A group of physicists courageously filled their lab with wine in the name of science to finally uncover why the delightful alcoholic libation runs down the glass in “legs” after being swirled. The study, published in the journal Physical Review Fluids, found that the effect so adored by wine connoisseurs was the result of shockwaves interrupting the halo of liquid that sticks to the glass, breaking it up and running down in lines.
If you’re ever in a restaurant desperately trying to look as if you know a thing or two about wine, try gently swirling your glass and observing how the liquid runs down the glass. In a good wine, “legs” or “tears” will appear as straight lines of fluid traveling down the glass. It’s thought by some that the presence of legs or lack thereof is indicative of the wine’s alcohol level and quality, but it’s actually a bit more complicated than that.
When you swirl a glass of wine, a thin film of liquid remains on the wall of the glass. This happens because the water in wine evaporates much faster than the alcohol and the change in surface tension sends the drink upwards as you swirl. After sufficient swirling, a ring of wine will be intact further up but it was previously unknown why this would flow back down in segments (legs).
Hangjie Ji at the University of California, Los Angeles, and her colleagues decided to finally suss out why this phenomenon occurs. The UCLA team performed a theoretical analysis that took into account the gravitational effects already established from previous studies, which explained why the film of wine climbs so readily in the glass. They established that the upward flow of the wine is characterized by a climbing wave that starts as a uniform consistency but once swirling ceases, the thickness drops off abruptly at the trailing edge. This segmented drop off down the inside of the glass is caused by a shock wave interrupting the ring of fluid stuck to the glass.
Their model suggests that the contrast between the flow of liquid up the side of the glass – due to surface tension differences mentioned earlier – and the downward force of gravity come together to form a shock wave. The wave they observed is called an undercompressive shock, an unstable type of shock wave that causes drops of wine to collect and eventually run down the glass like a tear. They tried swirling wine glasses in their lab and observed that the shock wave could be visualized as a ridge in the trailing edge. The same effect may also explain why liquid films influenced by wind run down in a similar way, as seen on airplane wings or car windows. Anyone who has ever pensively looked out of a passenger seat window on a rainy day pretending they're in a music video will be familiar with the result.
“Wine tears have been studied for over a century and it is remarkable that this is the first time that they have been connected to the instability of an undercompressive shock,” said Anette Hosoi, from the Massachusetts Institute of Technology, in a statement to New Scientist. “This study is a beautiful example of such shocks in a familiar setting.”