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A liquid floating upon another

  1. Nov 5, 2005 #1
    Hi all

    Just a quick (relatively simple) question..
    As high school physics/chem tought, a liquid of lower density will float uopn that of a higher density. This makes sense in most situations because of the pressure gradient in the higher-density liquid pushing the other fluid upwards. But what if we had a situation as so:

    A low density liquid is sitting in a beaker. We then somehow place a high density liquid PERFECTLY upon the lower density so there is a point in time where the beaker would look like:

    High density liquid
    ----------------- INTERFACE
    Low density liquid

    Now my question is what is it that would force the higher density liquid downwards? Because none of the liquid has actually penetrated into the low-density area yet (due to the interface) we can't use any pressure gradient differences. I was thinking it may have to do with Brownian motion causing a transfer of molecules across the interface which would eventually cause a total realisation of the pressure gradient. Does this make sense or is it wrong?

  2. jcsd
  3. Nov 5, 2005 #2


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  4. Nov 5, 2005 #3
    Thanks for the link mate.. very helpful.. wasn't aware of that
    So it is then right to say that a system like that can stay in equilibrium so long as it isn't touched/bumped etc? I guess that makes sense!
  5. Nov 5, 2005 #4


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    That's close, but not exactly right. The RT phase is a metastable phase, meaning that any perturbation will destroy it. However, it doen't need touching or bumping for this to happen. Ordinary thermal fluctuations at the ambient temperature will kill such a phase given enough time. The lifetime is hence inversely proportional to the ambient temperature.
  6. Nov 5, 2005 #5


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    Salt domes to thunderstorms --- you don't really need the "liquid" constraint. This is also the situation in which "the butterfly effect" is manifested, the "butterflies" being the fluctuations G. mentions.
    Last edited: Nov 5, 2005
  7. Nov 6, 2005 #6
    OK that makes sense.. thanks very much guys
  8. Nov 6, 2005 #7
    so it's a complex system?
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