Coffee ring
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In physics, a coffee ring is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine.
Writing in Nature, Robert D. Deegan of The University of Chicago and coworkers show that the pattern is due to capillary flow induced by the differential evaporation rates across the drop: liquid evaporating from the edge is replenished by liquid from the interior. The resulting edgeward flow can carry nearly all the dispersed material to the edge.
[edit] References
R. D. Deegan and others 1997. "Capillary flow as the cause of ring stains from dried liquid drops", Nature, volume 389, pp827-829
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