TL;DR: higher surface tension pulls wine up the sides of the glass creating a thin film → alcohol evaporates more rapidly at this interface between air and wine → the wine at this location has lower alcohol content (and thus higher water content) now that it has evaporated → this makes surface tension even higher allowing the wine to “fight” gravity and pull itself together until it gets to be a big enough droplet that it falls down when gravity wins out, little tug of war here). Also: Higher sugar content → higher alcohol content → more viscous wine → tears flow down sides of glass more slowly → greater observance of the Marangoni effect
Full lowdown:
In class, Alder mentioned that when he analyzes a wine, he swirls it and observes the “tears” of the wine. Or perhaps you’ll best remember him saying the “legs” of the wine. Whether you prefer the term legs or tears, like Alder said, this phenomenon arises because of some very cool fluid mechanical properties of the wine. To see this effect in action, swirl your wine and you’ll notice you create a thin film on the sides of the glass interior. At the interface of the air and your thin film (the thin layer of wine you swirled up on the sides of the glass), gradients in surface tension drive upwards flow of the wine molecules. We observe the movement of these droplets because the wine at the top of the glass forming a thin film has higher surface tension than the bulk liquid in the rest of the glass, which makes the part of the wine with higher surface tension pull upwards more strongly. Now, the reason the wine swirled on the sides has a higher surface tension is because the alcohol in it is beginning to evaporate at the interface with air. As the alcohol evaporates, the alcohol % decreases, making the water % increase in the composition of wine in that part of the glass. And if you took chemistry, you may recall that water is an extremely unique liquid because of its strong hydrogen bonding* properties, which create higher surface tension and thus greater ability to pull the wine upwards as the surface tension pulls the droplets together against the force of gravity, which is pulling the wine downwards back into the bulk.
Source: alyten.com
The air-liquid interface part here is key to allow for evaporation— in fact, if you shake a closed bottle of wine, you won’t see the tears because evaporation can’t happen in your closed system!
Source: Venerus, D., Nieto Simavilla, D. (2015).
For using this to actually judge your wine, know that wines with a higher alcohol content will display a higher density of tears in the glass and as you can hypothesize, so would sweeter wine (remember higher sugar content → higher alcohol content → more viscous → tears flow down sides of glass more slowly → higher chance of observing the Marangoni effect). However, don’t get too attached to observing this effect and relating it to the quality of the wine. A 2015 study examining the impact on the Marangoni stress of wine composition and temperature concluded that the contributions of composition and temperature gradients to the Marangoni stress have comparable magnitudes. This means that the tears of wine you see could equally be the result of the alcohol content and the result of the fact that you’re drinking on the beach or on the slopes!
Anyway, fluid mechanics was my second favorite engineering class here (after quantum mechanics) and there are a lot of cool fluid mechanical phenomena to observe with wine! Thanks, Alder, for suggesting this rabbit hole.
*H-bonding explains literally almost anything that is unique to water, like its high boiling point or how a lake won’t freeze over even if the temperature is below 0 degrees Celsius.
Sources
- Venerus, D., Nieto Simavilla, D. Tears of wine: new insights on an old phenomenon. Sci Rep 5, 16162 (2015). https://doi.org/10.1038/srep16162
- http://alyten.com/videos/the-marangoni-effect-explaining-the-tears-of-wine/ (great video, go to 3:37 to see how the drops continuously form over and over again as surface tension and gravity battle each other!)
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