Nut theory gets a shakedown
Physics of muesli almost passes lab test.
15 January 2003
PHILIP BALL
Whether the first person to open the muesli, or the one who shakes out the last of it, gets all the nuts depends on how vigorously you shake the packet, and how full it is, say researchers in Germany.
Ingo Rehberg and colleagues at the University of Bayreuth have been shaking glass, wooden and metal beads of different sizes in a glass cylinder at various speeds and by various amounts. This can make the larger ones either rise to the top or sink to the bottom, they find1.
Size segregation by shaking is called the Brazil-nut effect - in a box of muesli, all the big bits, such as Brazil nuts, are often at the top. In transit to and from the grocery store the nuts rise up through the oats and bran. The same effect can influence mixing in industrial processes involving sand, cement, gravel, foods, powdered drugs and pigments.
But the Brazil-nut effect is not easy to explain. Some argue that larger grains sieve smaller ones; others invoke friction between the grains and vessel walls; yet others suggest that several of these things happen at once.
In 1998 scientists found a further complication: sometimes the big grains go down instead of up. Three years later a team of physicists in the United States and Germany theorized that this 'reverse Brazil-nut' effect depends on the size and weight of the grains2. They proposed that the change from the 'Brazil-nut' to the 'reverse Brazil-nut' effect happens at a certain ratio and density of small and large beads.
In about 8 out of 10 cases the theory predicts this crossover point correctly, Rehberg and colleagues now find. But other factors also turn out to determine what sinks and what rises. For exampl the either outcome is possible in the same mixture, depending on how quickly the shaking makes the beads accelerate; this depends both on shaking frequency and amplitude.
And to get a reverse Brazil-nut effect, with large beads sinking downwards, the layer of small beads initially below must not be too deep. In other words, Brazil nuts might never make it to the bottom of a really tall packet of muesli.
References
1. Breu, A. P. J., Ensner, H.-M., Kruelle, C. A. & Rehberg, I. Reversing the Brazil-nut effect: competition between percolation and condensation. Physical ReviewLetters, 90, 014302, (2003). |Article|
2. Hong, D. C., Quinn, P. V. & Luding, S. Reverse Brazil nut problem: competition between percolation and condensation. Physical Review Letters, 86, 3423 - 3426, (2001). |Article|
© Nature News Service / Macmillan Magazines Ltd 2002 |
