The University of Chicago Magazine
Physics professor Sidney Nagel was about to enjoy his morning cup of coffee several years ago when he noticed something he had seen countless times before: a dried-up coffee stain.
This time around, the ring-shaped stain gave him pause. According to traditional explanations of solvents and solutes, a coffee droplet, dried upon a countertop, should display a uniform dispersion of the coffee particles within the water that conveys them. In other words, it should dry to an even brown across its entire plane.
Which is why the brown ring puzzled Nagel. "I couldn't figure out why the little grains of coffee would have all moved to the edge of the stain, rather than scattering uniformly throughout the whole stain," he told the Chicago Tribune.
Walking home from work, he raised the coffee conundrum with fellow physics professor Tom Witten, who soon admitted that he, too, was confounded. They took the problem to the department's Friday brown-bag lunch. Soon two students-Robert Deegan, SM'96, and Olgica Bakajin, AB'96; three professors-Nagel, Witten, and computer scientist and mathematician Todd Dupont; and (former) research associate Greg Huber were hard at work. Several years of experiments later, they had some answers, published in the October 23 Nature.
As the coffee drop dries, several processes come into play. First, and perhaps most important, is "pinning." If the drop lands on a surface with some irregularities-just about any surface in the natural world-its edges get pinned into place and remain, even as the droplet loses molecules to drying.
To keep the edges of the drying drop in place, the molecules at its edge-where the droplet has the fewest molecules to begin with-must be replaced as they evaporate. So molecules from the center migrate outward, creating the ring of color that marks the drop's original periphery.
Nagel and his group then tested whether this effect holds with other solutions-from milk to wine to salt water. As long as a drop's contact line is pinned, the drying proceeds as predicted.
The results have implications for industries that rely on the uniform deposition of solids suspended in liquid media-paint, for example. The ability to deposit dispersed solids in a controlled fashion could also allow for the creation of tiny electronic circuits or provide a way to store information at high densities.
For more details, see the Materials
Research Science & Engineering Center's Web site -C.M.
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