Q: Why do coffee stains dry in...
Q: Why do coffee stains dry in rings?
A: Sid Nagel, a physics professor at the University of Chicago, gave the answer in a recent report in the journal Nature. When coffee is spilled on a surface that will not absorb it, the liquid spreads quickly until it is stopped by surface tension. The edges of the stain are then trapped by microscopic irregularities in the surface of the counter, so that it cannot shrink.
As the liquid in the edge evaporates, it is replenished by liquid from the center, which carries the dissolved coffee with it. Eventually, the bulk of the dissolved coffee is deposited on the edge of the stain, forming a circle. Nagel and his colleagues proved this mathematically and duplicated the process with microscopic plastic beads.
