Why the currents in muesli put nuts at the top

The vexing problem of why the first person to open a packet of muesli gets a bowl full of Brazil nuts while the last helping consists of dusty oats may have been solved after decades of investigation.

The vexing problem of why the first person to open a packet of muesli gets a bowl full of Brazil nuts while the last helping consists of dusty oats may have been solved after decades of investigation.

A team of physicists has found a possible explanation for the so-called Brazil-nut effect, which describes how grains of different sizes separate when shaken during transit, with the largest ones on the top and the smallest on the bottom.

The phenomenon, which was first described in the 1930s, was thought to be the result of a sieving effect, with smaller grains falling into the spaces created beneath larger grains as a muesli packet is shaken.

This simplistic view, however, was modified subsequently by muesli revisionists who suggested that shaking a packet of cereal produced a "granular convection current" with small and large grains rising, but with only the smaller grains descending again near the sides of the cereal packet.

But Sidney Nagel and his colleagues from the University of Chicago in Illinois believe both theories fail to take into account the role of air pockets in separating the grains, which act as if they were a fluid by pushing the nuts to the surface. In an experiment reported in the journal Nature, the scientists explain how they observed the movements of a large "spherical intruder" – a hollow plastic ball filled with lead shot – in a jar of glass beads as they were shaken up and down.

The ease with which the intruder moved to the surface depended on the background air pressure and the relative densities of the ingredients.

The problem should be seen in effect as the interplay between two moving fluids, the grains themselves and the air spaces separating them. Air pockets in the muesli packet are just as important in the generation of the Brazil-nut effect as the nuts, raisins and oats.

Dr Nagel said: "The actual effect is more complicated than we thought previously. Air pressure plays an important part, irrespective of the density of the grains. Our results indicate an intricate interplay between vibration-induced convection and fluidisation, drag by interstitial air and intruder motion."

Earlier this year, another team found that the reverse happened – bigger grains fell to the bottom – when the mixture contained more larger grains than smaller ones. The reverse Brazil-nut effect until then had been overlooked because of the focus on mixtures containing a minority of bigger grains.