Why does spilled coffee leave a ringlike stain after it dries? After boggling a team of physicists, this conundrum has finally been solved. The researchers spill the beans in tomorrow's Nature and say that the finding could help make ultrathin wires.
Sidney Nagel, a physicist at the University of Chicago, was moved to ponder the deep question of coffee rings while staring at his stained countertop one morning. Perplexed about how the rings form, he started bugging his colleagues for ideas. One suggested that tiny electrical charges around the ring might be trapping dissolved coffee particles, but calculations indicated it would take hours for all of the particles to migrate to the border--much longer than the drying time. No chemical constituent of coffee seemed to explain the phenomena, as all kinds of spilled liquids--red wine, milk, tea, and soup, to name a few--leave rings. The rings would form under the most trying circumstances: Even drying the drop upside down or putting it on ultraclean silicon made no difference. "We were at sea," recalls University of Chicago physicist Thomas Witten. "We asked all the experts, and no one knew."
Then grad student Olgica Bakajin did what Witten calls "an innocent little experiment" that cracked the case. Before a drop dried, Bakajin covered almost all of it. A ring formed only where the drop had been exposed to air. Analyzing this quirky behavior, University of Chicago computer scientist Todd Dupont suggested that evaporation might happen more quickly at a drop's edge, where water molecules have a better chance of escaping into the air. Since surface tension maintains the drop's shape, liquid would tend to flow toward the edge. When grad student Rob Deegan videotaped a solution of tiny spheres in distilled water, they observed the spheres travel and accumulate into a ring, just as predicted.
The whimsical experiments may have a practical payoff. Witten says that by suspending gold particles in a drop, researchers may be able to fashion tiny micron-thick wires as the liquid dries. Paul Weiss, a chemist at Pennsylvania State University, ran to the lab bench when he heard about the work. "I saw [Nagel's] talk, and 10 minutes later we were doing experiments," he says. "I've never done that before." Weiss is now using the flow in evaporating drops to stretch out DNA for sequencing.