Curious Cook: The legs in your glass of wine


A swirl of wine with more than 11% alcohol content produces legs or tears flowing back down into the wine at the bottom of the glass. — AFP Relaxnews

In 2020, the global wine industry suffered a double whammy. Production was up, but consumption was down and as a result, the average price of wines had fallen worldwide.

It would be easy to blame the pandemic for these statistics, but 2020 was the third year in a row where wine consumption fell, and it fell by 2.8% globally. The closing down of hospitality venues such as bars and restaurants due to the pandemic obviously had an impact, as did the USA tariffs on EU wines. Importantly, the 17.4% fall in wine imports by China also accounted for a sizable chunk of the 2020 fall in global consumption as China is the world’s sixth largest wine importer.

Sparkling, festive wines such as champagne were the hardest hit in 2020, down disproportionately 15% by volume as fewer people found reasons to celebrate during a pandemic.

Against all that negative news, one curious spot was that French wine consumption was up 14% in 2020, possibly encouraged by the lockdowns where there were seemingly fewer things to enjoy except another bottle of wine. The quality of wines consumed was also higher as the French appeared to be paying more for their wine drinking at home.

This may be because quality wines were selling for less; a normal bottle of Margaux would normally retail for over 25 Euro (RM126) but I was buying them for 15 Euro (RM75) during the wine festivals. As a result, my cellar is now reasonably well-stocked to cater for a large wedding party if that ever became a necessity.

Legs or tears?

One thing quite intriguing and enjoyable is watching the interaction of wine with a normal wine glass. A swirl of any wine with more than around 11% alcohol content in a wine glass invariably leads to a fascinating inverted crown of liquid in the glass which has “legs” or “tears” flowing back down into the wine at the bottom of the glass.

By the way, it is important to note that this works only with wine glasses that have not been cleaned in a dishwasher using a rinse solution as many rinses add a thin layer of polymers inside the glass.

Legs or tears flowing back down the wine glass only work with wine glasses that have not been cleaned in a dishwasher using a rinse solution. — ELLY FAIRYTALE/PexelsLegs or tears flowing back down the wine glass only work with wine glasses that have not been cleaned in a dishwasher using a rinse solution. — ELLY FAIRYTALE/Pexels

Initially, one would assume this phenomenon is simply due to the wine settling back down after a swirl, but prolonged observations have confirmed that this is not the case. One reason is that the amount of liquid dripping back down the sides of the glass often continues unabated even after many minutes, hence it cannot be a gravity effect as more liquid falls back down into the wine than was displaced by the swirling of the glass. So, the question is: how does wine replenish itself up the sides of a vertical wine glass?

This phenomenon is known as the Marangoni Effect, discovered by James Thomson in 1855 and explained by Carlo Marangoni about 10 years later. And if you enjoy a good drop of alcohol, the reason for this gravity-defying effect might endear you even more to wines and spirits.

Typically, the Marangoni Effect leads to features such as the display of legs or tears in a mixture of liquids, or the movement of liquids in a closed system. These liquids first need to have some contrasting characteristics. In wines, the liquids involved are the combination of ethanol (the alcohol present in wines and spirits) and water. Ethanol has two important differences to water: (i) it has a lower surface tension than water, and (ii) it evaporates faster than water.

The difference between the surface tension of ethanol and water creates something called a surface tension gradient. Surface tension is both a tensile and a contractile force on the surface of liquids – the higher the surface tension tensile force (i.e. the ability of a liquid to form a film), the stronger also is the contractile force of the liquid (i.e. the ability of the liquid film to pull away from an area of low surface tension).

The simplest way to view the Marangoni Effect is via a dish of water where some finely-ground white pepper or nutmeg powder has been sprinkled on top. Touch the middle of the dish with a drop of washing-up liquid and the powder will shrink immediately to the edges of the dish. This is because the area around the washing-up liquid has a lowered surface tension; hence this drives the area of lowered surface tension to the area of higher surface tension due to the higher contractile force.

Note that this effect will be observed only if the amount of washing-up liquid used is not too high or diluted inside the water in the dish. Otherwise, there will be no Marangoni Effect visible as then the surface tension would be diminished to some common reduced point. This is a relevant point to remember in the following explanation.

Surface tension gradient

The surface tension gradient is the difference between the surface tensions of two liquids in the same mixture. In a wine bottle, this difference is not always apparent as the ethanol and water content is initially well mixed together, but the inherent surface tension gradient of wine is easily exposed by swirling it in a wine glass.

So, finally, this is how wine develops legs or tears.

Swirling the wine in a wine glass exposes the wine to evaporation and a surface tension gradient. — LOS MUERTOS CREW/PexelsSwirling the wine in a wine glass exposes the wine to evaporation and a surface tension gradient. — LOS MUERTOS CREW/Pexels

Swirling the wine in a wine glass exposes the wine to two important factors: evaporation and a surface tension gradient. The evaporation of ethanol is enhanced by the swirling as it exposes a greater surface area to air. The means the concentration of alcohol is reduced in the film of wine exposed by the swirling of the glass.

This increases the surface tension gradient significantly as there is now more water content in the film around the glass, instead of the original roughly balanced mixture of ethanol and water. That is because water evaporates slower than ethanol.

The net effect of ethanol evaporation is to pull more wine up the sides of the wine glass as the initial tangential stress (or increased contractile force) caused by the reduction in alcohol is higher than the force of gravity. Initially, the surface tension gradient is relatively low as the ethanol is mixed in well with the water content.

But when ethanol is evaporated faster than water, the surface tension gradient spikes up to the point it can surpass even the force of gravity. Hence more wine is pulled up from the bottom of the glass onto the sides.

The volume of wine dragged up the glass accumulate and form bigger and bigger droplets. After a certain point in time, the weight and volume of these droplets are unable to resist gravity and hence they start to flow back down the glass. These are the legs or tears one can observe in wine glasses.

So now one can understand why one cannot see legs or tears in a closed bottle of wine or spirits. There is not enough evaporation to reduce the ethanol content enough to develop the required tangential stress (or contractile force) to pull the bottle contents up the sides.

Therefore, from the above, it should be obviously not true that larger legs or tears always mean there is more alcoholic content in a wine. That is because an important factor is evaporation. This means the impact of the Marangoni Effect in wines is heavily influenced by factors such as humidity and heat.

However, where wines are compared in the same environment, then the alcohol content would be strongly correlated with the size of legs or tears observed. In such a case, a higher ethanol content will result in larger legs or tears.

So, I hope one now can appreciate not only the taste but also the complexity and beauty of the physics and chemistry in a glass of wine.

This is dedicated to Allan Cheah, a wine lover in KL who passed away from Covid-19 on 20 May 2021.

The views expressed here are entirely the writer’s own.

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Curious Cook , Chris Chan , wine , legs , tears , wine glass

   

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