The Physics of Wine Swirling

22 November 2011 1:58 pm

Martino Reclari

Shaken not stirred. By tweaking a few factors, grape enthusiasts can create a range of different waves in their wine glasses, including this triple-crested formation.

Meet the new flavor of wine: fruity with a hint of fluid dynamics. Oenophiles have long gotten the best out of their reds by giving their glasses a swirl before sipping. A new study has revealed the physics behind that sloshing, showing that three factors may determine whether your merlot arcs smoothly or starts to splash.

Twirling a wineglass gently creates smooth arcs in the liquid that then circle, coating the sides of the glass. The gesture isn't just for appearances, says study co-author Martino Reclari, who studies fluid dynamics at the École Polytechnique Fédérale de Lausanne in Switzerland. Scientists and enthusiasts alike have long known that the swirling motion mixes oxygen into a red, enhancing its flavor.

One evening over their own bottle of wine, Reclari and colleagues decided to tackle the physics of this oenological routine. The team filled up small cylinders in a range of sizes with different volumes of a cheap merlot, then set them spinning. To keep things uniform, the researchers employed gyrating machines, commonly used to mix liquids precisely in biology or chemistry labs. This week, at the annual meeting of the American Physical Society's Division of Fluid Dynamics in Baltimore, Maryland, the group reported a mathematical formula explaining how wine sloshes.

Unlike the flavor of a perfectly aged pinot, Reclari says, the factors at play aren't overly complicated. Three factors seemed to determine whether the team spotted one big wave in the wine or several smaller ripples: the ratio of the level of wine poured in to the diameter of the glass; the ratio of the diameter of the glass to the width of the circular shaking; and the ratio of the forces acting on the wine. Those forces affecting the wine were the centrifugal force pushing the liquid to the outside of the glass and the gravitational force shoving the liquid back down.

By tweaking these factors a notch—for instance, by pouring a bit more wine into a glass or shaking that glass in tighter circles—Reclari and colleagues mastered the art of unusual wine waves. Their creations in the video above included the wine lover's standard, a single, smooth crest, all the way to four miniwaves that built in quick succession. Curiously, however, if the researchers kept all three ratios identical, they began to spot the same waves forming again and again, even in cylinders of very different sizes. "If you have a very small glass or a very big glass and you put in the same parameters, you will have exactly the same shape of the wave," Reclari says.

He and colleagues also landed on another important discovery: how overly enthusiastic wine swirlers manage to splash their drinks, possibly staining their sweaters. Just like an ocean crest, wine waves begin to break, turning frothy, if they're moving too quickly, he says. The breaking acceleration for a merlot is about 40% of the force of gravity, the team concluded, or nearly 4 meters per second. That acceleration, in turn, is dependent on the volume of wine in the glass, the force of shaking, and other factors.

The team's formula is useful for more than just helping a wine taster "impress his friends," Reclari says. When growing bacterial cultures, biologists often mix cells in with nutrients in one big jar, then swirl, much like an aficionado over the latest vintage. That rotation distributes the bacterial food throughout the slurry and also removes excess carbon dioxide. Knowing just how liquids slosh in such jars may help lab technicians optimize their growing methods, he adds.

The team's analysis is "simple" but does "make sense," says Vladimir Ajaev, an applied mathematician at Southern Methodist University in Dallas, Texas. And the study illustrates well how seemingly everyday physics, such as the swirling of a glass of wine, might help scientists and engineers develop better lab tools: "At first it might seem like a matter of curiosity," he says. "But then it turns out there are some specific applications."

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