Brazil nut effect 697387 206019699 2008-04-16T14:03:48Z ClueBot 4928500 Reverting possible vandalism by [[Special:Contributions/151.205.243.50|151.205.243.50]] to version by Melchoir. False positive? [[User:ClueBot/FalsePositives|Report it]]. Thanks, [[User:ClueBot]]. (324099) (Bot) {{Refimprove|date=February 2008}} [[Image:Mixed nuts small white2.jpg|right|thumb|Brazil nuts ride on top of peanuts]] [[Image:Brazilnut.jpg|right|thumb|Brazil nut with shell]] [[Image:Mixed nuts.jpg|thumb|right|Mixed nuts]] 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 [[volume|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 [[buoyancy|buoyant]] or sink. *The phenomenon is an example of [[Parrondo's paradox]], an idea in [[game theory]]. 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 [[density|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 (geometry)|cylinder]] (such as a can of nuts) works well to counter the effect,{{Fact|date=February 2008}} while a [[cone (geometry)|cone]]-shaped container results in what is known as the '''reverse Brazil nut effect'''. The effect may also be of interest to [[geophysics|geophysicists]] or [[geologist]]s 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]] [[asteroid]]s, for example the asteroid [[25143 Itokawa]].<ref>[http://antwrp.gsfc.nasa.gov/apod/ap070422.html Smooth Sections of Asteroid Itokawa] NASA Astronomy Picture of the Day 22 April 2007.</ref> ==See also== *[[Cheerio effect]] ==References== <references/> ==External links== *[http://physicsweb.org/article/news/5/4/5 PhysicsWeb: The Brazil Nut Effect] *[http://www.iphy.ac.cn/English/news/..%5C..%5Cchinese%5Ckjdt%5C2003071702.pdf Physical Review Letters: Effects of Air on the Segregation of Particles] *[http://mtrenouf.googlepages.com/simulationresults The Brazil Nut Effect: Numerical Simulation] Example of the numerical simulation of the Brazil Nut Effect. [[Category:Granular materials]] [[Category:Convection]] [[cs:Efekt paraořechu]] [[de:Paranuss-Effekt]] [[zh:巴西果效应]]