Kids, why is it that the largest nuts in a can of mixed nuts always seem to be on the top when you open the can? The “Brazil nut effect” is a phenomenon in which the largest particles end up on the surface when a granular material containing a mixture of objects of different sizes is shaken. In a typical container of mixed nuts, the largest will be Brazil nuts.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
The phenomenon is also known as the muesli effect since it is seen in breakfast cereal that has pieces of different sizes but similar density, such as muesli mix. It seems counter-intuitive that the largest and (presumably) heaviest particles rise to the top, but there are several possible explanations:
- Smaller particles can fall into the spaces underneath a larger particle after each shake. Over time, the larger particle rises in the mixture. In other words: The center of mass of the whole system in a random state is not optimized; it has the tendency to be higher due to gravity. When a can of mixed nuts is shaken, gravity will make sure the center of mass of the system moves down and this can only be done by moving the Brazil nuts up. Gravity will also make sure they stay up.
- 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.
- Including the effects of air in spaces between particles, larger particles may become buoyant and rise.
This effect is of serious interest for some manufacturing operations; once a heterogeneous mixture of different sizes of granular materials is made, it is usually undesirable for them to segregate. Several factors determine the degree of the Brazil nut effect, including the sizes and densities of the particles, the pressure of any gas between the particles, and the shape of the container. A rectangular box (such as a box of breakfast cereal) or cylinder (such as a can of nuts) works well to counter the effect, while a cone-shaped container results in what is known as the reverse Brazil nut effect.
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
May 2010
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References: http://en.wikipedia.org/wiki/Brazil_nut_effect
This link contains a video, but it is a large file and takes a long time to download:
http://jfi.uchicago.edu/~jaeger/group/Mixed%20Nuts.html
