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        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.




        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.





        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.


URL of video:


Comments

        I thought this phenomenon in a different but simpler way: it's just the

difference between the affinity between particles. As shown in the movie above,

adding soap can make the elongated properties become similar to spherical.

That arrises from the van der Waals force changes. In other ways, the spherical

particals have ease to release the repulsion for its smaller surface area.


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