In the world of Sid Nagel and Tom Witten, a dried-up puddle of coffee on a kitchen counter is not so much sloppiness as it is a beautiful example of nature working in mysterious ways.
Most people wouldn’t find bad housekeeping’s telltale brown splotch mysterious at all, rather a slight mess easily cleaned up with a damp cloth. Five years ago, however, Nagel and Witten, physicists at the University of Chicago, noticed how puddled coffee on a flat surface dries up, leaving a dark, ridge-like ring at its outer edge instead of a more uniform stain.
Puzzled over what on the surface appeared to be a simple phenomenon, they soon discovered that nobody else had figured out an answer to it either.
So they embarked on an unlikely–to the layperson perhaps even ridiculous–quest to help them figure out why coffee stains form the way they do, along the way assembling a formidable team of physicists, chemists, mathematicians and computer scientists.
Incredibly, the answer they have come up with turns out to have implications for a range of industries and was deemed important enough to be a featured article in Thursday’s issue of Nature, the prestigious science journal. Normally squirreled away in their offices and laboratories at the university’s James Franck Institute, the two scientists spent much of Wednesday granting interviews on their discovery to newspaper and television reporters from as far away as England.
Understanding how spilled coffee–or any other fluid containing minute, solid particles–dries may one day be used to make better computer memory chips or more durable housepaint. That might mean money, and so, of course, many people might be interested in the process.
Why Nagel and Witten got interested in spilled coffee also offers a tantalizing glimpse at how top research scientists approach their work.
“I don’t even remember for sure how it began,” said Nagel, 49. “I was just standing bleary-eyed one morning at my kitchen counter, thinking about nothing in particular, when I noticed how a splotch of coffee from the day before had dried.
“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 through the whole stain. I kept thinking about it through the day, and that night I was walking home with Tom (Witten), and I asked him what he thought.”
Witten, 53, remembers it well.
“I started spouting all kinds of pompous theories,” he said, “but I soon had to stop and admit I didn’t have a clue. It went against normal principles of materials suspended in liquid spreading randomly and as uniformly as possible.
“The old saying goes, `Nature abhors a vacuum,’ but Sid’s coffee stain was saying something otherwise.”
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Some of the best science to be done, Witten said, is with a phenomenon so commonplace that nobody realizes it is occurring or worth studying, and Nagel’s coffee stain soon turned out to be a prime example of that.
“If you observe something in nature that you can’t explain, and if you discover that it is a widespread phenomenon, you have probably discovered something that influences a lot of things that happen around us,” Witten said. “Then you have something worthy of serious scientific inquiry.”
As a start, the two men took the coffee-stain mystery to the weekly bag lunch session that Franck Institute scientists established years ago to discuss research problems that individuals might be encountering in their work. The point is to bounce ideas off one another to stimulate solutions or different research approaches.
“The week we went in with the coffee-stain problem, there were only three or four other people there,” Nagel said, “but they got so excited by the description of the problem, it was the talk of the building.
“The next week, we had a full house at the lunch. People were pouring coffee all over the place. Everybody had explanations for it. More and more people got involved in coming up with theories and doing experiments– people from math, computer science, chemistry, theoretical and experimental physics and at all levels from undergraduate to senior faculty.”
While institute scientists went about their more conventional research, they also each kept pecking away at the coffee-stain problem. Weeks and months passed with no answers, however.
“This was very frustrating,” Nagel said, “until 1995, when I had an undergraduate, Olgica Bakajin (now a Princeton University graduate student), working in my lab.”
Bakajin, he said, took a sample solution and covered up all but a small, middle portion of the puddle, allowing evaporation to take place only there. In that case, the particles in the solution did not travel to the edge of the puddle, but settled in the drying center.
Armed with this new observation, several scientists devised new experiments, much of the important theoretical modeling carried out by Todd Dupont, professor of computer science and mathematics. Most of the physical experiments were carried out by a graduate physics student, Robert Deegan, who is given top billing by the other scientists in the Nature article (Bakajin ranked second billing and Dupont third).
The answer the joint scientific endeavor produced on the spilled-coffee mystery was this: The liquid puddles out until it is stopped or “pinned” by roughness on the surface it travels across. Evaporation along those edges becomes a capillary “pull” that draws tiny coffee grains dispersed evenly in the liquid out to the edge. By the time evaporation is complete, virtually all the coffee particles are at the edge, thus creating the telltale dark rim or ring.
Such discovery hardly ranks with the theory of relativity, of course. But it opens up new, practical avenues to explore.
Paint manufacturers, for example, will be looking at it because it might show them a better way to deposit liquid-suspended solids in a more controlled, uniform way. Electronic and computer engineers will be looking at it as a possible new technology in drawing infinitely fine lines for improved electronic and computer circuitry. And molecular biologists are looking at the process as a way of helping them understand enzyme structures through protein crystallization, Nagel said.
Finding serious science in commonplace phenomenon is nothing new to Nagel. He once started wondering why Brazil nuts always seemed to rise to the top in cans of mixed nuts. The underlying principle that he discovered has changed the way pharmaceutical companies make pills, construction companies build roadbeds and transportation companies ship and store granular cargoes like grain.
“I love the mystery and beauty of watching and discovering how nature works, like in a coffee stain,” Nagel said in an office cluttered with toys that demonstrate scientific principles. “That’s what I try to get students to see: the really beautiful aesthetic side to nature.
“I can’t help stopping and looking at any stain I spot on the street now.”