Tears of wine

The phenomenon called tears of wine is manifested as a ring of clear liquid, near the top of a glass of wine, from which droplets form and flow back into the wine. It is most readily observed in a wine which has a high alcohol content. It is also referred to as wine legs, curtains, and church windows.

Cause
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, colours, and flavours. 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.

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.

It is sometimes claimed incorrectly that wine with "lots of legs" is sweeter or 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 wine glass (which stops the evaporation of the alcohol). British physicist C. V. Boys argues that the biblical injunction

Look not thou upon the wine when it is red, when it giveth his colour in the cup, when it moveth itself aright.

refers to this effect. Since the "tears of wine" are most noticeable in wine which has a high alcohol content, the author may be suggesting this as a way to identify wines which should be avoided in the interest of sobriety.

Related phenomena
Other fluid phenomena that arise in alcohol/water mixtures include beading and viscimetry. These are more pronounced in liquor than in wine, and both phenomena are more pronounced in stronger liquor.

Beading refers to the formation of stable bubbles when liquor is shaken; this only occurs above 46%-50% alcohol, and is another example of the Marangoni effect. Shaking a whisky bottle to form beads is referred to as "beating the whisky".

Viscimetry is the formation of whorls when water is added to a high alcohol mixture.