Every once in a while you will see someone swirl
their glass of wine, raise it towards the light and watch with
bated breath for the wine's legs to appear, a mythical indicator
of wine quality. These legs, or tears as the French refer to
them, are the streaks of wine forming on the side of the wine
glass. The legs were once thought to be associated with a wine's
quality (the more legs, the higher the quality). However, the
legs have more to do with physics, the wine's surface tension
and alcohol content, than perceived quality.
Wine is a mixture of alcohol and water, the
alcohol has a faster evaporation rate and a lower surface
tension than water, effectively forcing the alcohol to evaporate
at a faster rate. This dynamic allows the water's surface
tension and concentration to increase, pushing the legs up the
glass until the surface tension pushes the water into beads.
Finally, gravity wins the battle and forces the
liquid to tear down the glass in a defeated streak. Still not
convinced that it's physics and not quality that drives this
phenomenon? Try covering your next glass of wine and see if the
legs present dramatically decrease when covered compared to when
open. No evaporation, no legs. Enjoy none the less.