Have you ever bought a bag of mixed nuts and noticed that Brazil nuts tend to be on the top of the pile? If you put nuts of various sizes in a bag and shake it, you will see that the larger nuts (such as Brazil nuts) will rise to the top slowly. This is strange as common sense dictates that heavier objects sink to the bottom. The strangest thing? No one knows exactly why this happens.
However, there are some persuasive theories. It has been suggested that the so-called Brazil nut effect is due to a phenomenon called granular convection. Convection is usually used to describe the movement of gases and liquids, where heated particles are more active and lighter, thus rising to the top. As the particles rise, they cool down and fall back to the bottom, creating a current. It seems to be that the same can be applied to solid particles, such as nuts. When the jar or bag of nuts is shaken, a vibrational force is applied. Nuts in the middle are pushed upwards by the vibration, with smaller particles filling the gap below. Once the nuts reach the top surface, the vibration pushes the nuts towards the side, where they are then pushed back to the bottom. However, larger nuts like Brazil nuts are too big to fit in this downward current so they stay on the top.
The Brazil nut effect is not exclusive to Brazil nuts. A similar phenomenon can be seen with any large particles surrounded by smaller particles, like pebbles in sand or coffee beans in ground coffee. The theory of granular convection has still not been fully understood, with various factors such as nut density and air pressure seeming to play a role.