The brazil nut effect is the phenomenon where big particles end up on the surface when a mixture of granular material of different sizes is shaken. The term was based on the observation that, when a container of mixed nuts is opened after it has been shaken, the brazil nuts tend to be on top. The same effect is seen in packets of breakfast cereal containing particles of different sizes, such as muesli.
There are several possible explanations for larger, heavier particles ending up on top (against intuition).
- When shaken, the particles move in vibration-induced convection flow: individual particles move up through the middle, across the surface, and down the sides. If a large particle is involved, it will be moved up to the top by convection flow. Once at the top, the large particle will stay there because the convection currents are too narrow to sweep it down along the wall.
- Smaller particles can fall into the spaces underneath a larger particle after each shake. Over time, the larger particle rises in the mixture.
- Including the effects of air in spaces between particles, larger particles may become buoyant or sink.
The effect is of serious scientific interest for industrial separation and mixing operations, and depends on the sizes and densities of the particles, the pressure of any gas between the particles, and the shape of a container. A rectangular box (such as a box of breakfast cereal) or cylinder (such as a can of nuts) works well, while a cone-shaped container results in what is known as the reverse brazil nut effect.
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