So surface tension increases as the length increases. I know that. But what doesn't make sense is if I have a bucket of water, and then suspend a needle on the top. The surface tension of the water should act like a 'membrane', the needle will then float on the top of the water. If I later add some soap to the bucket of water, the needle will sink because the surface tension of the fluid has been reduced.

My question is then, why can I not blow bubbles with pure water?

  • $\begingroup$ pure speculation: maybe because the surface tension is so high, it pulls the bubble in and collapses it? $\endgroup$ – MissMonicaE Mar 21 '17 at 19:58
  • $\begingroup$ You need to consider the stability of the bubble as it experiences small displacement disturbances. The disturbance amplifies such that the bubble breaks up into smaller droplets rather than returning to a bubble shape. $\endgroup$ – Chet Miller Mar 21 '17 at 20:04

Soap/Detergent is amphiphilic, having molecules hydrophobic ("water hating") at one end and hydrophilic ("water liking") at the other.
This design drags dirt (and indeed needles) into the water.

Soap/Detergent reduces surface tension by a factor of 2 to 3.

But it has a stabilising effect on soap films due to the Marangoni effect. Essentially the water will flow from regions where surface tension is low to regions where it is high. It flows away from the detergent.

As I interpret it, if there is an instability in the bubble, inter-molecular bonds in that area are stretching. This increases local surface tension, and the Maragoni effect will then flow water to that area, recovering it.

Intuitively it's hard to imagine that simply altering the tension would allow things like this... enter image description here It's a kind of negative feedback going on.


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