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When playing with soap bubbles in a bathtub, it seems impossible to bring two bubbles together to form a single larger bubble. When two floating bubbles meet, a wall is formed between them. Even when pressure is applied from both sides, they refuse to merge. Attempts to break the wall from below using my finger also always fail. I can even move my finger through the wall from one bubble to the other but no merging ever takes place. The bubbles easily pop when poked from outside, but don't seem to want to pop into each other. This seems to be true even when the bubbles are of different size.

So my question is twofold

  1. Why can't I get two bubbles to merge?

  2. Is there anything that I can do to force the two bubbles to merge?

I've read that bubbles minimize surface area, so my thinking is that this has something to do with it, but I don't understand how. It seems to me that joining the two bubbles would result in a smaller surface area than the two joined together, but I could be wrong. I also thought the internal pressure of the bubbles might play a role.

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I've read that bubbles minimize surface area, so my thinking is that this has something to do with it, but I don't understand how. It seems to me that joining the two bubbles would result in a smaller surface area than the two joined together, but I could be wrong.

No, you are right. Bubbles do minimize surface area. And joining the two bubbles would result in a smaller surface.

Looks like the consequence of these two statements is that if two bubbles meet they should immediately merge into a single bigger bubble. But it dose not happen. Sometimes they merge, sometimes no.

The reason is that in order to change the shape of soap water from "two attached bubbles" to "single bigger bubble" it's necessary first to increase the surface a little. If you create a tiny little hole inside the wall between the bubbles they would merge. But to do it you have to increase the surface and to spend some energy.

Even though the amount of required energy is very small, this is the reason why the bubbles wouldn't merge immediately.

And if you decide to do it there are some technical difficulties. You can easily poke the outer wall of one of the bubbles - but in this case one of the bubbles would explode. The wall between bubbles is "inside", you can't easily reach it.

When you "push" the wall with a wet finger you do not really push it. You move the finger and soapy film just changes the shape to minimize the surface given new external conditions. To break the film you need to touch it with something dry and/or hot.

If I really needed to merge two bubbles I would have taken a needle-shaped device, make it wet with soapy water, insert it through the outer wall inside one of the bubbles, turn it on to make the end of the needle hot, touch the inner wall with it so that it breaks and bubbles merge. After that turn it off and remove.

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  • $\begingroup$ +1. But could you please explain why "But to do it you have to increase the surface"? $\endgroup$
    – Hans
    Aug 11, 2021 at 4:33
  • $\begingroup$ @Hans Why a single bubble even exists? If we take all the liquid which form the bubble and make a small sphere from it - the surface will be much smaller! So, why the bubble does not immediately transform into a small drop? The thing is that "bubble" is a shape with a local minimum of surface. If you change the shape a tiny tiny little bit the surface will increase.To merge two bubbles you must destroy the wall between them and to do it you first need to create a very little hole. While size of hole is smaller than thickness of wall total surface of "wall with a hole" will be bigger than orig. $\endgroup$
    – lesnik
    Aug 11, 2021 at 7:16
  • $\begingroup$ Excellent point! I thought of your last sentence of your previous comment, too. What made me hesitate was this approach assumed the positivity of the thickness of the wall instead of the mathematical idealization which is the zero-thickness of the wall. Does zero thickness of the wall imply any soap liquid will just remain a point and the nonexistence of a bubble? $\endgroup$
    – Hans
    Aug 11, 2021 at 17:49
  • $\begingroup$ @Hans It is very difficult to reason about walls with zero thickness. Can it even exist in real world? In mathematics such a wall can exist, no problem with that, but in physics? I don't think so :) Using mathymatical abstraction (such as zero thickness wall) to describe real objects can be very useful, but sometimes does not work. If we want to describe process of destruction of a bubble the "zero thickness" abstraction is just not applicable. $\endgroup$
    – lesnik
    Aug 12, 2021 at 7:11

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