Shake, Rattle, or Roll?

by Donna Francis - Researcher / Chemistry and Materials Science
January 2002

Want to get the prize at the bottom of a box before having to eat all the cereal?

I've been spending a great deal of time thinking about this question recently.

Don't get me wrong, I'm not necessarily preparing for a wild cereal-eating spree; I've actually been learning about an entire group of materials which I've pretty much neglected until now.

Understanding the unusual behaviour of this group gives the answer to the title question.

This curious group is known as Granular Materials, and we are surrounded by them in our everyday lives. Granular materials are assemblies of individual solids—basically, anything which you can say "grains of" in front of such as:

• sand,
• sugar,
• or salt.

But it also includes:

• cereals,
• popcorn kernels,
• seeds...

Well, you get the idea.

When you have a quantity of individual solids together, they have the amazing ability to behave like solids, liquids, or dense gases at any given moment (depending on how you are using them).

Until recently, I viewed these materials simply as solids, having the same properties of other solids. However, there are a lot of things about the behaviour of granular materials which sets them apart. Research physicists at the University of Chicago's Granular Group have been working on determining why sand acts the way that it does.

The "Brazil Nut Effect"

One notable phenomenon they are studying is known as the "Brazil Nut Effect". This is the common term for what occurs when you take a can of mixed nuts and shake them up and down for a while—try this at home (unless you have nut allergies).

After a few shakes, the small peanuts end up at the bottom while the large Brazil nuts have found their way to the top.

Generally, we are used to larger, dense items settling to the bottom with lighter objects on the top. In granular materials, however, the difference is due to the fact that grains flow. Just like a liquid, sand pours from one place to another.

Convection flow

	View how the "Brazil Nut Effect" works through a series of animations.

Imagine a tall cannister of sand. When shaken up and down, the grains actually flow in a pattern, with individual grains moving upwards through the middle, across the surface, and down along the sides. This pattern is known as convection flow.

Now, if you add a marble, for example, to the sand, the larger marble will get caught up in the convection flow and move to the top of the sand. Once at the top, it will stay there, because the convection currents are too narrow for the marble to sweep downwards along the wall with the sand.

Change the effect

For years, physicists have experimented with variations on the "Brazil Nut Effect".

They have even found that the effect can be reversed depending on the shape of the container! Shaking a cone-shaped container will force a marble placed on the surface down to the narrow bottom because the convection flows in the opposite direction.

Variations in the density of the marble and the amount of air surrounding the sand can have a huge effect on its speed while rising through the sand. Reduce the air pressure, and the marble rises even faster.

Real life

What difference does this make in the real world? In industry, there may be occasions where a mixture of large and small particles of powders are mixed together and sent away. In the shaking of a delivery truck, by the time it reaches its destination, the mixture may have separated already.

What about everyday life?

This is where the cereal and the prize come in.

When I see that giant box of Rice Krispies with a large plastic prize buried at the bottom, I now know how to quickly find the toy without having to empty the box first.

Over the holidays I've found a number of situations where the "Brazil Nut Effect" came in handy:

• adding flavouring to popcorn at the movie theatre,
• getting my favourite snack from a bowl of party mix,
• and finding a non-delicate item in the midst of a box of foam peanuts,

to name a few.

Now that I know about the Brazil nut phenomenon, the world is wide open.