Physicists have worked out what causes 'the coffee ring effect,' at least on a droplet-by-droplet scale, and how to circumvent it.

Many of us have noticed that coffee drops, spilled on a counter, will not leave perfectly round smudges of color. Instead, the spilled coffee leaves a ring around the edge of places where the coffee droplets fell. This 'coffee ring effect' isn't a problem to wipe up with a sponge. But it becomes frustrating when you're dealing with paint or pigment samples that look fine when they're applied, but dry unevenly. Fortunately, physics rides to the rescue. Scientists at the University of Pennsylvania discovered that to change the effect, you simply need to change the shape of the particles involved. Here's how that works.

When a bead of water dries up it doesn't generally shrink into a smaller bead; it flattens. Most readers have observed water on a kitchen or bathroom counter slowly going from fat beads to shallower puddles to sheer slicks before drying up. The molecules of water can be attracted to certain surfaces. This means they hug close to the surface, the way the molecules in milk hug the sides of some containers so much that they 'climb' the walls slightly. Because of this, when the edge of the liquid evaporates new molecules of the liquid are 'pulled in' from the center to take their place. This makes the liquid flow from the middle to the sides of the drop, where it is finally evaporated. The liquid takes the pigment particles with it, leaving a clear middle and dark edges on a stain.


But not all particles go with the flow. The University of Chicago scientists who first discovered the mechanism of the coffee ring effect used perfectly spherical particles, which break away from the surface of the water easily, allowing them to be transported from the middle of the droplet to the edges. Ellipsoid particles, on the other hand, are resistant to such flow. They dry much more evenly. If someone were to manufacture coffee that somehow broke into ellipsoid particles when suspended in water, it would leave nice, even-colored stains.

Currently, there's no coffee company interested in something like that.


Via University of Chicago and Nature.