Take a container of mixed nuts, shake it and examine the contents. The chances are that the largest nuts, Brazil nuts for example, have risen to the top.
This phenomenon is called the Brazil nut effect and is known to depend on a number of factors, such as the container size and shape as well as the frequency and amplitude of the shaking. However, the general idea is that the shaking process creates voids that smaller, but not larger, particles can drop in to. This forces larger particles to the surface.
This kind of convection is thought to play an important role in granular media. For example, the texture of certain asteroids can only be explained by the Brazil nut effect in whihc larger particles are forced to the surface.
A key question in understanding this process is how the reduced force of gravity on other astronomical bodies might influence the Brazil nut effect.
Today we get an answer thanks to the work of Carsten Guttler from the Braunschweig University of Technology in Germany and a few pals.
With an admirable to duty, these guys have performed standard Brazil nut effect experiments in an ordinary lab but also on an A300 Airbus flying parabolic arcs to simulate gravitational forces on the Moon and on Mars. Their experimental equipment consists of a shaker and a camera to record what goes on inside their see-through experimental container.
Before each flight, Guttler and co placed a small number of 8mm green glass beads at the bottom of the container, which they then fill up with smaller 1 mm glass beads.
As the container begins to shake, they can see the movement of the bigger green beads in the container.
The results are straightforward (although a little ambiguous). They say that stronger gravitational forces lead to a bigger Brazil nut effect. In fact, they say that the relationship is roughly linear– more gravity causes the large beads to rise faster.
In other words, big nuts rise faster on Earth than they do on Mars or the moon (a phrase that’s not likely to find it’s way in to many scientific articles).
This is not an entirely whimsical result. A better understanding of the Brazil nut effect could improve our knowledge of the surface of Mars and the Moon. In addition, a good understanding of granular media and the way it behaves could be crucial when it comes to mining asteroids in low gravity conditions.
This last scenario may need a little extra work. Guttler and co say that when gravity is very weak, other forces become important such as cohesion between particles. So the linear relationship that they discovered may not extend into these low gravity regimes.
But that’s as good a reason as any to take to the skies fly a few more experiments on a vomit comet.
Ref: arxiv.org/abs/1304.0569: Granular Convection And The Brazil Nut Effect In Reduced Gravity