Scientists Crack The Physics Of Coffee Rings

by  Joe Palca

Scientists now know why coffee rings have dark, well-defined edges, as seen in the image above. The research finding may have implications on the development of inks and paints.

Marina Dominguez/NPR

A lot of simple things in science turn out to be quite complicated. Take, for example, coffee: You may have noticed that a spilled drop of coffee doesn't dry as a brown blob, but rather as a clear blob with a dark ring around the edge.

It's taken physicists more than a decade to figure out why this effect, known technically as "the coffee ring effect," happens. But now they think they have an answer.

The scientists who cracked the problem weren't initially studying the coffee ring effect at all. Peter Yunker and his colleagues at the University of Pennsylvania were studying how different-shaped particles — like spheres, egg-shaped, or even more elongated particles — pack together when the liquid they are in evaporates.

Ring Or No Ring?

Coffee, like many liquids, contains tiny, spherical particles (see the video below). When a drop of the liquid dries, forces push the particles toward the edge, where they are deposited in a thick line.

Peter J. Yunker and Arjun G. Yodh/University of Pennsylvania

When the particles in a liquid are elongated, they slightly deform the surface of a drop of liquid, changing the forces inside the drop. When it dries, a solid layer is formed, rather than a ring.

Peter J. Yunker and Arjun G. Yodh/University of Pennsylvania

So first they looked at what happened when liquids with spherical particles evaporated; these formed a ring like coffee does.

"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," Yunker says.

This was the aha moment: Maybe it was the shape of the particles that were responsible for the coffee ring effect. Coffee does have particles in it, but Yunker didn't know whether they were spherical or not.

So he did what any good scientist would do: "We went down to the building 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," he says.

Deforming The Drop

That might be enough proof for most people. But Yunker wanted to look at the effect in conditions he could control precisely, like the size of the particles and their exact shape. So he decided to make particles that could easily be manipulated.

"Our particles were made from polystyrene, so they're just plastic particles," he says.

Sure enough, when he let a drop of liquid with round particles in it evaporate, it formed a ring. When he tried it with elongated particles, he saw no ring. So why did the shape make a difference?

"When an elongated particle reaches the surface of the drop, it deforms the surface," Yunker says. Deforming the surface of the drop seems to be the key. "When spheres reach the surface of the drop, their shape does not induce the same deformation."

Without the deformation, the particles travel to the edge of the drop and form a ring. With the deformation that the elliptical particles cause, there's no ring. (You can see this effect in the video at the bottom of this page.)

Results of the study (which was not sponsored by Starbucks) appear in the journal Nature.

From Coffee To Printers

Arjun Yodh, director of the Laboratory for Research on the Structure of Matter at the University of Pennsylvania, was a co-author of the paper.

"At some level it was a curiosity, but then, actually, there's a lot of interesting physics about why it happens," Yodh says. And there are practical applications that go beyond coffee. "A lot of times when you're drying something, you'd rather make it uniform than to make it all congregate to the edge."

Think of a thin film of paint or ink from an inkjet printer — you don't want darker edges around each letter in a document.

Joan Curry, a chemist at the University of Arizona, says the new research appears to have solved that problem. "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."

Shape Matters

This video, produced by the University of Pennsylvania, shows what the coffee ring effect looks like under a microscope. Watch how round particles speed out to the edges while elongated particles stay in place as the liquid dries. Some of the footage has been sped up 25 times.