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Bubbles in a Pre-Boiling/Boiling pot of water

  1. May 21, 2013 #1
    How is it that bubbles form on the bottom of a surface of a pot of boiling water?

    I think that there is probably an elementary answer to this from chemistry. But I am thinking of something more along the lines of kinetics. I think that perhaps there is enough energy on the bottom surface of the pan to put some of the water molecules into a gaseous state. Perhaps this only happens to the layer of water very close to the bottom of the pot at first. Maybe the pressure of the vapor inside the bubble is balanced by the pressure of the fluid.

    Also, how is it that once a bubble acquires enough energy it is able to detach from the bottom surface of the pot and rise to the top?
     
  2. jcsd
  3. May 21, 2013 #2

    mfb

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    Bubbles form at the bottom if water is heated at the bottom.
    Right.
    Pressure cannot vary much there, indeed.
    Its buoyant force increases with its size.
     
  4. May 21, 2013 #3

    jbriggs444

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    Note that such a balance is not automatically a stable equilibrium. All things being equal, surface tension would tend to make smaller bubbles shrink and larger bubbles expand. So, all things being equal, it would actually be an unstable equilibrium.

    However, as the bubble grows, it would tend to expand toward a region farther away from the surface to which it is attached -- an area where the water is slightly cooler. That would tend to shrink the bubble. The result is a stable equilibrium.

    At least that's what I think.
     
  5. May 21, 2013 #4
    Thanks for the response.

    I wonder how the molecule content in the actual water itself affects the formation of the bubbles. Also I wonder what the content of the vapor inside the bubbles are, is it mostly water vapor, etc.?

    And yes I wasn't thinking about the buoyant force, nice. I wonder if the bubbles move through the fluid with a constant speed or if they have a significant acceleration? It kind of appears that they accelerate. Which I guess in terms of the buoyant just means the volume of the bubble increases. Thats just one simple way it could accelerate though.

    I think that the detachment of the bubble from the bottom heated surface is curious. I wonder if you could heat up a pot of water in some way such that a entire thin layer of gas remains on the bottom of the pot in a stable configuration. I don't know what the practicality would be but it'd certainly look cool.
     
  6. May 21, 2013 #5
    Thanks for the response. I somewhat understand what you are talking about with the surface tension. You are thinking that it could be the mechanism that causes the bubble to bounce around its equilibrium. I can agree with this. Do you think that surface tension is what will drive it to detach from the surface of the pot? That is, once the bubble expands to certain threshold the surface tension will drive the bubble to detach? I'm not sure..

    Also it seems like you are saying that it is an unstable equilibrium and a stable equilibrium at the same time. Maybe you could clarify for me if I'm understanding it wrong. It seems that whether it is stable or unstable greatly depends on the state of the fluid, i.e. heated or unheated. I have a feeling this is what you were saying.
     
  7. May 21, 2013 #6

    mfb

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    Water vapor, probably with some contributions from gases solved in the water.

    Depends on their size and the temperature of the remaining liquid.

    That would be unstable. You could probably do it on the ISS.
    It works with small drops of water on a hot surface, they get a thin layer of gas underneath while they boil away.
     
  8. May 21, 2013 #7

    jbriggs444

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    If surface tension alone were in play, a sufficiently small bubble would shrink entirely away while a large bubble would increase in size without bound. It would not fluctuate around the equilibrium; it would proceed away from that equilibrium.

    There are competing effects. Surface tension acts to pull the bubble away from an equilibrium size. You could look at it as positive feedback. A temperature gradient would act to push the bubble toward its equilibrium size. You could look at that as negative feedback.

    Negative feedback loops tend to result in stability.
    Positive feedback loops tend to result in instability.

    Yes. That's it. If the water is not heated, there is no temperature gradient, no stability and probably no bubbles. If the water is heated enough so that it is at boiling temperature throughout then again there is no temperature gradient and no stability. Just large bubbles forming, breaking off and boiling up. It is only while the pot is being heated that there can be temperature gradient so that you can have a stable equilibrium with small bubbles forming but not growing large enough to break off and boil up.

    I'm going to try to stay quiet and let mfb walk you through the relationship between surface tension and buoyancy that leads to bubbles breaking off and boiling up.
     
  9. May 21, 2013 #8

    QuantumPion

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    The phenomenon is known as nucleate boiling. The linked wiki article has more detail.
     
  10. May 21, 2013 #9
    In clean water in glass containers it is extremely difficult to form NEW bubbles of water vapour.
    New (small) bubbles are at high pressure and water needs to be heater to higher temperature to create new bubbles. When they are created vapour can form rapidly to create larger bubbles which break away. This causes an effect known as 'bumping'....the violent growth of bubbles.
    Bubbles usually form on particles of dirt or in scratches and are not new bubbles...there is usually a 'nucleation' site for the bubble to grow into.
    In some processes 'anti bumping granules' are added to water to act as nuclear ion centres for bubble formation. This reduces the likelihood of bumping when water boils.
     
  11. May 21, 2013 #10
    OK thanks again for the responses everyone. I am starting to understand what you are saying about the competing effects. Although now that I am trying to think about the surface tension in detail I am having trouble visualizing the effect. When the bubble is slightly larger than its equilibrium size I am not seeing why it would grow without bound.

    Lets say that there is no heat source on the bottom surface of pot containing water. However, maybe there are some bubbles at rest on the bottom surface. I realize that this is a somewhat different type of bubble as it was put there by filling up the container quickly with a faucet. Although I think they should behave the same in terms of surface tension. Those bubbles don't appear to move very much. Are you saying that if we slightly perturb those bubbles so that they are larger than the equilibrium size they grow without bound? Or is that only in the presence of the temperature gradient? Also what happens if we perturb it so its volume decreases, does it pop or something?

    I think I understand why the temperature gradient would act to shrink the bubbles. As the bubble expands into the cooler regions of the water the gas inside the bubble loses some of its heat to surrounding water and causes a drop in pressure inside the droplet which I think would induce the change in volume, very roughly.

    Thanks for the insight y'all
     
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