RumDood wrote:The glass gets its required swirling and a nice sturdy rim appears and quickly gives rise to very strong, nice looking legs.
Swirled about the glass and then set back down, the spirit holds firm, with legs slowly developing and then moving in a very slow manner back to the bowl. These legs, they walk, they do not run.
And, when the phenomenon is weak or absent, you can sense their disappointment. This can give the false impression that wines or spirits with "lots of legs" are of a better quality. In fact, the intensity of this phenomenon depends only on alcohol content, and it can be eliminated completely by covering the glass.[KitchenSavvy]Capn Jimbo wrote:The color is a clear dark amber with lovely slow legs. Nice.
Their intensity is also greatly affected by the shape and the smoothness of the tasting glass.KitchenSavvy wrote:To test this, swirl [...] the glass [...] 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.
The phenomenon was first correctly explained by physicist James Thomson, the elder brother of Lord Kelvin, in 1855. It is an instance of what is today called the Marangoni effect (or the Gibbs-Marangoni effect): the flow of liquid caused by surface tension gradients.[Wikipedia]
Wikipedia wrote:One important application of the Marangoni effect is the use for drying silicon wafers after a wet processing step during the manufacture of integrated circuits. Liquid spots left on the wafer surface can cause oxidation that damages components on the wafer. To avoid spotting, an alcohol vapor (IPA) or other organic compound in gas, vapor, or aerosol form is blown through a nozzle over the wet wafer surface (or at the meniscus formed between the cleaning liquid and wafer as the wafer is lifted from an immersion bath), and the subsequent Marangoni effect causes a surface-tension gradient in the liquid allowing gravity to more-easily pull the liquid completely off the wafer surface, effectively leaving a dry wafer surface.
Another manifestation of the Marangoni effect is "beading", the formation of stable bubbles in bottles of strong liquor when they are vigorously shaken. This occurs only in liquor that contains more than 46% alcohol.[wikipedia]KitchenSavvy wrote: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.
Legs are a quick but imprecise way of judging alcoholic strength. You cannot read much more out of it. It's not related to viscosity or quality of maturation. It's only a amusing phenomenon to observe while swirling and shaking strong liquors. That's all.
I was told a better indicator of quality is the characteristic amber colour of well-aged spirits that shifts toward green hues when you tilt the glass. This effect might be nearly impossible to simulate with caramel colouring. Can someone refute or confirm this?