Cause of Wine LegsWhile some people think wine legs are related to the quality, sweetness or viscosity of wine, they are really indicative of the alcoholic content of the wine and are caused by the interplay between adhesion, evaporation and surface tension of water and alcohol.
How Wine Legs WorkCapillary action draws a small amount of wine up the surface of the wine glass above the liquid. Both alcohol and water evaporate, but the alcohol has a higher vapor pressure and evaporates faster, producing a region of liquid that has a lower concentration of alcohol than the rest of the wine. Alcohol has a lower surface tension than water, so lowering the concentration of alcohol raises the surface tension of the liquid. The water molecules are cohesive and stick together, forming droplets that eventually become heavy enough to fall back down the glass in streams into the wine.
History of the Explanation of Wine LegsThe effect is called the Marangoni or Gibbs-Marangoni Effect, in reference to Carlo Marangoni's investigations into the effect in the 1870s. However, James Thomson explained the phenomenon in his 1855 paper, "On certain curious Motions observable at the Surfaces of Wine and other Alcoholic Liquors".
Test It YourselfThe Marangoni effect more generally refers to the flow of liquid caused by surface tension gradients. You can see this effect if you spread a thin film of water over a smooth surface and add a drop of alcohol to the center of the film. The liquid will move away from the alcohol drop.
Swirl a glass of wine or liquor and observe the wine legs or tears of wine on the glass. If you cover the glass and swirl it, wine legs eventually will stop forming because the alcohol will be unable to evaporate.