If you swirl wine in a glass and then set it down for a few seconds, you’ll see a weird thing happen. It defies gravity. First, a ridge of liquid rises up the sides of the glass, and then the liquid drips back down in little rivulets that form a sort of necklace around the inside.
These are called wine “legs,” or “tears.” They don’t always need a swirl to get started, either. The simple act of pouring a full-bodied wine or spirit into a glass can also get them going.
So, what are the legs on a wine glass, exactly? Do they indicate anything about the taste or quality of the wine?
What causes wine legs?
Scientists have tried to answer this question for centuries. In 1855, Scottish physicist James Thomson was the first to correctly describe the phenomenon, presumably based on years of scrutinizing his after-dinner Port glass.
An expert in fluid dynamics, Thomson’s qualitative study of the behavior of water and alcohol in glasses keyed on the role of surface tension and how it varies between the two liquids, causing movements.
The next big step didn’t occur until 1982, when Australia-born geophysicist Herbert Huppert created an equation that models the behavior of “shock waves” in solutions, like in wine legs. He poured liquids down a slope and measured their speed and height to formulate a model similar to those describing traffic flow.
One of today’s leading experts is Andrea Bertozzi, professor of mathematics at UCLA. “The story of tears of wine is an old story about a balance between surface stress and gravity,” she says. This year, she and her team published a paper, A Theory for Undercompressive Shocks in Tears of Wine.
Bertozzi compares the rising ring on a wine glass to the phenomenon of rainwater that rises up the windshield of a moving car, where wind creates a surface stress. In a glass, the instantaneous evaporation of alcohol around the edge of the liquid leads to lower alcohol in that region compared to the rest of the glass. This creates a difference in surface tension that pulls the liquid up the side of the glass in a ridge that Bertozzi calls a shock wave.
“It’s counterintuitive,” she says, “Alcohol as a chemical can change the surface tension of a liquid quite a lot. The stress is like a wind, pushing it from regions of lower surface tension to higher surface tension which means it’s going to be driving the flow up.”
The paper calls the upward flow a reverse undercompressive shock known to be unstable, according to a synopsis on the Physical Review Fluids website, where their study is published. So, any inconsistencies in the shock wave grow into drops that eventually fall as legs.
What do wine legs tell you about the wine?
The prominence of legs in a glass generally indicates higher alcohol content, and thus a richer texture and fuller body. That’s why they’re especially prominent in fortified wines and high-proof spirits. It’s also known that legs are diminished by chilling a beverage or diluting it with ice.
But don’t let anyone tell you that wine legs indicate wine quality. So far, research has made no such connection, so we’ll have to keep tasting wine ourselves to make that judgment.
Published: November 3, 2020