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Soap phenomenon

  1. Mar 23, 2003 #1
    To know what I'm talking about you'll have to do a little experiment....

    Go to your bathroom, take some soap and foam up your hands. Make a "bubble" with your index finger and your thumb, then reflect the light from a lamp in the "bubble". After a little while oil-like rainbowcolored patterns will appear. In the beginning, make sure you don't breath or blow on the bubble.

    In the oil-pattern you should see particle-shapes in various sizes traveling from one side to the other (or to the center). A few might shoot by in a random direction with high speed.

    HOW do these shapes form ? where do they come from [?] ... To me the whole thing looks sort of like a bubblechamber where particles enter and leave traces, except these particles would be enormous ! :smile:

    When the bubble is filled with colors and particle-shapes, try blowing VERY gently on it.

    What happens is that the patterns start freaking out ! .. They twist and turn very fast in seemingly random directions !

    Could someone please explain why this happens ? .. I can't see how such weak air turbulence could cause movement like that in the soap.
  2. jcsd
  3. Mar 23, 2003 #2
    You've just constructed a basic interferometer!

    What happens is that the thickness of the soap film determines which wavelengths of light will interfere constructively and which destructively; and hence which colors you see reflected. Very small changes in the thickness of the film -- a few hundred nanometers -- will cause a change in colors. So it's very sensitive; the patterns you see at the beginning are probably due to the bubble slowly thinning or flattening under its own weight or very slight air currents.

    This effect, interferometry, is widely used to make precision measurements.
  4. Mar 23, 2003 #3
    My enquiry was not about the colors, but the patterns & movement that arise in the colors. I don't believe aircurrents explain the particle-shapes that show up.

    I suggest you do the "experiment" so you'll see what I'm talking about :smile:.
  5. Mar 23, 2003 #4


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    hehe we were just learning about thin film interference in class this past week!

    What I guess happens is the bubble isnt uniformly the same thickness....so as the soap kinda swirls around the bubble (affected by outside forces like gravity and the air) the thickness changes slightly at various points, causing destrictuve/constructive to occur at various points..

    Again, this is just a hypothesis..but it sounds right.
  6. Mar 23, 2003 #5
    do the experiment then read my first post again. I'm not wondering about the interference.
  7. Mar 23, 2003 #6
    Hydr0matic, I just tried to reproduce your results. OK, I saw the 'particles': hundreds of them. They appeared white on a backgound of rainbow colors. They didn't move much, but if they did, then mostly downwards. They became less over a period of some seconds. Then, as almost all 'particles' had vanished, I blew on the film. Rainbow colors went turbulent.

    My interpretation: The particles are little bubbles of air enclosed in the water film. They can't (as an air bubble would in a liquid) move upwards. That's because, via surface tension & hydrogen bonds, part of the mass of water surrounding a bubble is attracted to the bubble, so it forms a massive object inside the water film. Which tends to move down.
    Blowing on the film, of course, causes air turbulence. Which induces the film to form erratic variations in thickness, making the rainbow colors go gaga.
  8. Mar 23, 2003 #7
    White ? you sure ? .. the ones I see are as colored as everything around 'em.

    You sure ? .. I've studied the phenomena a total of about 45 min, and I'm pretty sure most of the "particles" move upwards. Try tilting your hand back and forth. But you're right, some of 'em move downwards, but how come ? What's the difference between the ones that move up and those that move down ? And another thing - I've tried bulging the bubble outwards and inwards - and neither of these to caused the "particles" to move away from, or into the center. This would imply that their movement is not gravity related, like the "tilting of the hand"-effect suggests.

    Yes, airbubbles is the most likely explanation. But...
    1. How do you explain what I mensioned above ?
    2. Some of the "particles" emerge in the middle of the bubble, why ?

    Yes.. air turbulence.... Just help me understand the swirling.. Variations in thickness cause fairly linear and circular patterns, it doesn't cause swirling like that. The swirling must be an effect of the rotating air turbulence, but why does the swirlrotation shift direction so rapidly and intense ? The cause and effect does not seem proportional.

    Perhaps I'm making too big 'a deal out of this :wink:
    Last edited: Mar 23, 2003
  9. Mar 24, 2003 #8
    Surface Tension!
    Quantum Catastrophe!
    Do a search for surface Tension (and) Critical Points, failing that go do a search for Ulf Leonhardt, he is the one who is trying to create 'table top blackholes', using a 'orthoscopic cavity', or for the layman, bubble traps.
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