The liquid (water) with a large surface tension has a larger pulling force than the liquid (alcohol) with a small surface tension around it, resulting in a surface tension gradient. The phenomenon that liquid is pulled from low surface tension to high tension is called Marangoni effect.

In the video below, there is a droplet composed of water and alcohol, in which the evaporation rate of alcohol is fast, so the proportion of the remaining components in the droplet changes with the evaporation process. On the other hand, at the edge of the droplet, the liquid film is thinner and the surface area is larger, and the evaporation of alcohol is faster than that in the central area, and the proportion of alcohol decreases faster.

In addition, the change of droplet composition leads to the change of local surface tension. The alcohol concentration at the edge is low, and the surface tension changes greatly.

Therefore, the imbalance of the surface tension inside the droplet leads to the liquid flow: the middle part is "pulled" to the place with higher surface tension at the edge; At the same time, the "high surface tension zone" of the edge is difficult to maintain with evaporation, and it is broken into many small droplets to "shoot out".

This phenomenon often happens in life. Interested students can search for "wine tears" and "Marangoni effect".

Finally, leave a small question for everyone to think about: when the fate of the original big water drop comes to an end, how will the story of the small water drop develop?

Original links, including videos

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refer to

Marangoni rupture: evaporation-induced emulsification of two-component droplets. Physical review fluid. 3. 10 (20 18): 10050 1.