Discussion of the Question 12/96
The question was: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?
Jonathan Baugh (baugh@physics.unc.edu) wrote on 3/19/97:
If the surface that the drop sits on is not too smooth, then the diameter of the drop will remain relatively fixed over the duration of evaporation. Since the rate of evaporation is essentially uniform over the entire surface of the drop, the boundary can only remain fixed if there is a flow of liquid from the center outward. This flow tends to collect particulates at the edge, giving the familiar ring shaped stain. Anyway, that's my short, hand-waving theory.
Y. Kantor: I think that Jonathan is moving the right direction: the boundaries of the drop are indeed fixed during the process of evaporation. One needs, however, to get some more quantitative estimate regarding the evaporation rates from the different parts of the drop, and estimate what would be the current inside the drop. It seems that for small drops one can assume that the shape of the surface of the drop is parabolic.
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