Perhaps you've noticed a ring of clear liquid near the top of your glass of wine, with droplets forming and dropping back into the wine. These are "tears of wine," also known as wine legs or "fingers." Physicists have been intrigued by them for over 150 years, and while the basic mechanism is understood, new research indicates that shock-wave dynamics may also play a role in their formation.

UCLA mathematician [corrected] Andrea Bertozzi described her recent work on the subject today in Boston at the 2019 March meeting of the American Physical Society, the largest annual physics conference in the country. "There's been a flurry of activity over the last 30 years trying to understand more about this phenomenon, but nothing that really addressed the dynamics of the actual tear formation," she said. Adding shock waves into the explanatory mix "would explain why sometimes one sees tears of wine and sometimes one does not."

British physicist James Thomson (elder brother to Lord Kelvin) first noticed wine tears in 1855, although they're technically known as the Marangoni effect after Italian physicist Carlo Marangoni. The phenomenon is also responsible for the infamous "coffee ring effect," which has also generated much interest among physicists. It's most notable in wines (or other spirits like rum) with alcohol content at least as high as 13.5 percent. (That's because alcohol has a lower surface tension than water.) If you spread a thin film of water on your kitchen counter and place a single drop of alcohol in the center, you'll see the water flow outward, away from the alcohol. The difference in their alcohol concentrations creates a surface tension gradient, driving the flow.

Wine is basically water and alcohol, along with acids, dissolved sugars, and other compounds that lend color and flavor. The first step in creating wine tears is swirling wine to coat the inside of the glass—a common practice among oenophiles to enhance the flavor. Thanks to capillary action the wine will start to climb up the side of the glass. Both the water and alcohol evaporate as it does so, but since alcohol evaporates falser, the alcohol concentration gradually decreases. This increases the surface tension of that wine so more is drawn upward because the wine below still has a high alcohol content and hence lower surface tension.

Tears form and gravity kicks in when the droplets' weight exceeds the force of the effect, causing them to fall back into the glass. Bertozzi drew an analogy to driving in the rain. "You have water on the windshield of the car, and the wind creates a surface stress that pushes the rain up the front of the windshield," she said. "And gravity is pulling it back down."

That's the standard explanation, but some nuanced complications have emerged in recent years. For instance, a 2015 study looked into the impact of thermal effects at play (i.e., the temperature of the room). The study found that evaporative cooling is also a significant contributing factor to the formation of wine tears. The first quantitative study of the wine tears phenomenon appeared in 1992. But Bertozzi noticed that the equations typically used to describe wine tears didn't account for all the interesting physics—namely, they were missing such factors as the balance between surface tension, the surface tension gradient, and gravity, or the curvature of the glass.

For her own theoretical work on the phenomenon, Bertozzi drew on earlier work she'd done in the 1990s while at Duke University, with experiments involving silicon oil on a wafer. The wafer was placed at an incline, so when the oil was heated, it was colder on the top and warmer on the bottom—a thermal gradient, essentially the same kind of Marangoni stress as the surface tension gradient that leads to wine tears.

"Lo and behold, we were able to produce these unusual waves traveling up the plate against gravity—what we call undercompressive shocks," said Bertozzi.

The surface tension gradient behind terms of wine is essentially the same underlying mechanic dynamics. To test that theory, Bertozzi and her team used port wine in a martini glass with a 65-degree incline. They observed a circular wave forming—similar to the waves that formed in the silicon oil on a wafer—and traveling up the glass. "Fingers," aka tears of wine, then formed because of the instability of the wave, eventually draining back into the glass. "We believe such waves are some of the dominant effects when you see tears of wine," she said.

The next step is to partner with experimentalists to further test her theory about the complex dynamics of wine tears—which may or may not involve imbibing a glass or two.