Why brazils always end up on top

The big bits at the top: A puzzling natural phenomenon

By BBC News Online's Helen Briggs

It is a question that has vexed some of the top minds in physics since the 1930s. And it is almost as big a puzzle for those fond of snacking on muesli or a bag of mixed nuts.

You do not need to always have great particle physics accelerators to answer the questions of physics

Dr Sidney Nagel, University of Chicago

Why do the larger bits, like Brazil nuts, always find their way to the top of the packet when you shake it?

Existing theory suggests vibrations cause both big and small items to rise together, but that the big nuts or pieces of fruit get stranded on top as the smaller pieces go on mixing.

Now, Chicago researchers have added a touch more complexity. Their study indicates that there is also an interaction going on between the air and the smaller particles that helps separate out the different sizes.

Spaghetti boiling

Dr Sidney Nagel, a physicist at the University of Chicago, Illinois, US, told BBC News Online: "If you open a can of mixed nuts that has been shaken up on the highway, you will always find that the big nuts have risen to the top and the small nuts are at the bottom.

"This is known as the Brazil nut effect. One explanation, which is not wrong, is that convection forces raise everything to the top, then strand big particles at the top.

"Even within that explanation there is room for more subtleties to come."

Convection forces cause large and small particles to rise together but only the smaller ones can descend to the bottom in the very thin stream down the container walls.

Anyone who has watched a pot of spaghetti boiling will be familiar with such convection rolls in a liquid heated from below.

In that case, however, the downward moving liquid occupies as much volume as the upward moving liquid in the centre.

Glass spheres

Now, it seems that the air between the particles may also be important and give rise to a particle velocity that depends not only on the particle's size but also on its density.

The University of Chicago team has come to this conclusion by carrying out experiments on granular materials, such as glass spheres, in a container.

They added a larger (intruder) particle and watched how it behaved as its density was changed and as a vacuum was pulled on the container.

They demonstrated that how quickly a big particle reached the top of the container depended not only on the size of the rest of the particles in the container but on the interaction of two fluids - air and the smaller bits.

Defy intuition

Their complex equation may lead to applications in industries where the separation of particles, such as grains or minerals, is important.

But it applies equally in the kitchen, with what happens in a cereal packet.

"You do not need to always have great particle physics accelerators to answer the questions of physics," says Dr Nagel.

"This is a perfect example of one of those situations when you look at some aspect of everyday life and it raises interesting questions which defy our intuition."

The research is published in the journal Nature.