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Scientists crack the physics of coffee rings

from NPR

Two papers, one in Nature, the other in Physical Review Letters, provide a topic of conversation during coffee breaks, namely the coffee ring effect. Spill a drop of coffee on a substrate and after the stain dries you will see that the outer edge of the stain is darker than the rest of the stain. The question is, "Why does all the material suspended in a drop of coffee end up at the edge when the drop evaporates, considering that it started out dispersed across the whole drop?" The answer lies in the colloidal nature of the fluid and capillary fluid dynamics. more

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