Why do coffee spills dry into rings?
Capillary flow as the cause of ring stains from dried liquid drops
Why does a dried coffee drip leave a dark ring instead of an even splodge? Physicists found out.
A spilled drop of coffee dries into a ring with a darker edge. A team at the University of Chicago studied drying drops under a microscope to see what the coffee bits were doing.
The edge of the drop gets stuck in place, so as it evaporates, liquid flows outwards to refill the rim, carrying the coffee with it. Every mug ring on your desk is a tiny fluid dynamics lesson.
The coffee ring was never an accident. It was physics.