Today I offer you a new installment of the Quilo from my teacher, Miguel Pocoví Mieras, Professor of Biochemistry and Molecular Biology, already retired, but above all my former Biochemistry professor, with whom I now maintain a close friendship. As the good scientist that he is, Miguel is interested in a multitude of natural phenomena, but I dare say, with love and good humor, that fermented foods are one of his passions. If in the previous program Miguel talked to us about the smell of cheeses, and he will explain many curiosities about this food in future programs, today he is going to talk about wine, another fermentation product, specifically he is going to explain the phenomenon of his tears . Let’s hear what Miguel tells us about this interesting physical phenomenon that happens in good and bad wines alike, as long as they are in a clean, well-rinsed glass and have sufficient alcohol content.
The tears of a wine and the Marangoni effect.
Wine usually shows us an effect that we call “wine tears”, others say it is “wine tears” or “wine legs”. When we gently shake the glass of wine making small circles to make it easier for the wine to rise up the wall of the glass, after leaving it at rest, tears appear that slide gently down the glass.
Contrary to popular belief, tears do not depend on the glycerol or glycerin content of the wine, the effect is a consequence of the alcohol and the fact that alcohol has a lower surface tension and higher volatility than water. (The surface tension of a liquid is the resistance exerted by this liquid when its surface increases and is measured as force per unit length. The unit of force is the Newton and the unit of length is the meter)
Water has a surface tension of 0.07 Newtons/meter (N/m) and a boiling point of 100ºC; and ethyl alcohol, a surface tension of 0.02 N/m (3.5 times less than that of water) and a boiling point of 78ºC (22 degrees less than that of water).
Wine contains among its main components water and alcohol, the water/alcohol solution forms a thin film on the walls when the glass is shaken. Alcohol evaporates like water from this film, but it does so more rapidly due to its lower boiling point and higher vapor pressure (figure 2). As a consequence of this evaporation, the liquid in the film increases the surface tension (more water, less alcohol). In this way, the liquid on the wall acquires a higher surface tension than that on the bottom of the cup. The surface tension gradient that is generated forces the wine to rise up the walls of the glass and thus more liquid accumulates in the upper part of the walls. When the time comes, the force of gravity, due to the weight of the accumulated liquid, overcomes and that is when the tears are formed that slide down the walls of the glass. This is known as the Marangoni effect.
This Marangoni effect can also be easily demonstrated by spreading a thin film of water on a smooth horizontal surface and then dropping a drop of alcohol in the center of the liquid. The liquid will be displaced from the region where the drop of alcohol fell, because the tension surface of the water is greater than at the point where the drop of alcohol fell.
As we said, the force that is established between two liquids with different surface tension is known as the Marangoni effect and is due to the Italian physicist Luigi Carlo Giuseppe Marangoni (1840 –1925) (figure 3). Marangoni while developing his doctoral thesis at the University of Pavia, entitled “On the expansion of drops of a liquid floating on the surface of another liquid” and presented in 1865, discovered that this phenomenon is only related to surface tension. After his graduation, Marangoni moved to Florence and there he worked at the Museum of Physics. Later, in the same city, he worked as a Physics teacher for 45 years, until retiring at the Dante Lyceum. He investigated surface phenomena in liquids and invented the nephoscope (instrument to measure the direction and speed of clouds) also contributing to meteorology. Most likely, it was Marangoni who instilled a taste for Physics in the famous Albert Einstein, when he was 16 years old and spent his holidays harvesting with the Marangoni family in Casteggio.
Let’s go back to the crying of wine. There are other factors that can modify the shape of the teardrop, because the surface tension varies widely depending on: a) the temperature of the glass and the wine; b) Type of glass, wall thickness and glass composition; and c) presence of traces of detergent.
The presence of traces of detergent can substantially modify the formation of tears. If you want to check what a simple drop of detergent is capable of doing, do the following home experiment. Put water in a shallow container, for example, in a coffee saucer, and sprinkle with ground pepper, which will float on top of the water. Next, add a drop of dishwashing liquid or liquid detergent to the central point and you will see how the ground pepper particles shoot out towards the edges. This is due to the Marangoni effect and is a consequence of the different surface tension of the water (0.07N/m) and the detergent (0.025N/m) (about three times smaller than water).
It is impressive to observe the speed with which the pepper particles move in this experiment. Given this, imagine the effect that the remains of detergent can have on the wall of the glass.
Very small changes in temperature also affect the formation of tears in the wine. The temperature is also decisive in the formation of the beautiful figures (snow globes) that soap bubbles produce in a very cold environment when they are deposited on snow or ice. As it is difficult to carry out this experiment in warm places, you can look for videos on the internet where these figures are shown, for example in Mike Shaw Frozen Ice Bubbles.
The ice plumes that form in the film of the bubbles are also a consequence, in part, of the Marangoni effect and of the very small changes in temperature that occur locally when the ice forms, which modify the surface tension.
When the base of a soap bubble in a very cold environment comes into contact with ice or snow, the freezing process begins. Small sometimes invisible ice crystals, nanocrystals, are formed that grow and move (figure 4). The change from liquid to solid at constant temperature emits heat*. Let’s explain the latter.
*When ice melts at a constant temperature, heat needs to be provided and we call this heat the latent heat of fusion (Latent from Latin: hidden or hidden), so to melt one gram of ice at 0ºC we need to provide 80 calories. The opposite is the latent heat of freezing, that is, when ice is formed from liquid water at 0ºC, heat is released, specifically 80 calories.
Therefore, this small amount of heat that is emitted in the lower part of the soap bubble (transition from liquid to solid) produces a decrease in the surface tension of this area and since the highest part of the bubble has a temperature lower, that area has higher surface tension. Therefore, a current or Marangoni flow is established, which forces the small ice crystals to move towards the top, as they grow, forming the “ice feathers” and various figures.
Let’s go back to the wine and its tears.
The number of tears in a wine is an indication of the amount of alcohol it contains, which is useful information in the case of a blind tasting, as long as the same conditions are maintained regarding the type of glass, temperature of wine and of the cup and cleansing of the cup.
There are those who maintain that when we observe that a wine shows dense and slow tears, it is indicating that this wine has more body, a high alcohol level, volume, strength, while if we observe fine tears that do not last long, the wine shows us that it is young and less evolved and probably less mature. I do not agree with these statements, the volume, the texture, the sensations in the mouth of a wine are very complex and do not depend, far from it, on the alcohol content.
However, remember that to taste a good wine it is convenient to use the five senses and each one of them will contribute to a greater or lesser proportion in the pleasure of the tasting. Once we have uncorked the bottle and poured a little wine into the glass, the first thing we will do is, as in love, start with the view, observe its appearance: color, layer, brightness…. We will shake the cup and observe their tears…. We will smell it…. At the end of the whole process, the important thing is that it is so good that “we cry”, enjoying the pleasure of that wine and good company.
Miguel Pocoví (14-01-2021)
References:
Ahmadi, S.F., Nath, S., Kingett, C.M. et al. How soap bubbles freeze. Nat Commun 10, 2531
Thomson, (1855), On certain curious motions observable at the surfaces of wine and other alcoholic liquors Londres, Inglaterra: The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science.
Braco C. Albert Einstein and the Marangoni family. Proceedings of the 37th SISFA meeting, Bari, Italy, 2017.
Theodoros Kolydas Marangoni effect experimental demonstration.