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Shock waves cause the tears of wine

The legs, church windows or tears of wine. Many a wine drinker and scientist have swirled the wine through their glass and watched this phenomenon with amazement. What causes the tears? And what do they say about the wine? For a long time, science has not been able to explain exactly how the tears come about, but surface tension, temperature, ridge instability and shock waves (!) seem to give the long-expected complete answer.

Alcohol evaporation

Wine is primarily a mixture of water and alcohol. In a wine glass, due to capillary action, the wine crawls up slightly at the side of the glass. The bend wine surface that is formed is called the meniscus (see figure below). The alcohol evaporates faster in the meniscus than in the rest of the glass. This is because the meniscus has a larger surface area in proportion to the volume underneath. As a result, the wine in the meniscus contains less alcohol and relatively much water compared to the wine in the rest of the glass. Water has a higher surface tension than alcohol and therefore “pulls” harder on the surrounding liquid. The gradient that arises from the high-alcohol wine in the glass to low-alcohol wine in the meniscus causes a difference in surface tension. The higher surface tension of the watery wine in the meniscus makes that more wine is pulled up from the glass, resulting in a film layer of wine on the side of the glass.

In 1865 the Italian physicist Carlo Marangoni described this effect in his thesis, and a few years later Josiah Willard Gibbs gave a theoretical thermodynamic description in several articles entitled “On the Equilibrium of Heterogeneous Substances”. Since then, the formation of the film layer of wine on the side of a wine glass has been known as the Marangoni-Gibbs effect.

The emergence of the tears of wine

The formation of a wine film layer on the side of a wine glass due to the Marangoni-Gibbs effect. The evaporation of alcohol provides a gradient in the surface tension (λ), and a gradient in the temperature (T) of the film layer on the wall of the glass. The tears of the wine form under the influence of gravity on the ridge of the film layer.

Temperature difference

Recent research shows that the Marangoni-Gibbs effect can not only be attributed to the evaporation of the alcohol and the difference in surface tension that arises. Venerus et al. showed in 2015 that the evaporation of the alcohol also causes the wine in the film layer to cool down. The importance of this temperature difference for the Marangoni effect has always been overlooked.

The resulting temperature gradient in the film layer causes the wine to “flow”. Just like the warm Gulf Stream that brings warm sea water from the Gulf of Mexico to the northern part of the Atlantic. Similarly, the temperature gradient in the film layer ensures that wine flows up the side of the glass. As such, the temperature difference contributes to the Marangoni-Gibbs effect. The flow rate of the wine in the film layer is therefore dependent on the gradient in surface tension AND the temperature gradient, and both are caused by the evaporation of alcohol1.

Venerus, 2015 Infrared tears of wine

Infrared photo of the tears of wine. The color scale indicates the temperature of the film layer of wine on the glass. The white arrow indicates the direction of the flow of the wine caused by the Marangoni-Gibbs effect.
Adapted from Venerus, 2015 via CC BY 4.0

This information further clarifies the Marangoni-Gibbs effect, and explains how it is that wine flows up against the side of the glass. But what makes the wine drip down again? Gravity? Yes, but that is a very simplistic representation of reality. How is it that wine specifically forms drops at regular intervals that as tears flow back into the glass?

Plateau-Rayleigh-Taylor instability

The ridge, on top of the film layer, falls apart into tears under the influence of gravity. In a 2018 study, Nikolov and his colleagues show that the way in which the ridge of the film layer falls apart corresponds to the theory of the Plateau-Rayleigh-Taylor instability2. This is a mathematical description of the instability that occurs when a liquid with a light density pushes against a liquid with a higher density. In the case of our wine film layer, the film layer is the liquid with a light density that pushes against the wine that forms the ridge and has a higher density. At a certain point the downward pull of gravity on the heavy liquid (that is pulled down more than the light liquid) outweighs the upward force of the light liquid. The heavier liquid breaks through the barrier created by the upward flow of the light fluid and forms tears that stream into the glass. The ridge stability is therefore a balance between gravity and the upward pressure of the light liquid.

Rayleigh-Taylor instability

A simplistic representation of the Plateau-Rayleigh-Taylor instability on the basis of which the wine tears form according to Nikolov et al.. The ridge of the film layer (dark red) has a higher density than the film layer (light red) that under the influence of the Marangoni effect is formed and flows up.

However, the last question that remains is: what makes that the film layer has two different densities, creating a ridge that falls apart into tears?

Shock waves

The film layer is created by the Marangoni effect, and the tears are caused by the instability of the ridge of the film layer. But how is it possible that this ridge is created at the top of the film layer? According to Dukler et al. of the University of California, a shock wave through the film layer causes the formation of the ridge and the subsequent tears3. A “reverse undercompressive shockwave”, that is. Dukler and his colleagues have developed a theoretical model that shows how this shock wave (in theory) arises from the evaporation of alcohol in the film layer and moves from the meniscus to the ridge of the film layer. A characteristic of this atypical shock wave is that the density of the liquid behind the wave is lower than before the wave. A situation thus arises that the ridge of the film layer has a higher density than the film layer below. And let this be THE starting point for the Plateau-Rayleigh-Taylor instability as described above.

“After removing the cover, [from a glass filled with port wine] evaporation quickly increases, inciting a “reverse” front to climb out of the meniscus, followed by the formation of wine tears falling back into the bulk. The forming front is characterized by a depression, i.e. the film ahead of the front is thicker than the film behind it. It is in a sense, a “dewetting” front that leaves a thinner layer behind it.”

[Dukler, 2019]

The researchers tested their theory by looking at the formation of tears in a stemless Martini glass with Port wine. The shock wave was perceptible (see the figure below) and preceded the tears of the wine. The researchers applied some simplifications for the model, but also for the experiment. For example, a martini glass is used because this glass has a constant angle, and the model assumes a constant gradient in the surface tension (caused by the evaporation of the alcohol). The effect in different wine glasses with a convex surface (and in particular the theory behind it) needs to be further investigated.

Tears of wine by shock waves Dukler, 2019 Fig 10 + 11 adapted

The ridge of the film layer and therefore also the tears of the wine are caused by a shock wave. The top four photos show from left to right how a wave front forms out of the meniscus and stabilizes in the tears of the wine after 10 seconds. In the lower left photo you can see a close-up of the shock waves that run to the ridge at the top of the film layer. The term “rarefaction” indicates the zone in which the density of the wine is lower than in the rim due to the shock wave. The figure below on the right shows the stemless Martini glass as used by Dukler et al. to test their theory.
Adapted from Dukler, 2019 via non-exclusive distribution license

What do the tears say?

As the theory above shows, the speed with which the alcohol evaporates is particularly important for the formation of tears. This evaporation depends on the amount of alcohol in the wine, but for example also on room temperature, wine temperature, humidity and air pressure. In general, the more alcohol the wine contains, the more tears there are. However, it is difficult to keep all these conditions constant, and in a mountain village, or on a rainy day, the formation of tears will proceed differently than on a beautiful sunny day at the beach.

The speed with which the tears flow back into the glass says something about the viscosity of the wine. The slower the tears drip down, the syrupier the wine. This syrupiness depends among other things on the amount of alcohol, sugar and glycerol in the wine. Alcohol, but especially glycerol and sugar, increase the viscosity.

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All these different variables make it very difficult to conclude anything based on the tears of the wine about the contents of the wine glass. Provided that you do not only have the alcohol content as a criterion, then the tears of the wine say nothing at all about the quality of the wine.

Wine glasses

The tears are the secondary effect of the evaporation of alcohol from the wine. With the evaporation of the alcohol, however, all kinds of aromas also evaporate. The wine derives its scent (bouquet) from the evaporation of these aroma substances. Knowledge about the formation of tears, and therefore the evaporation of alcohol in the wine glass, can contribute to the development of wine glasses. Wine glasses that optimally support the film layer on the wall of the glass, due to their shape, or perhaps even a coating, contribute to the release of aroma components and therefore to the bouquet of the wine. Therefore, the knowledge gained by the tears of the wine may eventually lead to happy faces.

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References
1. Venerus DC, Nieto Simavilla D. Tears of wine: new insights on an old phenomenon. Sci Rep. 2015 Nov 9;5:16162. https://doi.org/10.1038/srep16162
2. Nikolov A, Wasan D, Lee J. Tears of wine: The dance of the droplets. Adv Colloid Interface Sci. 2018 Jun;256:94-100. https://doi.org/10.1016/j.cis.2018.05.001
3. Dukler Y, Hangjie J, Falcon C, Bertozzi AL. A theory for undercompressive shocks in tears of wine. arXiv 2019 Sept. https://arxiv.org/abs/1909.09898

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