The drops that run down the inside of a glass after wine is swirled 鈥� called 鈥渓egs鈥� or 鈥渢ears鈥� 鈥� are caused by a shock wave interrupting the ring of fluid that sticks to the glass.
We know that a film of wine can聽flow up the side of a glass after swirling because the alcohol in wine evaporates faster than the water, creating a difference in surface tension that drives liquid upward. But exactly what caused wine tears to form was a mystery until now.
Hangjie Ji at the University of聽California, Los Angeles, and her聽colleagues have built a model that considered the effects of gravity, the shape of the glass, the wine鈥檚 alcohol concentration and the motion of swirling. The model suggests that the contrast between the flow of liquid up the side of the聽glass聽鈥� due to surface tension differences聽鈥� and the downward pull of gravity could lead to the formation of a shock wave.
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They tested the idea by swirling wine in glasses in the lab, and saw what is called an undercompressive shock wave forming as a ridge in the liquid climbing the side of the glass.
This type of shock wave is unstable, which is why it causes the聽formation of thick drops that eventually fall down as tears, rather than as a continuous flow of liquid.
鈥淲ine tears have been studied for over a century and it is remarkable that this is the first time that they聽have been connected to the instability of an undercompressive shock,鈥� says Anette Hosoi at the聽Massachusetts Institute of聽Technology. 鈥淭his study is a beautiful example of such shocks
in a familiar setting,鈥� she says.
Ji says the formation of liquid films driven by wind, such as on car聽windscreens or aeroplane wings, could also be explained by聽these unstable shock waves.
Physical Review Fluids
