Ever been at a wine tasting, dinner party, or other event featuring wine and some know-it-all (probably with a beard and wearing a sweater) swirls his glass vigorously and then tilts the glass sideways to observe the streaking wine (wine legs)? Don’t feel bad if you believed the explanation that followed explaining that this was a good wine because it had “nice legs” or some other nonesense. This is one of the most persistent wine myths around. Read the explanation of why wine “has legs” (or “tears” for our francophile readers) after the jump.
The fact that a wine has more legs tells you absolutely nothing about the wine’s quality. Let’s just make sure that’s clear from the get go.
This answer, from Wikipedia, is pretty thorough and explains the science behind what you see happening in the glass.
The effect is a consequence of the fact that alcohol has a lower surface tension than water. If alcohol is mixed with water inhomogeneously, a region with a lower concentration of alcohol will pull on the surrounding fluid more strongly than a region with a higher alcohol concentration. The result is that the liquid tends to flow away from regions with higher alcohol concentration. This can be easily and strikingly demonstrated by spreading a thin film of water on a smooth surface and then allowing a drop of alcohol to fall on the center of the film. The liquid will rush out of the region where the drop of alcohol fell.
Wine is mostly a mixture of alcohol and water, with dissolved sugars, acids, colourants and flavourants. Where the surface of the wine meets the side of the glass, capillary action makes the liquid climb the side of the glass. As it does so, both alcohol and water evaporate from the rising film, but the alcohol evaporates faster, due to its higher vapor pressure and lower boiling point. This change in the composition of the film causes its surface tension to increase – this in turn causes more liquid to be drawn up from the bulk of the wine, which has a lower surface tension because of its higher alcohol content. The wine which moves up the side of the glass then forms droplets which fall back under their weight.
So there you go. Nothing at all to do with the quality of the wine. Feel free to stop the phenomenon and drive your point home if anyone doesn’t believe you by covering the wine glass with a coaster or menu and watch the legs of the wine stop developing. Do keep swirling the wine, however, as this actually aerates the wine allowing you a better smell.
The only question left is what to do with your newly acquired information? Do you speak up and put the know-it-all on the spot? Do you just let it go and say nothing with a smug smile on your face?