Why the Brazil nuts are on top: Size segregation of particulate matter by shaking

When a can containing one large ball and a number of smaller ones is shaken, the large ball rises to the top, even when the larger ball is more dense than the others. Similarly, a mixture of different sized particles will segregate by size when shaken. An adaptation of the Monte Carlo method is used to study this size segregation. The results show the local, geometric mechanism by which the segregation is produced. Segregation by size is to be distinguished from the more obvious sifting process which occurs when tiny grains filter down through the interstices between large particles.