Q: What does it mean when people say that a wine has “legs”?
In wine, “legs” refer to the way that wine swirled in a glass runs down the inside surface. Fill a wine glass one quarter full and gently move the glass in a circular motion so that the wine swirls up the sides. Hold the glass still and look at the wine as it runs down. It may flow down in a fairly even sheet, or it may pull into more concentrated rivulets. These are what are referred to as legs, or sometimes tears.
While some people believe that the presence and thickness of legs relates to the sweetness, viscosity or quality of wine, none of these is correct. Legs form because of the alcohol content of the wine and the effects of surface tension, adhesion and evaporation. The alcohol, because it has a lower surface tension, tends to crawl up the glass. At the same time, it evaporates faster than the water in the wine because of its lower boiling point. As more alcohol evaporates, the water concentration increases. The greater surface tension of the water causes the wine to pull together into a teardrop that then runs down the inside of the glass.
This was first explained by James Thomson in an 1855 paper, "On certain curious Motions observable at the Surfaces of Wine and other Alcoholic Liquors". However it was later attributed to Carlo Marangoni based on work he did in the 1870s. It is refered to as the Marangoni or Gibbs-Marangoni Effect.
To test this, swirl a glass of wine and observe the formation of legs. Now cover the glass and swirl it again. In a few tries, the legs will usually quit forming. This is because the air inside the glass contains enough alcohol vapour to prevent more from evaporating and the formation of legs stops. Uncover the glass and legs will start to form again.
Because a wine’s “body” is affected by the alcohol content, there is some relationship between legs and body, but there are so many other factors involved that legs are a poor indicator of quality.
But, if that is the case, you might ask why connoisseurs still swirl their wine? Because it helps to release aroma molecules from the wine which in turn helps to taste and smell the wine, because swirling increases the surface area, which increases oxygenation of the wine, which in turn affects the flavour, and because they like looking at the legs even if they know they don’t mean anything!