The Physics of Coffee Rings

Here's another example of the "mundane" curiosity that I love very much.

This time, it is on the coffee rings that we typically see after a drop of coffee dries. And the physics is important enough, and has important applications, that something that appears as mundane as this gets published in Nature, no less.

The edges of a water drop sitting on a table or a piece of paper, for example, are often "pinned" to the surface. This means that when the water evaporates, the drop can't shrink in circumference but instead flattens out. That flattening motion pushes water and anything suspended in it, such as coffee particles, to its edges. By the time the drop fully evaporates, most of the particles have reached the edge and are deposited on the surface, making a dark ring.

There's also a video that accompanies this:

Zz.