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In granulometry, say I have a tube with an heterogeneous mix of particles of different sizes (sand and stones, by instance). The tube is vertical in the usual gravity field and is placed in the air. If I randomly shake the tube, the smallest particles will eventually be at the bottom, and the largest ones at the top. So the particles are now sorted by size. Why?
A friend of mine considers a volume of the bottom part of the tube has a greater density (or mass) than of the top's. And denser things go downward.
I consider however, by shaking the particles, small ones can fall through the space left between larger ones.
What is the correct explanation?