Dear navig75-ga,
This effect applies to many granular materials: nuts, sand, sugar,
seeds and salt. It is known as the ?Brazil Nut Effect? and the reasons
are not yet completely understood.
Donna Francis - Researcher / Chemistry and Materials Science, Ontario
Science Center, gives the following explanation:
"Imagine a tall cannister of sand. When shaken up and down, the grains
actually flow in a pattern, with individual grains moving upwards
through the middle, across the surface, and down along the sides. This
pattern is known as convection flow.
Now, if you add a marble, for example, to the sand, the larger marble
will get caught up in the convection flow and move to the top of the
sand. Once at the top, it will stay there, because the convection
currents are too narrow for the marble to sweep downwards along the
wall with the sand."
There is further information on this page.
http://www.ontariosciencecentre.ca/scizone/brainz/francis/granular.asp
Researchers at the University of Chicago have been studying this
effect and have produced an animation on the Brazil Nut Effect with
some detailed information on the physics involved. Keep clicking play
progress through the animation (Macromedia Flash required)
http://jfi.uchicago.edu/~jaeger/group/brazilnut.html
The University Magazine reports that,
"In the November 15 issue of Nature, Nagel and colleagues pointed out
flaws in previous attempts to understand the "Brazil-nut effect," or
why the first person to open a box of muesli gets all the big pieces
and the last helping contains only crumbled oats. Theorists since the
1930s have blamed smaller grains for slipping into the spaces created
beneath larger particles. Others claim that everything rises when
shaken but only the smaller bits find room to descend. The Chicago
physicists suggested that we can no longer simply blame the little
guys; the problem is far too complex.
Not only must grains, nuts, and fruit be considered, Nagel and his
colleagues suggest, but also the air between particles. "Our results,"
they conclude, "indicate an intricate interplay between
vibration-induced convection and fluidization, drag by interstitial
air, and intruder motion." In other words, both the smaller particles
and the air between particles act like fluids, so variations of air
pressure within the box alter how the nuts "float." Despite this
discovery, no one has yet developed a pressurized cereal box."
http://magazine.uchicago.edu/0202/features/physics.html
This is the summary of the Nature article,
"Mounting evidence indicates that differences in particle density
affect size separation in mixtures of granular particles. We show here
that this density dependence does not follow a steady trend but is
non-monotonic and sensitive to background air pressure. Our results
indicate that particle density and interstitial air must both be
considered in size segregation."
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v414/n6861/abs/414270a0_fs.html
Additional information with an experiment you can try yourself.
http://www.seed.slb.com/en/scictr/lab/brazilnut/
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder
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