Designation (also legs, legs, windows, tears) for the liquid formations on the inner wall of a wine glass, which are (can be) formed when the glass is swivelled in a circle. A more or less high, vertically arranged film of liquid (wine) is formed by the swivelling. At the upper edge the film begins to thicken and contracts into viscous, tear-like drops. These then flow back down again to the liquid level. The decisive factor for the phenomenon with wine is the mixture of water and alcohol, but it would also work with any other mixture of two liquids with different boiling points. With pure water or pure alcohol alone it would therefore not occur. In the right picture you can see the tears or church windows in the shadow (see arrow).
Liquids are held together by the attraction of their molecules, the so-called adhesion. The alcohol type ethanol has a significantly lower boiling point than water at 78 °C. When parts of the alcohol evaporate, the surface tension of the film is changed, the remaining liquid contracts and flows down in the form of more or less large drops. The droplets flowing down form windows. Depending on whether the distances between these tears are narrow or wide, one can draw conclusions about the alcohol content. The smaller the distances between the tears are (in "pointed arch style"), the more viscous (thicker) the wine is and the higher the alcohol content. The wider the distances between the tears (in "round arch style"), the lower the proportion.
The effect is more pronounced from an alcohol content of at least 12% vol. It is sometimes claimed that the church windows also indicate a high content of glycerine. However, this is not correct, as glycerine has a much higher boiling point than water at 290° C. It is almost exclusively the alcohol content that is decisive. There are alcoholic content measuring instruments that use this effect. These consist of a glass capillary with an attached funnel and a measuring scale. At the top a little wine is poured in, which flows out again at the bottom. Due to the adhesion some wine remains in the capillary, which does not flow out. By means of a table the alcohol content can be determined from the height and the temperature. The procedure is accurate to 0.5%.
Already in 1855 the British physicist James Thomson alias Lord Kelvin (1824-1907) recognized this phenomenon, which he called "tears of a strong wine". The discovery is often erroneously attributed to the Italian physicist Matteo Marangoni (1840-1925), who published it in 1871 and therefore it is also called Marangoni convection. See also under Wine Address, Wine Evaluation and Wine Enjoyment.