Why Do Coffee Rings Form? Because the Grounds are Round

By Veronique Greenwood | August 17, 2011 1:00 pm

coffee ring
Those dark rings in the bottom of your cup arise from fundamental physics.

What’s the News: Some of the most mundane things in life—drinking through a straw, for instance, or washing your hands with soap—are the results of some really neat physics. Today, scientists are adding another item to that list: The ring that forms around a drying drop of coffee. A team at University of Pennsylvania has discovered that that brown ring is a result of the shape of the particles floating in your coffee—and if you squash them out a little, the coffee ring disappears.

How the Heck:

  • The ring forms from particles moving towards the edge of the drop as it dries. The drop’s edge is firmly stuck to the surface of, say, your desk, and as the water evaporates, the fluid that’s left moves outwards from the center, flattening the drop and taking the particles with it.
  • Now, zoom in on the surface of the drop. The particles of bean in your coffee, like many particles in nature, are spherical. This means they don’t bend the drop’s surface very much, so, drawn by capillary action, they can easily flee down the drop to the edges and congregate in that fine black ring. See this in action in the video below.
  • ellipses
    Spherical particles form a ring (right). But ellipsoid particles
    are spread evenly across the surface (left).

  • But if you stretch them a little into ellipsoids, it’s a different story. Ellipsoids bend the drop’s surface such that it doesn’t make energetic sense for them to slip down to the edges; instead, they begin to clump together on the surface. The end result is that when a mixture in which the particles are ellipsoids dries, the particles are distributed all over the stain, instead of at the edges. Using plastic particles of different shapes and sizes, the team found that this happened even when larger spheres were mixed in with the ellipsoids, though tiny spheres would still dodge around the clumps and make a break for the edge. Even a very small change in shape can have this effect, says Arjun Yodh, the senior author. Stretching a particle so that it’s 20% wider than it is long is enough.
  • To verify that surface deformation was indeed the culprit behind the ellipsoids’ behavior, the team added a surfactant, a substance that disturbs the surface tension, like soap. Sure enough, with the surface no longer tense enough to be deformed, the ellipsoids shot down to the drop’s edge and formed a ring.

What’s the Context:

  • The coffee ring effect, as it’s known, isn’t just for armchair physicists. It’s been quite a nuisance for scientists designing inks and paints, since, as you can imagine, a painted wall that dries with all the color bleeding to edges or a printed page where each letter is white in the middle is not a success. This hurdle has been addressed with workarounds like additives to paint that hold particles in place as they dry or printing dots so small that the effect isn’t noticeable. But this work shows that tweaking the shape of the particles might be enough, suggesting that manufacturers could swap out hazardous solvents, for instance, for ellipsoid particles.
  • A landmark 1997 paper in Nature ($) showing that capillary action was responsible for pulling the particles down to a drop’s edge sparked many scientists’ interest, and the past decade has seen a number of papers picking apart this phenomenon. This is the first time, though, that particle shape has been implicated.

The Future Holds: Changing the shapes of paint or ink particles would certainly have its difficulties, but attaching ellipses to the particles, or even just sprinkling in a handful of them, might be enough to counteract the coffee ring effect, says Peter Yunker, the lead author of the study. That’s another subject for research. And though these experiments demonstrate this effect qualitatively, there’s still a great deal of quantitative work to be done, says Yodh. The equations that govern particle shape and the coffee ring effect are still waiting to be fleshed out.

Reference: Yunker, Still, Lohr, and Yodh. Shape Dependent Capillary Interactions Undo the Coffee Ring Effect. Nature, 18 August 2011.

Image credit: Lightsurgery / flickr and Yunker et al., Nature.

CATEGORIZED UNDER: Physics & Math, Top Posts
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