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Surface Tension

  1. Dec 21, 2009 #1
    I don't really understand surface tension. I understand that the molecules at the liquid surface will be attracted downwards due to the resultant force caused by the attraction between molecules. However, this statement in my textbook baffled me.

    "Consequently molecules in the surface have potential energy."

    What does it really mean by that? Is potential energy between molecules due to the intermolecular attraction?

    There's another statement on wiki :

    "Another way to view it is that a molecule in contact with a neighbour is in a lower state of energy than if it weren't in contact with a neighbour"

    Why is that so? Why is that the molecules at the surface have higher state of energy and what does state of energy mean? Is it potential energy?

    Furthermore, why are there forces acting upwards on a needle due to surface tension? Can anyone explain it more clearly? I understand the water will have indentations on it due to the extra force caused my the weight of the needle but what effect does the molecules on surface has on the needle which resulted in the upward forces?

    One last question. Why is it that there are two forces acting upwards on the needle instead of a single one?

    Thank you in advance. Surface tension has been plaguing my mind for a long time and I could not comprehend it clearly.
  2. jcsd
  3. Dec 21, 2009 #2


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    Staff: Mentor

    Molecules on the surface are pulled in only one direction, molecules inside are pulled in all directions at the same time - hence difference in their potential energies.
  4. Dec 21, 2009 #3
    To get a molecule from the bulk of the liquid out to the surface, you have to do work on it. This work is needed because you have to oppose the forces pulling the molecule back inwards. Potential energy is work done against this opposing force. Just like gravitational potential energy is equal to the work done in lifting an object up against the opposing gravitational force. Hence the expression "surface energy" in a liquid.
  5. Dec 21, 2009 #4

    Andy Resnick

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    While the above are somewhat reasonable interpretations using atoms, 'surface tension' (or interfacial energy) is an energy which is ascribed to a Gibbs dividing surface between two phases. It is manifested by curvature of the interface: a macroscopic quantity.

    The surface tension represents the amount of energy required to create an area of interface.
  6. Dec 21, 2009 #5
    What do you mean by "It is manifested by curvature of the interface"?

    So how does this surface tension create an area of interface? Is it by the attraction between molecules, thereby making the surface "hard" like a table?

    Thank you everyone for replying
  7. Dec 21, 2009 #6


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    The tension is in the liquid surrounding the vapor, since a vapor/gas implies separated atoms/molecules, so they cannot experience tension. A vapor bubble forms in a liquid as more or less a spherical cavity, or on a heated solid surface, the vapor bubble forms a section of a sphere, until the bubble expands to form a spherical bubble.

    The liquid at the boundary of a vapor bubble or cavity forms an area of interface, the area surrounding the vapor volume. Liquid is not hard - as in a solid - but it does support or experience tension through intermolecular forces.
  8. Dec 21, 2009 #7
    hmm.. Okay, thanks.

    So according to wiki,

    "Its weight, Fw, depresses the surface, and is balanced by the surface tension forces on either side, Fs, which are each parallel to the water's surface at the points where it contacts the needle"

    This means that there are a lot of surface tension forces on the needle since the area of contact between the needle and the water is a curve. Am I right to say that? So surface tension forces always acts parallel to the area of contact? IF that is so, what about the soap film experiment.

    "imagine a flat soap film bounded on one side by a taut thread of length, L. The thread will be pulled toward the interior of the film by a force equal... "

    There's the upper and bottom surface of the film. I assume that the thread is on the side of the film, not the upper and bottom. But why is the force caused by surface tension acting inwards?
  9. Dec 22, 2009 #8

    Andy Resnick

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    Astronunc did a nice job clarifying, but I also wanted to point out a few subtleties:

    1) A solid does not have a unique specified interfacial energy because a solid can maintain stress without flow.
    2) The interfacial energy of a material (for example, pure water has 72 ergs/cm^2) is technically defined in terms of vacuum; experimentally this is tricky to determine due to vapor pressure.
    3) wetting (for example, Young's equation) involves one fluid displacing another. How this occurs microscopically is still unknown.
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