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Brazil nut effect



     

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The Brazil nut effect is the name given to 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.

The phenomenon is also known as the muesli effect since it is seen in packets of breakfast cereal containing particles of different sizes but similar density, such as muesli mix.

It may be counter-intuitive to find 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 (containing the mixed nuts) in an arbitrary state is not optimal (because of the space close to the Brazil nuts); it has the tendency to be higher due to gravity. When the nuts are 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 or sink.

The effect is of serious interest for some manufacturing operations; once a heterogeneous mixture of granular materials has been produced, it is usually undesirable for the different particle types to segregate. Several factors determine the severity 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, while a cone-shaped container results in what is known as the reverse Brazil nut effect.

The effect may also be of interest to geophysicists or geologists when studying the effect of vibrations on sand or other loosely contacting materials.

In astronomy, it is also seen in some low density, or rubble pile asteroids, for example the asteroid 25143 Itokawa.[1]

References

  1. ^ Smooth Sections of Asteroid Itokawa NASA Astronomy Picture of the Day 22 April 2007.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Brazil_nut_effect". A list of authors is available in Wikipedia.
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