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And as NPR's Joe Palca explains, it's taken them more than a decade to figure out why it happens.
JOE PALCA: First, they looked at what happened when liquids with spherical particles evaporated. These formed rings like coffee does.
PETER YUNKER: But when we evaporated the drop with the elongated particles, instead of forming a ring, they were spread out across the entire area covered by the drop.
PALCA: And this was the Aha Moment: Maybe it was the shape of the particles that were responsible for the coffee ring effect. Coffee has particles in it, but Yunker didn't know whether they were spherical or not. So he did what any good scientist would do.
YUNKER: We went down to the building coffee machine, put, you know, 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 in fact spherical in shape.
PALCA: That might be enough proof for most people. But Yunker wanted to look at the effect in conditions he could control precisely - the size of the particles and their exact shape.
YUNKER: Our particles were made from polystyrene, so they're just plastic particles.
PALCA: Sure enough, when he let a drop of liquid with spherical particles in it evaporate, they formed a ring. When he tried it with ellipsoid particles, no ring. So why did the shape make a difference?
YUNKER: When an elongated particle reaches the surface of the drop, then it deforms the surface.
PALCA: Deforming the surface of the drop seems to be the key.
YUNKER: When spheres reach the surface of the drop, their shape does not induce the same deformation.
PALCA: Without the deformation, the particles travel to the edge of the drop and form a ring. With the deformation like the one the ellipsoid particles cause, there's no ring. So there you have it. These results appear in the prestigious international science journal Nature.
ARJUN YODH: At some level it was a curiosity, but then, actually, there's a lot of interesting physics about why it happens.
PALCA: Now, this research was not sponsored by Starbucks. Yodh says there are practical applications that go beyond coffee.
YODH: A lot of times when you're drying something, you'd rather make it uniform than to make it all congregate to the edge.
PALCA: Joan Curry is a chemist at the University of Arizona. She says the new research appears to have solved that problem.
JOAN CURRY: They found a variable that they can tweak - apparently it's not too hard to do - and they can change whether this film is uniform or not.
PALCA: Joe Palca, NPR News, Washington.
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