Ringing up a coffee stain.

When a spilled drop of coffee dries on a countertop, it leaves behind a powdery residue concentrated in a ring marking the drop's edge. The stain's distinctive appearance suggests the action of some physical or chemical process that segregates particles, initially dispersed throughout the entire drop, as the liquid evaporates. Conventional mechanisms such as diffusion, convection, and wetting fail to account for the phenomenon.

Robert D. Deegan, Sidney R. Nagel, and their coworkers at the James Franck Institute of the University of Chicago have now discovered that the widespread effect is due to a particular type of capillary flow. The edge of the drop typically gets pinned in place by surface irregularities, so liquid evaporating from the edge, where the drop is thinnest, must be replenished by liquid streaming in from the drop's interior. "The resulting outward flow can carry virtually all of the dispersed material to the edge," the researchers report in the Oct. 23 Nature.

To demonstrate the effect and test their mathematical model, Deegan and his colleagues suspended polystyrene spheres, I micrometer in diameter, in a water droplet on a glass microscope slide. They then tracked the concentration and motion of the spheres as the water evaporated and the spheres collected at the drop's edge.

Because the measurements fit their model, ring deposition can be predicted and controlled without knowing the chemical nature of the liquid, the particles, or the surface, the researchers say. Indeed, ring deposits are common wherever drops containing dispersed solids evaporate on a surface. The phenomenon may someday serve as a method for printing a thin line or depositing a fine pattern on a surface.