Why Are Dried Coffee Stains Darker At The Edges Than In The Center?
If you spill coffee anywhere near as often as I do, you may have noticed something peculiar: When a puddle of coffee is allowed to dry undisturbed, the brown sediment collects almost entirely on the edge of the spill rather than being spread out evenly. It was not until recently that we could explain why a drop of coffee (or wine, or ink) dries this way. The theory of coffee ring formation was published in 1997 by a group of University of Chicago physicists. Lest you think it entirely academic, this curiosity of fluid dynamics is a problem in the world of inkjet printers, and there is serious technological interest in overcoming it.
The actual mechanism is somewhat like this: Water evaporates faster from the exposed edges of a coffee drop than from the interior. For this reason, you might expect that the drop would shrink in area as it dries. In practice, however, the drop edge gets pinned by bits of solid material or by the texture of the table, and this prevents the edge from receding. In order to accommodate the slightly faster evaporation at the edges, the liquid at the center flows outwards. The molecules and solid bits floating in the water get swept along in this outward flow, and as the drop continues to dry, they pile up at the edge like windblown snow against a fence.
Researchers trying to turn off the coffee ring effect have largely done so by counterbalancing it with surface tension effects that recirculate the particles during drying. Recent work from the University of Pennsylvania has shown that irregularly-shaped particles are able to resist ring formation by linking up into big floating rafts. Not everyone is trying to squelch the coffee ring effect, though. Several research articles have shown that it can be employed to gently lay out arrays of particles on surfaces, making it a possible tool for micro-manufacturing.
If you set out to watch the coffee ring form on your desk, you'll find that the whole process is about as fast as, well, watching coffee dry. Fortunately for us, our friends at Penn captured the process in time-lapse in a video explaining their recent work:
Andrew Koltonow is a graduate student in Materials Science and Engineering at Northwestern University.