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Soap bubbles are never "too small" or "too large". What defines the range of possible diameters of a soap bubble?

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Why is the pressure inside a soap bubble higher than on the outside,
What is the physics behind a soap bubble

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  • $\begingroup$ I thought it would be the amount of liquid that goes into making it, too little and it will be small and pop, to large and it won't hold and pop- the destruction of the bubble in any case, so sad. $\endgroup$
    – Harry David
    Commented Nov 2, 2014 at 23:32
  • $\begingroup$ Mark H is right. Surface tension wants to contract but as we try to expand, the former weakens as a result the binding force simultaneously the thickness decreases as a result of which it breaks. $\endgroup$
    – user36790
    Commented Nov 3, 2014 at 3:34
  • $\begingroup$ Faq: Contrary to the belief that water has no definite shape , it can be said that in absence of gravity, if you turn a bucket ful of water upside down, the water will take the shape of huge sphere! Surface tension and surface energy!! $\endgroup$
    – user36790
    Commented Nov 3, 2014 at 3:38

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At the lower limit, if the bubble is very small the pressure inside will be so large that the gas inside can dissolve into the shell of the bubble, and from there diffuse out to the atmosphere. That limits the life time of small bubbles.

On the large side, huge bubbles (several meters diameter) are certainly possible. These tend to be unstable because they require extremely low surface tension, and thus they don't "keep their shape" very well. Over time they evaporate, or the soap molecules migrate causing an uneven distribution of surface tension along the surface. With insufficient surface tension in some places the bubble will once again burst.

In the absence of evaporation and instability, the ultimate size limit of a bubble may have to do with gravity: the film somehow needs to support the "column of soap bubble" above it, and if you think about hanging a soap film from a thread, you can see that it would be thinner at the top (which is supporting more weight) and thicker at the bottom. There will be a limit beyond which the film cannot support its own weight - I don't know how to compute it.

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  • $\begingroup$ why small bubble means high pressure? $\endgroup$
    – Sparkler
    Commented Nov 3, 2014 at 15:23
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    $\begingroup$ Surface tension creates force along the circumference. This force $2\pi r \gamma$ divided by the area of the sphere cross section is the pressure increase - the ratio scales with $2\pi r \gamma/\pi r^2=2\gamma /r$. So smaller bubble = higher pressure. $\endgroup$
    – Floris
    Commented Nov 3, 2014 at 16:09
  • $\begingroup$ so if I understand correctly, what you mean is: (1) the smaller the bubble, the higher the surface tension, (2) the higher the surface tension, more internal air pressure is required to keep the bubble size in equilibrium, and (3) high internal air pressure causes more of the internal air to diffuse outside the bubble. is that so? $\endgroup$
    – Sparkler
    Commented Nov 3, 2014 at 17:01
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    $\begingroup$ Almost. The "surface tension" (which is a property of the material) is the same, but the force relative to the area is greater - thus larger pressure difference, and diffusion. $\endgroup$
    – Floris
    Commented Nov 3, 2014 at 17:03
  • $\begingroup$ @Sparkler See this answer: physics.stackexchange.com/a/127709/45164 $\endgroup$
    – Mark H
    Commented Nov 3, 2014 at 21:14
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I don't know what you define as too large, but soap bubbles can reach very large sizes.

https://www.youtube.com/watch?v=5bjggctu3kw

It's a balance between the surface tension being weak enough to allow the film to stretch to large size but strong enough to not tear as it flexes. The soap added to water is for weakening the surface tension.

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Why soap bubbles have larger sizes than water bubbles?

For the same mass of air m in a water bubble and soap bubble, the soap bubble will have low internal pressure than the pressure in the water bubble. It is because of large diamter and low surface tension. In other words, the effect of an increase in diameter & decrease in surface tension is counterbalanced by decrease in the internal pressure (Pi) of bubble. Po is atmospheric pressure. st is surface tension and d is diameter of bubble. Pi-Po = (8*st) / d

Thanks and have a nice day.

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