Surely you've noticed the slow-dripping streaks encircling the inside of your wineglass after swirling a Port or a hearty Cab: That melancholic vinous phenomenon we call "wine tears" (or "wine legs"). We know a bit about what causes weepy wine—and that the streaks have nothing to do with the wine's quality. But Prof. Andrea Bertozzi of the UCLA department of mathematics realized there was more to it—and it involves little shock waves going through your wine.
Bertozzi told us that the study produced by her and her team, recently published in the journal Physical Review Fluids, was inspired by a lecture on wine tears she had planned previously. “I thought that it would be really good for the students to have a fun lecture. I knew the tears of wine literature,” Bertozzi told Unfiltered. “So I brought in wine and glasses, and some cheese and crackers.” But leading up to the lecture, Bertozzi noticed that previous research hadn’t quite nailed it. “I realized that there was a gap in the literature … they were missing some physics that I thought was actually really important.”
Previous research informed us that wine tears were caused solely by the “Marangoni effect.” For those of us not in the know: As alcohol evaporates from a glass of vino, the surface of the liquid is pushed up the sides of your glass, which is why higher-alcohol wines and spirits have heavier tears. But Bertozzi suspected there was more to it, so, using Port-like wine and a stemless martini glass, the UCLA team got to work.
After glasses were poured and analyzed, the team discovered the specific cause of these weepy patterns: “Reverse undercompressive shocks,” shock waves in which liquids move against the direction of the wave. “It’s caused by a combination of three very simple types of physics,” Bertozzi explained, “One is gravity, the other is the Marangoni stress, and the third thing is the bulk surface tension.” Marangoni draws the wine up, gravity and surface tension pull it down, leaving the sad pattern we all know and love; you see a similar effect on your car’s windshield when driving through rain and wind. Interestingly, it seems that if it weren’t for this effect, the trails in your glass would look more like fingers than tears.
“We sort of discovered [these shocks] 20 years ago,” Bertozzi noted. “But now we have them in wine, which is very, very cool.” More proof that wine and science pair well, and we're a little closer to fully understanding what’s in our glass. Before we've dried those tears along with the rest of it, anyway.
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