Coffee and wine

The tears of wine can be more easily seen in the shadow here
The tears of wine can be more easily seen in the shadow here

Perhaps of an evening, rather than sit with a cup of coffee, you relax with a glass of wine.  Looking carefully at the wine glass reveals the ‘tears of wine’ effect that you can see in the photograph.

A first explanation of this effect was given by James Thomson (brother of William Thomson, Lord Kelvin) in 1855 who had evidently spent a fair amount of time observing his wine.  It has to do with surface tension effects and particularly the fact that the surface tension of alcohol is much weaker than that of water.  Consequently, pouring a small amount of alcohol into a thin layer of water (or coffee), results in the water surrounding the alcohol drop to ‘pull back’ against the weaker surface tension of the alcohol region.  As the water recedes, a dry-ish spot is left in its place.  This is what is going on in the video shown here:

This movement of liquid from regions in which there is low surface tension to regions in which there is a higher surface tension is known as the Marangoni effect, after Carlo Marangoni (1840-1925).

For the case of the tears in wine, capillary action drives the wine up the sides of a wine glass forming a thin film of wine on the sides of the glass. This may be easier to see if you first swirl the wine around the glass to thoroughly ‘wet’ the sides of the glass. Alcohol evaporates faster than water and so this wine-film becomes less alcoholic than the wine in the glass.  Consequently it has a higher surface tension than the rest of the wine and the water-y wine climbs further up the sides of the glass, pulled by this surface tension effect. As more water-y wine climbs up the sides, the mass of water increases until it becomes too great and gravity pulls the water down in the droplets which we know as the tears of wine.

This effect can be used for some other fun experiments which can be done on the kitchen table top such as a soap powered boat, see the link here. Yet it also has relevance for the industrial growth of semiconductor crystals (which are grown as liquids) and other technologies that require liquids during their development. Semiconductors are used throughout the electronics industry and are in many devices that you use every day, including whatever you are using to read this. Consequently it is very important to understand how the Marangoni effect works without the complication of gravity which is always present in any experiments done on earth. Therefore, in a series of experiments due to finish in 2015, NASA are exploring the Marangoni effect in space where the effect of gravity is weaker and so not so much of an influence in the experiments. More information regarding NASA’s work can be found here.