PROVIDENCE, R.I. [Brown University] — Artisanal makers of mezcal have a tried and true way to tell when the drink has been distilled to the right alcohol level. They squirt some into a small container and look for little bubbles, known as pearls. If the alcohol content is too high or too low, the bubbles burst quickly. But if they linger for 30 seconds or so, the alcohol level is perfect and the mezcal is ready to drink.
Now, a new study by a team of fluid dynamics researchers reveals the physics behind the trick. Using laboratory experiments and computer models, the researchers show that a phenomenon known as the Marangoni effect helps mezcal bubbles linger a little longer when the alcohol content is around the sweet spot of 50%. In addition to showing the scientific underpinnings of something artisans have known for centuries, the researchers say the findings reveal new fundamental details about the lifetimes of bubbles on liquid surfaces.
The study, a collaboration between researchers at Brown University, Universidad Nacional Autónoma de México, Université de Toulouse and elsewhere, was published on July 3 in the journal Scientific Reports.
When Roberto Zenit, a professor in Brown’s School of Engineering and the study’s senior author, first heard about the bubble trick, he said he was instantly intrigued.
“One of my main research interests is bubbles and how they behave,” Zenit said. “So when one of my students told me that bubbles were important in making mezcal, which is a drink that I really enjoy with my friends, it was impossible for me not to investigate how it works.”
The researchers started by doing experiments to see how changing the alcohol level of mezcal changed bubble lifetimes. They watered down some samples of mezcal and added pure ethyl alcohol to others. They then reproduced the squirting trick in the lab while carefully timing the bubbles. They found that, sure enough, alcohol level dramatically affected bubble lifetimes. In unaltered samples, bubbles lasted from 10 to 30 seconds. In both the fortified and watered-down samples, the bubbles burst instantly.
Having shown that bubbles really can be a gauge of alcohol content, the next step was to figure out why.