What are 'wine legs'?
When you swirl a glass of wine and look at how it runs down the glass, you're looking at the legs. Look at the ring of wine near the top of the glass and how the droplets form and run back down into the wine. These are the legs. The French call them tears of wine, some people call them fingers, curtains and even church windows. But we know them as legs.
They usually occur in high alcohol wines and you'll see the link between alcohol and wine legs shortly. Some people say you can pick wine quality by the legs. This is just not true.
The legs occur due to the Marangoni effect. It comes from the fact that alcohol has a lower surface tension than water. Wine is mostly a mix of water and alcohol plus a few other parts such as sugars, tannins etc. As you swirl your wine around the glass and leave some on the side it starts to evaporate. Alcohol evaporates faster than water due to its lower boiling point and higher vapour pressure. The changed alcohol to water mix now has different surface tension areas. The wine forms droplets in the areas of least / most resistance which fall back down the glass under their own weight.
The quickest way to see this for yourself outside of a wine glass is by mixing alcohol and water on a flat surface. Put some water on a flat surface and then add a few drops of alcohol. Watch how the water races away from the alcohol drops. This is the lower surface tension of the alcohol interacting with the water.
Physicist James Thomson (Lord Kelvins brother) first identified this behaviour in 1855. The effect is named after an Italian physicist called Carlo Marangoni from the University of Pavia. Marangoni published his doctoral thesis on this phenomenon in 1865.
As you can see the legs don't have nothing to do with wine quality. They're nice to look at and they're a pretty good conversation starter. But as a wine quality indicator, I'm afraid they miss out altogether.
* Reproduced with permission from Peter Svans at The Gurdies Winery