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Capillary forces, intermolecular forces and surface tension questions

  1. Jan 19, 2008 #1
    I tried posting a similar question before, however, I've found the 1 reply is actually wrong. Hopefully with better illustrations, someone will know.

    The situation in the picture is a drop of water entering a capillary tube. It is located here: http://img207.imageshack.us/my.php?image=capillaryaction2yi2.png

    Correct me if I'm wrong, but the drop is drawn into the tube by the adhesive forces of the water to the capillary tube. Once the entire drop is drawn in, the forces equal out and the water stops moving up: the bottom water molecules are pulling down with the same force as the top is pulling up.

    Question #1: as shown in the zoomed-in red square, what is making the top water molecule move up? If it isn't moving up, how are other water molecules leap frogging it and pulling more water up with them?

    In the bottom diagrams, we have cone-shaped capillary tubes. I know that the capillary pressure is directly related to the radius of the capillary tube.

    Question #2: It's my understanding that the water would move up to the top in the cone capillary tube because of the pressure difference. But why is there more pressure when the radius becomes smaller? Why would a water molecule that's stuck to the side at the bottom in the 3rd tube want to move up? It can't be the cohesive forces of the water pulling it up--the adhesive forces are stronger than the cohesive forces, that's why we have the capillary action.

    Thanks for any help.
  2. jcsd
  3. Jan 20, 2008 #2
    Should I post this in the advanced forum? Does anyone know someone who might be able to answer?
  4. Jan 24, 2008 #3

    Andy Resnick

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    One conceptual problem here is mixing continuum concepts (interfaces, wetting) with atomic concepts (molecules 'leapfrogging').

    There has been some work on molecular modeling of wetting (Joel Koplik), but don't concern yourself with that yet.

    The curved interface represents a pressure jump across the interface, and the contact angle represents a force balance between the two fluids and the two fluid-solid interfaces. Don't worry about what individual molecules are doing, it's not relevant.
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