Wine aficionados are known for gently swirling their wine in the glass before tasting, and it isn't as pretentious as it seems. (Well, maybe a little.) They claim the rotation mixes in oxygen and enhances the flavor. Physics backs them up, specifically a mini-subfield dubbed "oenodynamics." The swirling action—technically called "orbital shaking"—creates a rotating gravity wave in the direction of the swirling force being applied, churning up the liquid in the process.
It's often said that science begins with someone noting an unusual effect and thinking, "Huh… that's funny"—and then investigating to find out more. Such was the case for a coterie of wine-swirling physicists. Several years ago, Martino Reclari, then a graduate student at the École Polytechnique Fédérale de Lausanne in Switzerland, was out to dinner with colleagues when the conversation turned to the different kinds of wave dynamics they could produce by swirling their wine glasses: one big smooth wave, a series of smaller ripples, or a splash, for instance.
Not content to leave it at the dinner table, they performed a series of experiments in the ensuing weeks by filling small cylinders of varying sizes with different volumes of a modest merlot. They used gyrating machines commonly found in chemistry labs to swirl the wine in the cylinders, duly noting how various factors affected the behavior of the wine.
They identified three factors at work: the ratio of the level of wine in the glass to the glass's diameter; the ratio of the glass's diameter to the width of the circular shaking; and the ratio of the primary forces acting on the wine: one pushing the liquid to the outside, the other forcing the liquid back down. So by tweaking how much wine you pour in a glass and whether you swirl it in wide or tight circles, you can create different kinds of wave dynamics in the glass (detailed in the video below).
That work became Reclari's PhD thesis. It's not limited to analyzing your glass of wine, either. The formula he and his colleagues devised works for any fluid container size, most notably the bioreactors used to mix large batches of nutrients when cultivating cells in the lab.
A recent paper by a team of Parisian scientists extended the analysis to beer, with surprising results. Unlike wine, beer has a head of foam, and while you might expect that foam to move in the same direction as the swirling motion, the opposite is true. It rotates in the opposite direction. The same thing happens with cappuccino foam, tea scum, and a powder floating on the surface, like cinnamon, under the right circumstances.
The original 2011 paper on the physics of wine swirling inspired a limerick contest (physicists love a good limerick almost as much as they love a good wine) in the Annals of Improbable Research, which holds the Ig Nobel Prizes every year. Here is our favorite entry:
They use several metrics to show
The circular motion and flow
Of wine as it swirls,
As it whirls and it twirls,
Although why, I am too drunk to know.
DOI:Europhysics Letters, 2018. 10.1209/0295-5075/122/34002 (About DOIs).
[h/t FYFD] [contf] [contfnew] 
Ars Technica
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