How does altering the surface tension of water affect noncoalescene?

Question

I have recently seen many videos that talk about the noncoalescence of water droplets; however, I do not understand the purpose of adding surfactants into the water. From my knowledge (high school), objects will stay on top of the water due to water's cohesion, resulting in a high surface tension. This is why in an experiment of a paperclip floating on water, when you add soap, the paperclip sinks - due to the decrease in surface tension. What I am really confused is that the noncoalescence of water droplets works BETTER when surfactants are added. It would be great if anyone has an answer, I am really puzzled...

Answer 1

Looks like the mechanism of the effect of surfactants on coalescence is quite complex. According to Phys. Fluids 20, 040802 (2008), "It is known that the addition of surfactant inhibits coalescence in the sense that the time required for film drainage to the point of film rupture is significantly increased. Although there is a direct effect on the rate of film drainage due to Marangoni effects within the thin film, we find that an equally important effect is due to the fact that the hydrodynamic force pushing the drops together is increased, hence causing the film to be more strongly deformed into a dimpled configuration that slows the film drainage process."