How Come?: Coffee's brown ring

Tea stains on your shirt. Saltwater dried on a sidewalk. Red wine spilled on a white carpet. Milk spilled on a dark carpet. The familiar result? Rings.

But if there's one ring to rule them all, it's the coffee ring. In fact, the phenomenon of particle-laden liquids leaving stain rings behind is known as the Coffee Ring Effect.

So why do dried-out drips leave circles instead of solid splotches? As it turns out, the coffee stain problem wasn't solved by scientists until 1997. Then, University of Chicago researchers did a series of experiments, using particles suspended in liquids.

Once liquid is spilled, evaporation begins. At the point where liquid meets air -- say, at the surface of a drop of coffee -- molecules make their escape. Shaking off their chains to their chums in the drop, some energetic water molecules break free, jetting off into the air in all directions.

The researchers found that the edges of drips evaporate more quickly than the bulging centers. They also discovered that any bit of surface roughness caught or "pinned" the particle-laden edge. (Metal, plastic, glass -- all common surfaces tested caused the pinning effect.) So as the liquid evaporates, the edge doesn't shrink back to the center.

As the edge holds steady, it continues to lose liquid to the air. But the liquid is replaced, at least while the drop lasts. How? The water in the center of the drop is drawn to the evaporating edge in a kind of "capillary" motion, carrying particles (say, of coffee) along for the ride. So the center fades to beige, even as a brown ring of marooned particles builds up. Depending on where the coffee plopped (say, the front of your white T-shirt), a hard-to-remove stain may be the result.

But coffee (and other) rings don't just make for fun physics experiments. Scientists have found that different-sized particles suspended in water can form separate rings within rings. This may lead, researchers say, to medical tests that can rapidly separate bacteria, proteins and human cells in a watery mixture, usable even in areas without electricity.

Now, a new study at the University of Pennsylvania, published just last week in the journal Nature, shows how coffee-type rings can also be defeated.

The researchers say that the familiar ringed stains we see -- from dripped coffee to splashed saltwater -- form when roughly spherical particles flow from the center of a drop to the edge. Change the shape of the particles from round to oblong, they found, and the motion changes, too. The stretched-out particles, or ellipsoids, deform the surface of the liquid drop. So instead of streaming to the edge, the particles form clumps across the middle. Making for a splotchy stain, rather than a telltale ring.