Brazil-nut effect

Settled in 1987 by Anthony Rosato, Katherine Strandburg, Friedrich Prinz, Robert Swendsen; later extended by Marcélo Möbius, T. C. Lauderdale, Sidney Nagel, Heinrich Jaeger.

The tendency of Brazil nuts to end up on top of a shaken can of mixed nuts is old kitchen and industrial-packing folklore. In 1987 Rosato, Strandburg, Prinz and Swendsen gave the first quantitative mechanism using Monte Carlo simulations: shaking opens small voids beneath a large particle that small grains preferentially percolate into, ratcheting the large particle upward even without any density difference. Subsequent work, notably Möbius, Lauderdale, Nagel and Jaeger in 2001, showed that container-wall-driven convection currents are often an equally or more important mechanism in real shaken beds, and that air interstitial pressure and vibration parameters matter too. The phenomenon itself is considered solved in the sense that its main physical mechanisms (percolation and convection) are established, though which one dominates depends on the specific experimental conditions.

A jar of mixed nuts spontaneously sorts itself by size using nothing but random shaking, a purely mechanical form of self-organization.

Explanations that circulated

• Big particles rise only because they are effectively 'lighter' or more buoyant than the small ones — false; the effect occurs even when the large particle is denser, since small grains percolate into the voids that open up beneath it during shaking, and wall-driven convection can independently push large particles upward.

References

• Why the Brazil nuts are on top: Size segregation of particulate matter by shaking, A. D. Rosato, K. J. Strandburg, F. Prinz, R. H. Swendsen, Physical Review Letters, 1987
• Brazil-nut effect: size separation of granular particles, M. E. Möbius, B. E. Lauderdale, S. R. Nagel, H. M. Jaeger, Nature, 2001