Coffee-ring effect

Settled in 1997 by Robert Deegan, Olgica Bakajin, Todd Dupont, Greg Huber, Sidney Nagel, Thomas Witten (University of Chicago).

Anyone who has left a coffee spill to dry has seen the dark ring form at the perimeter, but the quantitative cause was only identified in 1997 by a University of Chicago group led by Sidney Nagel and Thomas Witten. They showed that once a drop's contact line pins to the surface (as most real, non-ideal surfaces cause), evaporation is fastest at the thin edge, and capillary flow from the drop's interior rushes outward to replace that lost liquid, carrying nearly all suspended solids to the rim. The paper derived a power-law growth of the ring mass with time that is independent of the specific fluid or substrate, and the mechanism is now standard textbook material with major applications in inkjet printing and DNA microarray manufacturing.

One of the cleanest examples in physics of a big, universal effect (a scale-independent power law) explaining a phenomenon everyone has seen on their kitchen table.

Explanations that circulated

• That the ring is simply where the liquid 'runs out to' last (a vague evaporation-front idea) rather than the specific mechanism: a pinned contact line forces a capillary outflow that continuously replenishes evaporative loss at the edge, sweeping nearly all suspended particles there.

References

• Capillary flow as the cause of ring stains from dried liquid drops, R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, T. A. Witten, Nature, 1997