TRUTH AND NUTS COME TO THE SURFACE

Take a pop-corn bowl. Fill it half with rice. Take hand-full of peanuts. Put on the rice. Mix them well. Now shake the bowl. Slowly, all the nuts will come to the top and the rice will settle at the bottom. This is called 'Brazil nut effect or granular convection'.

Why this happens? There are two reasons.
1. The system wants to lower the center of gravity and to occupy minimum energy level.
2. Small particles easily slip through the gaps between the larger particles and reach the bottom.

In the case of salt and sand. sand rises to the top.
Suppose, you want to reverse Brazil-nut effect, there is a way. Use a 'cone' shaped container. In cone, smaller particles come to the top because the gaps or the space decreases as you go down. The principle can be used for filtering particles according to size.

The plowing of a field always brings up new stones and boulders due to 'granular convection'.
Gravity is the driving force in this 'effect'. Stronger the gravity, faster the convection. Granular convection will be slow on the moon.
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