (affects packing) all contribute to which particle moves what direction in a matrix of some other particles. Mixtures of unlike solid particulates (cake mix, raisin bran, Lucky Charms) have been subjects of intense study regarding particle surface treatments, sizes, shapes, and mixing times necessary to produce a "homogeneous" (on some specified sample size within some larger batch size) mixture that does not separate or unmix.

You can float steel balls on sand, you can sink ping pong balls in dense media (don't ask me for the trick on this one right now), and you can get wildly different density, or sized, particles to refuse to unmix.

 

[ek]

This was a question on who wants to be a millionaire the other day. I had never heard of it until then. Quite the "nutty" name!

 

[imabug]

The field that covers that kind of thing is known as granular physics. Googling it ought to bring up a number of sites that talk about it.

It's a growing area, with a lot of really interesting phenomena that people haven't quite figured out yet and covers a pretty wide range of things.

 

[billy_boy_999]

thank you guys...

the density seems a simple satisfying explanation for the brazil nuts and for the ping pong ball...but is the steel ball lighter than the same volume of sand? the shells lighter than the soil? now i am going crazy...i will try googling on granular physics, thank you...

 

[russ_watters]

thank you guys...

the density seems a simple satisfying explanation for the brazil nuts and for the ping pong ball...but is the steel ball lighter than the same volume of sand? the shells lighter than the soil? now i am going crazy...i will try googling on granular physics, thank you...

Density is not the answer. Percolation is. Think of it this way: if you have a bunch of billiards balls in a big container and pour sand on them, and shake, what happens? The sand, being smaller, finds empty spaces and works its way to the bottom of the container. Repeat with BB's and the same thing happens. Repeat with marbles and the same thing happens. As it turns out, it doesn't take much difference in particle size for the small particles to settle to the bottom and the big ones to percolate to the top.

 

[tozhan]

This is used to great effect in coffee percolators. The coffee is placed in a compartment above the water. The water is boiled and the vapour will travel to the top and condence back to a liquid. This liquid runs though the coffee back to the bottom taking with it any disolvable coffee particles. This happens again and again to made a lovely fresh cup of steaming hot coffee! ...but i use instant...

Russ explained it well but i thought id give you an everyday use...

 

[Mk]

This effect was proven on Scientific American Frotiers on PBS or the National Geographic channel depending on where you live: density is not the answer by percolation, the smaller particles fall through the bottom, and the larger ones stay on top or are displaced, as russ said. Also, another strange effect occurs, I don't know if its the same effect.

Imagine a cross section, about 9/13 of the diagram (in the middle) is the section where smaller particles go down, and 2/13, 1/13 one each side, have particles going up. Why does this happen?