Readers who occasionally contemplate the bottom of a wine glass will surely be familiar with one of the greater mysteries of the universe. This is the centuries-old observation that wine can sometimes travel up the surface of a glass and then form “tears” as it dribbles down again. How come?

Today the puzzled is solved—at least in part—thanks to the work of Yonatan Dukler and colleagues at the University of California, Los Angeles. These guys have selflessly devoted their research to this phenomenon and say they have hit on an answer.

Let’s start by filling our glasses—a small amount of high-alcohol wine in a martini glass. Observe how a thin layer is drawn up the glass surface above the main body of liquid.

This effect is well understood, having understandably attracted some of the greatest minds in physics. The American scientist Willard Gibbs published a complete theoretical treatment in 1875.

This was based, at least in part, on the work 20 years earlier of James Thomson, the brother of Lord Kelvin. Thomson discovered that the effect comes about because wine is a mixture of water and ethanol, with water having the greater surface tension.

The wine is initially drawn up the surface of the glass by capillary action. This occurs when surface tension forces the surface of a liquid up the vertical walls of a glass container.

In a wine glass, this thin layer immediately begins to evaporate, with the alcohol evaporating faster than water. As a result, the layer becomes less alcoholic and the greater concentration of water leads to a higher surface tension than that of the main body of wine in the glass.

This difference in surface tension is important. It creates a force that drives more wine up the walls of the glass. And this leads to a constant flow of liquid up the glass, driven by the evaporation of alcohol.

All this is well understood—it is known as the Marangoni effect, after the Italian physicist who studied it in the 1860s.

However, there is another force involved—gravity, which pulls the liquid back down again. When this happens, the wine forms into the famous tears that roll down the inside of the glass.

The outstanding puzzle is why the wine forms downward-flowing tears and not some other kind of flow. It is this problem that Dukler and co have tackled, using a comprehensive theoretical model and a great deal of experimental work. They have observed the tears forming in a way that has never been outlined before.

The new work is based on the theory of shock waves. A shock wave is a disturbance that forms a sharp boundary in the properties of the liquid. Shock waves are usually driven by disturbances that travel faster than sound in the fluid, such as a supersonic jet. The wave continues for as long as the disturbance remains supersonic.

But shock waves can form when the disturbances are not supersonic—for example, when evaporation causes fluid flow. These waves are known as undercompressive shocks. It is these that Dukler and co focus on.

They point out that the theories based on Gibbs’s work suggest that the wine ought to form finger-like shapes as it travels up the glass. “We argue that the actual wine tears, which drain down the glass, in contrast to the well-known fingering instability of driven fronts, which travel in the same direction of the front, arise from an instability of a reverse undercompressive shock,” they say.

And their experiments seem to back this up. They place a mouthful of port with an alcoholic content of 18% in a martini glass with sides at an angle of 65%. To make the observations easier, they swirl the port to pre-coat the glass with fluid. The tears then form easily.

These tears clearly demonstrate the wave properties that the team’s model predicts. “We illustrate the existence of non-classical undercompressive shocks for the first time in the context of tears of wine,” say Dukler and co. “We argue that, in the case of a pre-coated glass, the famous ‘wine tears’ emerge from a reverse undercompressive shock originating at the meniscus.” 

That’s interesting work that many readers will want to celebrate with some “research” of their own.

However, it leaves several questions unanswered.  The way Dukler and co pre-coat the glass is important, because it creates a layer over which another layer of fluid can flow. The Marangoni effect is easy to create in these circumstances.

But it leaves open the question of how the tears arise on an uncoated glass.

Which means there is much more work to be done. Let’s get to work—cheers!

Ref: arxiv.org/abs/1909.09898 : A Theory for Undercompressive Shocks in Tears of Wine



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