At first glance, the coffee rings that form underneath your mug are nothing more than nuisances that challenge your knowledge of stain removal. Take a moment before wiping them away, though, and consider why the outer edges of the rings are darker than the inside. Stumped? Scientists were too, until University of Pennsylvania physicists discovered that the darkened outer edges are caused by something as simple as the shape of the particles in coffee and other similar liquids.
According to NPR, the physicists were researching how liquid particles of different shapes clump together as the liquids evaporate. When they realized that liquids with spherical particles tended to form strongly outlined stains similar to coffee rings, they woke up and smelled the coffee, so to speak: They went down to their office coffee machine, "put 35 cents in, got a cup of coffee, went back upstairs to the microscope, put it on a slide, took a look, and, at least on the micron scale, the particles that we saw were spherical in shape." Their hypothesis confirmed, the group began to play with particle shapes and sizes to determine why the shapes mattered.
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As it turns out, spherical particles tend to migrate to a liquid's edge as the liquid dries, which then forms the dark outline. In contrast, oval- and oblong-shaped particles clump together and spread towards the center of the liquid, leaving a more even stain. The video below shows in microscopic detail how particles in a drop of coffee move as the liquid evaporates. And yes, this is far more exciting than watching paint dry.
Explaining a coffee's rings is just a secondary benefit of the scientists' discovery. As they point out, knowing the science behind the ring effect may help engineers improve printer ink, paint, and other liquids so that they coat surfaces as evenly and uniformly as possible.
The results of the study were published in the most recent issue of Nature. As for those coffee stains, a little baking soda goes a long way.