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The Brazil Nut Effect

The Brazil Nut Effect

By B. James McCallum and Stuart M. Smith

It's one of life's little frustrating truths - the raisins are always at the bottom of the cereal packet. But why? Well the explanation is rather more complex than you might expect. James McCallum and Stuart Smith go in search of the Brazil Nut Effect.

Have you ever opened a can of mixed nuts and found that the bigger ones have all shifted to the top, while the smaller, less costly nuts have in fact shifted to the bottom? That shouldn't happen, right – the larger nuts are heavier, they should be on the bottom. A can of nuts isn't an antigravity device, is it? Well, no, it’s not. As it turns out, what you’re witnessing is a well-described but surprisingly complex phenomenon called the "Brazil Nut Effect". The same phenomenon can occur with breakfast cereal, which has caused a competing appellation: the "Muesli Effect", but both refer to the same actual circumstance.

So what goes on inside a container housing items of different shapes and sizes? At least three mechanisms come in to play. The first is easiest to understand, thanks to gravity, smaller particles filter through gaps between larger pieces. The larger particles are then left behind making it appear as if the have risen to the top, when, in fact, they have remained stationary. This part of the phenomenon is known as "percolation", and is enhanced by the vibrations normally encountered when a container undergoes shipping.

Click image for the Brazil Nut Effect in pictures
What is a little less intuitive is the fact that the larger particles are actually driven upward by "convection". As a container is bounced around during shipping, particles are forced to move upward and downward within the confines of the space allowed. In the case of a cylinder or box, these motions result in a large upward flow in the middle of the container with smaller rivulets of force around the outskirts of the enclosed space. Large particles can travel upward in the large central column, but are unable to travel downward in the small columns around the edges. Eventually, one gets a heap of the larger material in the middle of the container, and a pile of the smaller materials at the bottom.

Interestingly, this isn't the case with all container shapes. By sloping the container walls, you can actually cause the reverse of the Brazil Nut effect, producing downward movement in the middle with small upward movements around the periphery of the container. This results in the larger particles moving towards the bottom of the container, and the smaller particles moving towards the top.

Even more confusing is a third mechanism of action known as "condensation". For the percolation and convection to occur a certain amount of energy needs to be applied to the system, usually thanks to wobbles and shakes whilst the packages are being moved. This critical level of energy depends on the size to weight ratios of the particles. If this energy isn’t applied then particles actually condense out at the bottom of the container, becoming tightly packed.

So little space exists between particles that even quite vigorous movement of the container causes them to only vibrate against each other - they are unable to change places. As a result, they are basically on the bench as far as the dynamic portion of the mixture is concerned, affected by neither convection nor conduction.

As interesting as this may all be, where, outside of the narrow world of package science and engineering, is this knowledge of any use? Apparently almost everywhere. It has played a role in moving viruses to the surface of a polymer in bioengineering. In astronomy, it has been used to help explain the composition of the atmosphere of Titan, Saturn's largest satellite. In building and manufacturing, it helps to explain why granular materials are sometimes difficult to work with. And it explains while there aren’t any raisins in the bottom of the cereal box.

How other things work:

- Musical - Didgeridoos
- Historic - Old-style TVs
- Astronomic - Dark Energy
- Futuristic - Wi-tricity
Image: Pablo Ferrat

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14 Aug 2009
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