Explore the complicated forces of breakfast cereals and bar snacks — and find out why Brazil nuts always rise to the top of shaken jar of nuts.
Sometimes you feel like a nut. Sometimes you don't. Whether you do or don't, doesn't it always seem that the biggest nuts always come out to greet you? What's true in social life is also true in cereal or in jars of mixed nuts. When a jar is shaken enough (through vertical vibration) the brazil nuts always pop out on top. This form of self-segregation by size is found in all kinds of particulate mixes. But what causes it?
The movement of small particles in large groups is called convection. Usually the term is applied to gases or fluids, but granular convection is much studied as well – especially by people who transport or store food. They've found that particles in a cylinder or rectangle subjected to vertical vibrations tend to get forced upwards from the center of the container. As they travel upwards they leave empty spaces, and particles from the side slide inwards. They, in turn, leave empty spaces, which particles above and to the sides of them slide in to fill. (In this way, the particles resemble the convection of heat. The air in a fire is heated and travels upwards, while more air slides in from the side, feeding more oxygen to the fire.)
Since particles from the center are always pushing upwards, once they reach the top and can't climb higher, they tumble to the sides. Since the particles on the sides of the container are falling down to fill the space left by the upward-moving particles in the center, once the particles tumble to the side, they get drawn down to the bottom of the container again.
Ah, but not the biggest of them. The biggest of them stay right on top. Sometimes, this has phenomenon has to do with density. If the large particles are less dense than the smaller ones and they pop to the top like a cork in water. But other time's it's just the granular interaction. In a phenomenon shown with coffee beans in a big patch of coffee grounds, or different-sized glass beads, or the eponymous Brazil nuts, the big particles rise to the top through normal convection. They stay on top not because they're less dense, but because they can't fall back down. One popular theory of why is that the large particles – the brazil nuts – can't fit back down at the sides of the container. Smaller particles can slip down the small holes left by other small particles heading downwards, but unless a large pit opens up it would be like trying to shove a house down a prairie-dog hole.
But there's a wrinkle. Studies have shown that density does have a role in the Brazil nut effect. If the large particles are much less dense than their surrounding particles, they rise to the top and stay. If they are much denser than their surrounding particles – they also rise to the top and stay. Those that have a small difference in density from the other particles tend to remain mixed. And all this density dependence stops if the particles are in a vacuum. It seems that the simple course that a Brazil nut takes through life is dependent not only on its surrounding nuts, and on its density, but on air pressure as well. Sometimes physics really does feel like a nut.