SHAKE a can of mixed nuts and you鈥檒l generally expect to see the largest ones rise to the top. But shake hard enough, and you can actually produce the reverse effect. Apart from ensuring you don鈥檛 find all the peanuts at the bottom of your party selection, the discovery should lead to a better understanding of how granules mix together and help improve industrial processes such as making cement, granulated food or powdered drugs.
The so-called Brazil-nut effect, which has puzzled cocktail party guests for decades, occurs when smaller nuts fall down into the voids created as the bigger nuts jiggle around, forcing the larger ones to the top. But in 2001 Daniel Hong at Lehigh University in Bethlehem, Pennsylvania, predicted that under the right circumstances large particles could also sink. His trick was to think of the particles like molecules in a gas and to determine 鈥渢emperatures鈥� at which certain sizes of particle would 鈥渃ondense鈥� like steam in cold air and fall to the bottom.
Now Andreas Breu and colleagues at the University of Bayreuth in Germany have demonstrated the effect with a mixture of 4-millimetre glass beads and 10-millimetre polyurethane beads (Physical Review Letters, vol 90, p 014302). With only very slight shaking, analogous to low temperatures, the beads did not have enough energy to reorganise. 鈥淓ffectively the material has crystallised,鈥� says Breu.
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When the researchers shook the container harder, the familiar Brazil-nut effect occurred. But shaking the container even more violently, simulating high temperatures, gave the small particles enough energy to push up past the larger ones 鈥� the reverse Brazil-nut effect that Hong had predicted.
However, Breu says there is still a long way to go before the effect can be precisely controlled in industry. 鈥淥ur experiment with two types of particle is only the most basic system.鈥�