Did you notice how good quality wine flows down the sides of a glass? It trickles in dense streams and flows down slowly. Do you know why? Can you wash up a glass properly? Water flows down easily and there are no drops when the glass is really clean.
The two phenomena are explained by so called surface tension: the forces that interact between water particles and between water particles and container walls. The same surface tension forces a dew drop into a form of a sphere (however flattened a little thanks to gravity). This surface tension makes water climb the walls of a drinking trough for canaries.
And the surface tension of wine is half the tension of water. Wine, especially sweet wine, flows wide down the walls of a glass. While grease on the walls does not allow water to flow easily and some drops stick to it. If you don't drink wine, try water with some juice. Sugar, similarly to alcohol, lowers the surface tension of water.
Water is so-called polar liquid (its molecules have strong electric dipole moment) and it has a high coefficient of surface tension. This coefficient drops only slightly with the rise in temperature, from 75.6 units (dyn/cm) at 0°C to 69,6 junits (at 40°C) However, it drops considerably when water is mixed with other liquids.
Adding small amount of, for example ethyl alcohol, considerably lowers surface tension. Thus, 10% of alcohol (as in a white wine) reduces surface tension to 48.25 units (at 40oC) and 24% (as in liqueur) - to 35.5 units.
Surface tension for given liquids can be assessed upon observation of the shape of their drops on a clean glass plate (see the picture).
The surface tension determines also the velocity of the waves propagating on the surface of liquids. Therefore, as shown here, the pattern of waves changes in wine as compared to water. This in another method for checking if the wine is good.
