By Christopher Dombrowski, Ars Technica

Ever wonder why a drop of coffee leaves a ring behind when it dries? Physicists did. In 1997, they came up with a theory of how it works. It turned out to be such a universal theory that it shows up in a number of problems related to deposition of material. Since then physicists have been trying to find a way to get around it and stop making rings. Now a group of physicists in Chicago have done it.

So what is the coffee-ring effect? When a drop of coffee dries, its outer edges are pinned, so the radius does not change even as the amount of liquid shrinks. As the volume of the drop decreases from evaporation, the contact angle of the edge of the drop also decreases. This causes a radial capillary flow that carries coffee particles from the center of the drop to the edge, where they are deposited, forming a ring.

What researchers have shown is that the coffee-drop effect can be negated if the particles are not spherical. When ellipsoidal particles are transported to the drop edge, they form loosely packed structures that can resist the capillary flow. When the drop has completely evaporated, these particles are more or less evenly distributed. The more elongated the particles, the more uniform the deposition, providing a way to control the distribution of material.

The coffee-ring effect crops up when dealing with many methods of depositing materials. Having the ability to control the uniformity of deposition will be useful in fields such as coating and printing.

Image: Dano/Flickr

Source: Ars Technica

Citation: “Suppression of the coffee-ring effect by shape-dependent capillary interactions.” Peter J. Yunker, Tim Still, Matthew A. Lohr and A. G. Yodh. Nature, Vol. 476, Pg. 308–311, Aug 18, 2011. DOI: 10.1038/nature10344

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