MadSci Network: Chemistry

Re: How do weather conditions have an effect on soap bubbles?

Date: Thu Mar 11 16:29:57 1999
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 921115211.Ch

Often a question put to a forum like this is looking for an explanation of some puzzling effect that has been observed. Your question gives no idea about what particular effect you may have noticed -- it is a very general question. So I will just have to try to think of and cover every way that weather conditions might make a difference to the behaviour of soap bubbles. I am also unsure about whether you are talking about a raft of soap bubbles on the surface of a body of water, or about bubbles freely floating that you have blown from a bubble pipe.

I am therefore almost certain to miss out on something, and Murphy's law dictates that it will almost certainly be the very effect that you are most interested in. So my apologies in advance!

There are three parts to a soap bubble. There is the trapped air inside. That will not enter into things very much. It has a fairly constant volume (varies just a little bit with pressure and temperature), and a pressure just slightly higher than the air pressure outside. Around that trapped air volume, there is a layer of water (or, more accurately, of soap molecules dissolved in water). On both surfaces of that water, inside and outside, there is a layer of aligned soap molecules, just a single molecule thick, that protects and adds stability to the structure.

Let's think about weather in four headings, that I think just about cover it:

  1. wind;
  2. precipitation (i.e. rain hail or snow);
  3. temperature; and
  4. humidity (which has some overlap with each of the others).
The main effect of wind is pretty simple and obvious: it puts a mechanical strain on the bubble, and is likely to burst it. This is increasingly the case for larger bubbles, and for stronger and more turbulent and eddying winds. With a raft of bubbles it is likely to pile them up at one edge of the water, leaving most of the surface bubble-free. A second effect of wind is in helping evaporation -- I'll talk a bit more about this under 'humidity'.

Hail is totally destructive of bubbles, for good mechanical reasons, as is heavy rain. Gentle rain will make a thick raft of bubbles thin right down, by diluting and draining the soap solution from the top layers. But the last layer of bubbles in the raft it may sometimes preserve, because it can form just as many new ones by stirring up the top layer of the water as it destroys by bursting or draining. What happens to soap bubbles in a snowstorm is completely beyond my experience -- perhaps you are in a better position to try that one out!

If the temperature becomes too low, there are two possible things that might happen to make soap bubbles less stable. Firstly the soap might become less soluble in water, to the point where there would not be enough soap molecules around to form a proper film. Secondly there might be a change in the state of the surface film from the normal expanded state of a soap film, where the molecules spread over the whole surface, to a condensed state where the soap molecules would huddle together, leaving part of the surface uncovered, and thus collapsing the bubble. Either of these changes would result in a fairly sudden collapse of soap bubbles, at a particular temperature, which would depend on the exact nature of the soap. For many soaps, neither will happen. Going to higher temperature, on the other hand, increases the solubility of a soap, and increases the thermal motions more generally. So the soap bubble would become weaker as the temperature increased, because soap molecules would be moving away from the stabilizing surface film into the solution generally, and because the surface film itself would be more agitated, and therefore weaker. The change with increasing temperature would be a very gradual weakening, quite unlike the sudden collapse that might or might not happen at low temperature.

Finally, the effects of humidity relate almost entirely to the water solution part of the bubble. A soap bubble has a relatively large surface area for the amount of water it contains. Evaporation can therefore happen quite rapidly. As water evaporates from the bubble, it thins, loses strength, and soon collapses altogether. Evaporation is helped by warmth, low humidity, and a steady gentle breeze. On the other hand, in fairly cool damp conditions with damp fog or dew forming, water from the air can be taken in to bubbles, and this again can lead to draining and dilution of soap from the top of a bubble raft, which soon destroys the bubbles. On this occasion, unlike the gentle rain case, there is no mechanical agitation to make new ones.

That is all that I can think of. I hope it has covered what you wanted.

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