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I have this bowl of sodium polyacrylate (Insta-Snow) that has been drying out for a few weeks. At this point it consists of solid pieces ranging in size from 3 or 4 mm across to much smaller ones that form a fine powder. I can stir them around and mix them but if I hold the bowl at a slight angle and shake it for few seconds they sort themselves out with the smallest particles at the higher part of the bowl and the largest ones at the bottom. The behaviour reminds me of seeing sand grains sorted on a beach by the action of waves.
How does this self sorting happen?
If I put the particles in a cylindrical cup and shake them they sort themselves the opposite way with the largest particles on top but in the bowl it's the finest particles on top. This seems to be a case of the reverse Brazil nut effect. Clearly interactions between the particles and the container are part of the explanation.
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