What causes the formation of “legs” or “tears” in a glass of wine? Is it really and indicator of quality?
Contrary to popular belief, the “legs” or “tears” that can often be seen on a glass of wine are not an indicator of quality. These so-called legs are formed when a glass of wine is tipped or swirled such that the sides of the glass become coated with a thin film of wine: a thin ring of liquid remains at the highest point the film reached, and small droplets fall back from this ring into the pool of wine below, leaving behind elongated droplets resembling tears or legs. Even without tipping the glass, capillary action will make some wine climb up the surface of the glass at the wine-glass interface, leading to the same dripping ring, but the effect will not be as pronounced.
This phenomenon, so often mistaken for an indicator of quality, can actually be explained with some very simple physics, and it occurs in both magnificent and terrible wine. The ring and ensuing droplets are caused by something called the Marangoni Effect, which describes the flow of liquid between two fluids due to a surface tension gradient. In this case, a small amount of wine flows away from the bulk of the wine up towards the thin film of wine on the surface of the glass, accumulating at the top of it in a ring. This occurs because the bulk of the wine has a lower surface tension than the film of wine on the sides of the glass, leading to a surface tension gradient between the two.
The reason for this difference in surface tension is simple. Wine, which is predominantly a mixture of alcohol and water, will evaporate faster from the thin film on the glass than the pool of wine below it, as it has a larger surface area exposed to air. In turn, alcohol will evaporate faster than water from this thin film of wine, as alcohol has a higher vapour pressure (and hence evaporation rate) than water. As a result, the thin film of wine is left with a lower concentration of alcohol (and a higher concentration of water) than the pool of wine below it. Since water has a higher surface tension than alcohol, the water-dominated film is also left with a higher surface tension, creating the necessary difference in surface tension between the film and the bulk of the wine.
The surface tension eventually pulls the rising liquid into spherical drops (a sphere minimises surface area, which minimises a drop’s potential energy). As the drops become larger, the surface tension becomes too weak to overcome the force of gravity, and it pulls the drops back down to the bulk of the wine, forming legs. The higher the alcohol content of the wine, the more prominent the legs will be, as the surface tension gradient will be stronger. Hence, wine legs are indicative of alcohol content, not quality.
An easy experiment to prove that the Marangoni Effect is indeed responsible for the appearance of wine legs is to cover a glass of wine whilst swirling it. If it is well covered, legs will eventually stop forming because the alcohol cannot evaporate.