FOR years physicists have studied the 鈥淏razil nut effect鈥� seen in mixtures of
large and small nuts or beads. When you shake such a mixture, the large ones
rise to the top (New 杏吧原创, 24 May 1997, p 25). Researchers thought
that denser beads should rise faster because they have more inertia to carry
them through the smaller grains.
But it turns out that the large beads actually rise more slowly until their
density passes a certain level, report Matthias M枚bius and colleagues from
the University of Chicago. Surprisingly, the researchers believe that drag
caused by air in the mixture is to blame, and the effect went away when they
sucked the air out (Nature, vol 141, p 270).