The seemingly-alive dancing droplets are back in a new video from Veritasium. These droplets of food coloring attract, merge, and chase one another due to evaporation and surface tension interactions between their two components: water and
propylene glycol. Because the droplets are constantly evaporating, they are surrounded by a cloud of vapor that helps determine a drop’s surface tension. These localized differences in surface tension are what causes the drops to attract. The chasing is also surface-tension-driven. Like any liquid, the drops will flow from areas of low surface tension to those of higher surface tension due to the Marangoni effect. Thus drops of different concentration appear to chase one another. This is a relatively simple experiment to try yourself at home, and Derek outlines what you need to know for it. (Video credit: Veritasium; research credit: N. Cira et al.; submitted by @g_durey)