The beautiful droplets called "tears of wine" are actually related to the change of surface tension.
shake a glass of wine, form a thin liquid film on the wall of the glass, and then place it for a while, there may be a circle of liquid droplets like tears on the wall of the glass. This is what people call hanging the cup. How did this thing come from?
below is a dynamic picture formed by hanging a cup (this is not red wine, and 70% of the ethanol solution has a red dye, purely as a demonstration)
to add a picture taken by myself:
(this is a bottle of Bombay sapphire kept at home for several years, in order to see clearly, the picture is accelerated, it takes about 50 seconds)
this is actually a matter of surface tension. This is an example of the Marangoni effect, which means that when there is a gradient difference in the surface tension of different regions of the liquid, the liquid is driven by this difference to flow spontaneously.
when it comes to the hanging cup of wine, the whole process is like this: first, when shaking the glass, a thin film of wine solution will be formed on the wall of the glass, and then the liquid film will gradually volatilize. Alcohol volatilizes more easily than water, so after volatilization, the alcohol concentration decreases, while the part of the liquid film volatilizes faster because of its larger surface area than the wine concentrated under the glass, so the alcohol concentration decreases more significantly here. On the other hand, the surface tension caused by alcohol and water is different, the surface tension of the place with high alcohol concentration is lower, and the surface tension of the part with more water is higher. The liquid will be "pulled" from the place of low surface tension to the place of high surface tension, so the upper boundary of the liquid film does not go down, but the droplets gather on it. Also under the action of surface tension, with the volatilization of alcohol, the remaining liquid films gradually converge into droplets, and then they slowly gather and flow down under the action of gravity, which becomes the appearance of tears.
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the key to the formation of hanging cup is that there should be a significant difference in alcohol concentration between the liquid membrane part and the lower part. If the alcohol concentration is very low, this effect is naturally not obvious, so it needs to have a certain degree.
hanging cup is often used as a standard to measure the quality of a kind of wine, but simply looking at it to judge the quality is not correct. after all, you can see that the alcohol solution mixed with pigment hangs so well, it is not a good wine.
finally, as a second-hand healthy editor, there are many examples of Marangoni effect that remind you to drink less alcohol. For example, in the small experiment, the dynamic color effect of milk + pigment + detergent is derived from the change of surface tension, and so is the boat that can run on the water with a little soap or detergent.
Mini experiment: magic of Milk + detergent
Mini experiment: drive a boat with detergent