A.D. Rosato, K. J. Strandburg, F. Prinz, R. H. Swendsen, “Why the Brazil Nuts Are On Top: Size Segregation of Particulate Matter by Shaking,” Phys. Rev. Lett. 58, 1038-1042 (1987)

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 large 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.

Here are some simulations that go with the paper.

Tap magnitude is ¼ of the small disk diameter. | |

Tap magnitude is ¾ of the small disk diameter. | |

Tap magnitude is the same as the small disk diameter. | |

Tap magnitude is 1¼ of the small disk diameter. | |

Tap magnitude is the same as the diameter of the small disks. The large disk diameter is 1½ times the small disk diameter. |