Next time you spill a little, let the coffeedry before wiping it up. You'll notice that after it dries there is a darkish ring around the very edge of the spill, whereas the middle disappears almost entirely.
What's going on here?
Robert Deegan and Sydney Nagel at the University of Chicago looked into this phenomenon, and they discovered something pretty interesting. The coffee puddle leaves a ring because it doesn't simply dry where it sits.
As the spill begins to dry, it actually creates its own outward motion, drawing the insides of the puddle toward the edges.
Why is that?
It's because the edges of the spill are where the fluid is at its thinnest. A spill sitting on a flat surface is something like a pancake. Big in the middle and thin on the rim.
As the thin edges evaporate, they are replenished by liquid and particles of coffee from the denser interior. That starts an outward push going in the puddle. The more it dries, the more particles get drawn out to the edges. When all of the liquid has evaporated we're left with a ring of coffee particles making a stain.
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