Question 12/96

When a coffee drop dries out, the coffee doesn't stay uniform; instead it concentrates in a ring around the boundary. The same thing happens when almost any drop dries. What makes the coffee become so nonuniform?

In case you have never seen a stain of a dried drop, R. Deegan provided for you the pictures of dried coffee (left) and dried latex (right):

This question was asked by R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel and T.A. Witten

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