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How can a soap film be stable?

  1. Aug 31, 2012 #1
    It is said that a soap film will find the surface with the minimum area. If the two interfaces between the soap and the air meet, the soap film can effectively reduce its area by bursting. You would think that if you simply hold a soap film up vertically, all the liquid would want to go to the lower parts of the film (since this reduces the potential energy), hence leaving very little liquid or no at all at the top, making the two interfaces meet which would lead to the bursting of the soap film. However, this doesn't happen, and nature continuously keeps defying this logic since it usually takes quite some time before the soap film bursts, even if it is held up vertically.

    So how can a soap film be stable? Is there some force that keeps the two interfaces apart?
     
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  3. Sep 1, 2012 #2

    Simon Bridge

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    The soap film does eventually burst though - and, left otherwise alone, what you describe is a major factor in this (as well as drying). What slows it down is the inter-molecular interaction within the film. A soap bubble is not stable. If you hold up a sheet of soapy film (in a hoop of wire for eg) and shine monochromatic light through it onto a screen, you can watch the film get thicker towards the bottom until it bursts.
     
  4. Sep 1, 2012 #3
    Yes. But it is still takes a very long time before the soap film bursts. The logical thing would be that the liquid would travel to the bottom of the film with free fall speed, since there is seemingly no net force keeping the liquid up. This means that the soap film would burst immediately when the film is angled, which it doesn't. Therefore, there has to be some force keeping the liquid up, and that force is not the surface tension since that pulls the liquid in all directions along the interface, leaving a net force only in the normal direction of the interface (unless the surface tension increases the farther up on the film you get, which I don't think seems likely).
     
    Last edited: Sep 1, 2012
  5. Sep 1, 2012 #4

    Simon Bridge

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    I think you have this backwards - it seems that there is an upwards force slowing the fall, you just don't know how it could arise.
    But does it have to pull equally in all directions? In a bubble, that is what makes the bubble globular - but we are talking about a flat film. Where does the surface tension come from?


    There are properties of a liquid besides surface tension.
    The film has a volume as well as a surface.

    Soapy water is gloopy and slimy. Glycerine in the water also helps.
    So there is more going on than surface tension.
     
  6. Sep 1, 2012 #5
    Think about viscosity. I'm no expert but I'm guessing that viscosity of the film and thermodynamics (soap molecules forming a somewhat stable bilayer) have something to do with it.
     
  7. Sep 1, 2012 #6
    What are those forces? I understand that there has to be some, but I don't know which. What other forces are there that could play a role here?

    Do you mean that the surface tension can be different in different parts of the film? In that case it could definitely be the reason the liquid isn't falling down. But then the question is, why does the surface tension vary, does it depend on the thickness of the film because it senses the opposite interface somehow? Or do you mean something else, that the surface tension can vary depending on in which direction you measure it?

    I know that the viscosity for a soap bubble is very high, about 4 million times as high as for clean water, but is that enough to prevent the liquid from falling down? Viscosity tends to damp shear velocity and fine details in the velocity field, but those are only a small part of the motion inside of a soap film, meaning that just being very viscous is not enough for stopping the liquid from falling down. Watch this video for example:

    https://www.youtube.com/watch?v=DZLoxBBI4xc

    The liquid seems to flow almost unhindered by the viscosity.

    Besides, the liquid can be seen to move in a circle motion, as if it would want to move to places where the amount of liquid was not very high and prevent the film from getting to thin. This supports the theory that there is some force keeping the two interfaces apart, but which??
     
    Last edited: Sep 1, 2012
  8. Sep 2, 2012 #7

    Simon Bridge

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    I gave you a few examples.
    These are elastic forces from distorting a viscous material - they arise from polar effects in the molecules and from the geometry of the polymer part - polymers tend to get tangled up and so resist being pulled apart. If the film were rubber, you would not be at all surprised to find it hangs against gravity - so where does the elastic force from rubber come from?

    Now imagine a weaker elastic material - where the Hook limit is easily exceeded.

    Now and even weaker one.

    Really weak ones may tear under their own weight - but not right away. Why not?

    Now you are on your way to understanding the soap films.
    It looks like you need to understand more about how the forces act in a film before you will be able to see where they come from more clearly.

    Take a simple case - just a plain wire loop.
    If an elastic membrane were stretched across it instead of soapy water you'd have no trouble understanding it right? Now hold the loop so it is horizontal. How does the tension vary across the elastic sheet?

    Notice that the sheet bulges downwards in the middle?
    This should tip you off that the tensions are not uniform across the sheet ... consider: the lower surface is more stretched than the upper surface.

    Lets use an easier model - do it in 1D ... model the sheet as an elastic cord strung between two points.
    To keep it simple - we'll rig it so the un-stretched length of the cord is the same as the distance between the points - then let gravity stretch it.

    Then the cord kinda loops down and back up like cables between power-poles.

    Pick a small length dl on the cord and draw a free-body diagram for the small mass dm there.

    You get gravity being balanced by two forces (must be since it ain't falling down) which act at different angles. If you pick a dl in a different place, the free-body diagram looks different.

    To work out the magnitudes and directions you need the natural principle that covers this situation. That principle is called "least action" (minimize the difference between kinetic and potential energy). This principle puts the cord into a special shape called a "catenary" ... the horizontal components of the tension is the same everywhere, but the vertical components are not. You can look this up.

    If you turned the model through 90deg - what happens is the center of mass of the cord ends up below the midpoint ... there is no horizontal tension - vertical tension increases as you get to the top. Below the center of mass, the cord is mostly compressed - so the force there is repulsive.

    The bottom and top fixed points have to provide a net upwards "reaction" force balancing the entire weight of the cord - otherwise the whole rig would be accelerating
    If there were no ficed point at the bottom, then the very top dl would have to stretch to support the entire weight of the cord.

    Where does this tension come from in the elastic cord?

    Why, "intermolecular forces" of course.

    The details depend on the substance. In rubber, the long polymers are all tangled up in knots and so on, so when you pull one end you picture lots of tiny knots getting tighter, letting the whole thing get longer. When you let go, the knots and tangle loosen up again. Why? Because the outside of the polymers are made of electrons and the electrons from one will repel the electrons of the other. It's the same reason Major General Albert Stubblebine III never managed to walk through his office wall even though both he and the wall are mostly empty space.

    If you pull very hard, the polymer strands will actually slip with respect to each other - so, when you stop pulling, the whole does not relax into the same shape as before. This is called "exceeding the elastic (or Hook) limit".

    The fine details are very involved and usually need advanced study: what level did you want the explanation on? It is also why I'm trying to get you to do some of the work thinking this through: I am not going to spoon-feed you pat answers - there are none - all I can do is point you in the right direction.

    Now - back to the soap film. Soap has a polymer structure - but it is also polar - you get an electrostatic effect as well as the prev mentioned polymer-tangling stuff. That is why soapy water is viscous and slippery... ie: slimy. It is also how it dissolves in water (despite being oily) and how it can get oils to dissolve in water (so you can use it to clean stuff).

    Given all this, is it really surprising to find soapy films have some funny behaviors?

    Hopefully you now realize that:
    1. the tension in a film is not just "surface tension".
    2. tension need not be the same throughout a film.

    Remember the note about how, without the bottom support, the top dl of cord has to support the whole weight?
    If you remove that point - the cord stretches out more because there is nothing to push the bottom up.
    Some of the upwards force slowing the soap film down comes from the bottom arc of the wire loop ... and that the charged parts of the soap repel each other.
    The surface tension - two surfaces remember - just holds the sides in.
     
    Last edited: Sep 2, 2012
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