Do the Biggest Nuts Always Come on Top When Shaken?

The name comes from the fact that Brazil nuts, in a container of mixed nuts, always come on top when the container is shaken, even though they are the largest and heaviest in the bunch. This phenomenon also happens with other large-sized elements in a mix, like cereals in a muesli mix, but nobody has managed to predict or control the process.

Somehow, a system of particles different in size and density manages to sort itself through a variety of mechanisms, which makes the smaller one migrate downwards and the larger ones upwards. However, a team of physicists led by Matthias Schroeter at the University of Texas at Austin claim to have found that this doesn't happen all the time.

In fact, they say that by shaking a can with a mixture of particles for a long time, abrupt transitions occur that can reverse this unusual process. They experimented on a glass tube filled with a mixture of small glass spheres, only 1.4 millimeters in diameter and larger ones, made of brass, with a diameter of 2.4 millimeters.

By violently shacking the tube up and down, at a rate of 20 shakes per second, generating an acceleration five times bigger than that of gravity, for one hour, they observed that, though the system had initially reversed the Brazil nut effect, 25 hours of continuous shaking later, the larger brass spheres suddenly started coming all at the top of the test tube.

Curiously, the system then reverted to the initial, "positive" state, after they resumed the shaking, where it remained for an additional 180 hours of the experiment. Schroeter thinks that this sudden change was in fact caused by increased friction between the spheres, which became scratched and scuffed after hours of vibration.

His conclusion is that there is a continuous competition that manifests between gravity, which pulls the largest particles downward and convection, a phenomenon believed to be responsible for pushing them upward. Increasing friction actually increases the rate of convection until this becomes the dominant force in the system, thus reversing the process.

So, they proved for the first time that there is no absolute law that makes the larger and heavier particles come to the surface, but rather a combination of factors that could be altered to change the result of their combined actions.