Ever notice that when you spill coffee over the edge of your cup it always produces a ring under the bottom edge?
There is a rather complex reason for this, but it can be summed up somewhat easily. Two main factors are at play: surface tension of the molecules of the liquid and the temperature of the surrounding environment.
When a drop of coffee is splashed outside of the cup, it has an initial “pinned” spot, and from there the surface tension within the liquid causes the molecules to spread and draw more liquid away from it.
The temperature of the surrounding area then comes into play as a difference in temperature between the liquid and the air causes evaporation to begin. When an evaporating drop is checked under a microscope, there is a strong outward flow of material as the particles stream toward the edge, rather than moving around randomly. As the process continues, the molecules of the liquid continue to draw towards the edge and, because of their surface tension, they continue to draw more molecules towards them to replace liquid that has already evaporated. This continuous flow piles the material up at the edges, where it eventually dries and forms a ring.
No matter what type of liquid or different types of surface on which the liquid is spilled, all combinations still produce rings. Scientists who have recently studied this phenomenon believe it has implications for industries that rely on the uniform deposition of solids suspended in liquid media (i.e., paints) and that dispersed solids could be deposited in a controlled fashion such as by creating tiny electronic circuits or providing a means of high-density information storage.