The Science of Coffee Rings

Picture:Marina Dominguez/NPR

NPR has an interesting article on the science of coffee rings. Simple things like coffee rings can seem like they're easy to understand but in fact they're complicated. It's taken physicists more than a decade to figure out why this effect, known technically as "the coffee ring effect," happens.

Coffee, like many liquids, contains tiny, spherical particles. When a drop of the liquid dries, forces push the particles toward the edge, where they are deposited in a thick line. We love this thick line of coffee, and when it happens it's because spherical particles evaporated instead of long particles which just produce spread stains with no ring.

"When spheres reach the surface of the drop, their shape does not induce the same deformation." Without the deformation, the particles travel to the edge of the drop and form a ring.

Check out this video to see the particles in action:

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