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Mixing fluids

  1. Dec 26, 2005 #1

    Mk

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    If I stir cream into my coffee, what prevents me from stirring counter-clockwise, and the cream seperating again?
     
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  3. Dec 26, 2005 #2

    Tide

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    There are several more or less sophisticated answers one can give to such a question but basically it's the same reason why you cannot ungrate cheese by reversing the motion of your hand on the grater.
     
  4. Dec 27, 2005 #3

    Danger

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    I saw a right freaky demonstration a few decades ago. There were two very specific chemical combinations that did separate when the stirring motion was reversed. I have no idea where I saw it, or what the compounds were. At the time, I thought that it was a serious experiment, but I suppose that someone might have been reversing the film. Anyone out there familiar with it?
     
  5. Dec 27, 2005 #4

    Mk

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    If I have a pool of water, make a wave, then put opposite wave on it, the pool will be tranquil again.
     
  6. Dec 27, 2005 #5

    Bystander

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    Yes. How desperately did you want a reference? And, strictly speaking, it wasn't/isn't perfectly reversible.
     
  7. Dec 27, 2005 #6

    Tide

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    I don't think so. Once the wave is generated and propagating away from the source your "opposite wave" can't even catch up let alone cancel it out.
     
  8. Dec 28, 2005 #7

    Mk

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    Mhmm, you're right. My plans for world domination have been foiled again! Curse you!!
     
  9. Dec 28, 2005 #8
    The cream has basically diffused into the coffee (it ain't black no more.) You can't reverse time by reversing your hand movement so it won't return to its original state of being seperate.
     
  10. Dec 28, 2005 #9

    Danger

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    Thanks, Bystander. I don't actually need a reference; I just wanted to make sure that I hadn't either imagined it or been taken in by a bogus demonstration. I was just a wee innocent lad at the time. Knowing that someone else is familiar with it is sufficient. :approve:
     
  11. Dec 28, 2005 #10

    Integral

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    I believe that the demo you are referring uses glycerin to fill the region between a pair of concentric cylinders. A streak of dye is injected into the glycerin then the interior cylinder is slowly rotated, apparently mixing the dye into the glycerin. Reversing the motion of the inner cylinder "unmixed" the dye. It returns nearly perfectly to its original steak like condition.

    This is actually a demo of laminar flow of a fluid, the dye is not really mixed, just stretched out. Over time the dye will diffuse into the glycerin, truly mixing it.
     
  12. Dec 28, 2005 #11

    Danger

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    Aha! Indeed, Integral, that is exactly the thing that I was thinking of. Most of the details had leaked out of memory. Thanks.
     
  13. Dec 29, 2005 #12

    Mk

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    I saw it on television, then they explained how it realted to quantum mechanics. How?
     
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