What Is the Brazil Nut Effect?
What happens if you shake up a can of mixed nuts?
Do the Brazil nuts rise to the top?
Do you still have any Brazil nuts left over from Christmas that you can shell?
- 1 cup peanuts
- 1 Brazil nut, shelled, whole
- 1 straight-sided, narrow drinking glass
- 1 cup water
- Pour half the peanuts into the glass
- Set the Brazil nut on top of the nuts
- Pour the other half of the peanuts into the glass
- The glass should be about 2/3 full
- Cover the glass with your hand and shake the glass up and down.
- Count how many times you shake it.
What Should Happen?
When my grandson and I did it, it took 90 shakes for the Brazil nut to appear on top of the peanuts.
It should happen between 40 and 100 shakes.
Next, repeat steps one through three, burying the Brazil nut in the middle of the peanuts again.
Then, pour the water into the glass.
Cover the glass with your hand and shake it up and down.
Count how many shakes it takes for the Brazil nut to appear on top of the peanuts.
When we did it, it took eight shakes.
What Is Happening?
There are several theories about what is happening.
The most common is that the smaller peanuts fall below the larger Brazil nut, take up space underneath it and prevent it from falling back down.
The effect is called granular convection.
When you add water, it speeds up the process by reducing the friction and adding buoyancy.
However, the first time I tried this experiment, with a golf ball and dry grains of rice, it failed.
This could have been for several reasons:
- I didn’t shake it long enough
- I filled the glass with too much rice, not leaving enough room for the rice to shake out
- The golf ball is too heavy, relative to the rice.
And, Then, There’s the Reverse Brazil Nut Effect
As it turns out, while scientists are trying to nail down why the Brazil Nut Effect happens, they’ve discovered that sometimes the reverse happens.
The Brazil Nut goes to the bottom.
They call this, descriptively, the Reverse Brazil Nut Effect.
It can happen if you tip the container over on its side, then shake it vertically, from side to side instead of up and down.
One scientist calculated that the relative size of nuts matters.
If the Brazil nut were even larger, compared to the peanuts, it would sink instead of rise.
What About the Shape of the Container?
The shape of the container may matter.
A cylinder shape, such as a mixed nut can, dampens the effect.
So also, does a rectangular shape, like a cereal box, stabilize the effect.
This is why the toy buried in the bottom of a cereal box stays there.
A cone shape, however, reverses the effect, sending a large Brazil nut down to the bottom of the cone, in line with what you would expect if only gravity were operating.
I invite you to try these variations.
For my part, it was all I could do to conduct one experiment before my grandson ate the peanuts.
Is This Effect Useful in Real Life?
Let’s say you’re a skier or snowboarder.
You’re caught in an avalanche.
You pop your backpack into an inflatable balloon to keep you near the top of the snow.
You can thank the Brazil Nut Effect for your life.
Thanks to flyingcircusof physics.com for this activity.
Carol Covin, Granny-Guru
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