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What cause surface tension ?

  1. Jun 28, 2012 #1
    Hi everyone.

    I have a very basic question here. What exactly cause surface tension?

    On hyperphysics website, it say surface molecules develop stronger bond with each other. 'Those on the surface have no neighboring atoms above' But what cause this stronger bond? Stronger bond is a result of something.....Does liquid surface have higher density??

    I can understand why droplet tend to minimize its surface area, but how does this relate to surface tension? this is a pressure inward, not a force tangent to the liquid surface. And this cannot explain why the force is proportional to the length but not the area of the contact.

    I just cannot persuade myself..

    Thanks
     
  2. jcsd
  3. Jun 28, 2012 #2

    Simon Bridge

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    The bonds are electrostatic ... since the surface molecules have fewer surrounding molecules to "hold hands" with, they have more hands to hold on to each other with.
     
  4. Jun 28, 2012 #3
    At a boundary between a liquid and the air the average separation of molecules at the surface of the liquid is greater than in the bulk of the liquid due to molecules evaporating from the surface. When the separation between molecules increases there is a resultant force of attraction between the molecules and it is this force in the surface that is surface tension.
     
  5. Jun 28, 2012 #4
    Oh dear you are right those explanations don't really cut it.

    You need to be aware of two things.
    Firstly surface tension is an average effect. The result of lots of small contributions adding up.
    Secondly it is a result of symmetry (or lack of it).

    Look at the atached sketch.

    I have greately simplified the model and am assuming that every interior molecule is surrounded by on average (remember my average?) 4 other molecules.

    This is molecule A in the sketch.
    As a result whatever the inermolecular forces there is no net force on molecule A because of symmetry.

    Now look at a typical surface molecule, molecule B.
    The intermolecular forces are still symmetrical sideways along the surface so have no net effect.

    However there is an asymmetry between the pull into the interior and the lack of pull from the outside.
    So every surface molecule experiences a net pull into the centre of the liquid.
    So it does what every body that is subject to an unbalanced force does.
    It moves in slightly until the forces are again in balance.
    This tightening, all around the surface, constitutes an extra ring force, like the skin tension in a balloon and we call it surface tension.

    does this help?
     

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  6. Jun 28, 2012 #5
    Density has nothing to do with it. Liquid has tension throughout due to attractive forces between molecules. However, at the surface, that tension is skewed in the direction towards bulk of the liquid, resulting in a "drop".

    Look up Van der Waals force.
     
  7. Jun 28, 2012 #6
    Hi. thanks for the reply

    Yes, but what cause them to hold more hands? I thought bond strength only depend on distance, chemical properties, and number of molecule present....(the greater the number, the stronger)

    There is less molecule at the surface then how can the bonds be stronger?
     
  8. Jun 28, 2012 #7
    Look carefully at what you wrote.
    "this is a pressure inward, not a force tangent to the liquid surface."
    Forces are vectors, not scalars. The have direction. Draw your force diagram not for a flat surface, but for a concave surface. Assume that there is a constant tension along every point of the concave surface, just as you would assume for a rope.
    If the surface was perfectly flat, with no curvature, your statement would be absolutely correct. However, surface tension becomes significant only if the surface is not flat.
    If the surface is concave, then a force tangent to the surface can have a component that is positive in the direction inward to the concave surface. So by stretching the surface of the fluid tangent to the surface, a force is being applied inward.
    This is why the surface tension minimizes the surface area. The surface tension is tending to flatten the surface, not pull it inward. The flatter surface has less area.
    You can work the problem backwards to understand the cause of surface tension. Yes, the molecules in the fluid are pulling the molecules on the surface inward. However, if the surface is concave then the inward forces have a component parallel to the surface. So the inward force is equivalent to a tension.
    If a rope is lax, then it has a curvature. However, if you apply tension then the rope straightens out. The tension has components inward and tangential to a curved surface.
     
  9. Jun 28, 2012 #8
    THanks for the reply....This explanation is a little bit different to the ordinary explanation...But it makes some sense.. Can you verify this 'the average separation of molecules at the surface of the liquid is greater than in the bulk of the liquid '. Thanks
     
  10. Jun 28, 2012 #9
    Thank so much for the reply and the picture...
    I guess I have made some progress... Maybe I should look more carefully into the word 'tension'. By the way, do you mean that in the liquid, neighbour molecule actually develop repulsion force, but molecules further will develop attraction force? so that a surface molecule can move down until the force can again balance?

    Thanks
     
  11. Jun 28, 2012 #10


    I thought this tension is tangent to the surface... people say water surface is like an elastic sheet..
     
  12. Jun 28, 2012 #11
    Hi, thanks for reply... I think I have understanded what you say.

    Thanks
     
  13. Jun 28, 2012 #12

    Simon Bridge

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    Basically the cause is that more hands are available... the only available hands to hold are "down" or "inwards" so the outer bits get tugged inwards more than outwards.

    It's just a dramatic picture that can help get you the idea.
    I suspect you are getting caught up in ideas about the words used that are not part of the physical concept.

    I think the others have covered this accurately and in more detail. I'll defer to their explanations.
     
  14. Jun 29, 2012 #13
    No the nature of the interaction between the molecules does not change. Darwin has it.

    The inward pull introduces curvature. That is molecule A moves inwards slightly. In my diagram the surface molecules to the left and right of A now have an outward component which balances the otherwise unbalanced inward pull of the interior molecules.
     
  15. Jun 29, 2012 #14
    Tomz: If you have access to text books 'Advanced level physics' by Nelkon & Parker gives a good explanation which translates greater separation into greater potential energy if you want an 'energy' approach.
    'Materials and mechanics' by T Duncan states: 'molecules in the surface of a liquid are farther apart than those in the body of the liquid i.e. the surface layer has a lower density than the bulk liquid. This follows because the increased separation of molecules which accompanies a change from liquid to vapour is not a sudden transition. The density of the liquid must therefore decrease through the surface'
     
  16. Jun 29, 2012 #15
    The surface energy approach does indeed make matters easier.

    But I would caution against the statement by Duncan.

    Surface tension is also active in the (solid) container/ liquid interface where there is no vapour.
    Further solids themselves exhibit a surface tension.
    If you look into metal alloys there is a surface energy associated with the grain boundaries.
     
  17. Jun 29, 2012 #16
    Another reference: 'Understanding physics' by Jim Breithaupt.
    'Molecules inside a liquid are spaced on average at the equilibrium separation. The molecules move about but the average force on a molecule inside the liquid is zero, because such molecules are at equilibrium separation. But molecules near the surface have a bit more space than those inside the liquid. So the average separation between surface molecules is a little greater than the equilibrium separation.Therefore weak attractive forces link surface molecules due to their increased separation. So molecules near the surface are linked in a state of tension'
    This is a very common explanation of surface tension in LIQUIDS.....this is the basis of the original post.
    'A level physics' by Muncaster, like Nelkon & Parker, considers potential energy differences due to atomic spacing to explain surface tension in liquids.
     
  18. Jun 29, 2012 #17
    I won't verify this because it isn't true. The average separation of molecules at the surface of a liquid are not necessarily greater than at the bulk of the liquid. The issue is a matter of the balancing of forces on a molecule. It is a matter of direction, not spacing.
    The forces on a single molecule in the bulk are balanced. A single molecule in the bulk of a liquid is surrounded by ambient molecules on all sides. For every ambient molecule on one side of the single molecule, there is another ambient molecule on the opposite side at the same distance. So the sum of forces from ambient molecules is zero for a single molecule in the bulk of the liquid. Two adjacent molecules next to each other in the bulk of the liquid aren't pulled part.
    The forces on a single molecule at the surface aren't balanced. For every ambient molecule inside the bulk of the liquid, there is an empty space opposite the surface.So there are fewer ambient molecules surrounding the single molecule on the surface. The average spacing is the same, but there are fewer molecules.
    Two molecules on the surface of the liquid are pulled apart if and only if the surface is curved. This is the surface tension. The surface tension is the force pulling molecules on the surface of the liquid apart when the surface is curved. Because of the curvature, there is an empty space on the opposite side of the single molecule.
    An environment is isotropic at a point if the magnitude of forces on a body at the point are the same in all directions. A molecule in the bulk of a liquid is in an isotropic environment. An environment is anisotropic if the magnitude of forces on a body at that point is different in some directions. A molecule on the surface of the liquid is in an anisotropic environment. The curvature at that point on the surface causes an unbalance force that points inward.
    Most books describe surface tension as being like the skin on the surface. However, it is not a real skin.
    Vector decomposition is a very useful tool for describing forces. Unfortunately, no vector decomposition is completely unique. That is why surface tension can be described both as a skin on a surface and a pull by the bulk. Surface tension is best described in terms of force diagrams.
     
  19. Jun 29, 2012 #18
    The rules of these forums state that responses should be traceable to standard text books.
    Anything out of the range of standard text books should be backed up with some reference.
    Text books have stood the test of time and we should be very very reluctant to assume that unmoderated contributions to a forum can override recognised text books.
    If your response does not fit with conventional text book explanation you are probably wrong..... look again.
    It would be an improvement if all responses were accompanied with a backup independent reference in these forums.
    Too much is opinion.
     
    Last edited: Jun 29, 2012
  20. Jun 29, 2012 #19
    The best explanation of surface tension I have ever seen was produced by Unilever as one of their science booklets and an accompanying film.

    I further remember in my GCE practical one question was to measure surface tension by floating a needle or razor blade on a dish of water and then dragging it up the meniscus with a magnet. I forget how we actually deduced the force now.

    As a matter of interest here is my definition of surface tension.

    Imagine a 1 metre line drawn on the surface of the liquid.
    The ST is the force in Newtons at right angles to this line and tangential to the surface required to part the surface along this line.

    truesearch

    I could not find any reference to ST in N & P (2nd ED) could you provide references?

    I am also concerned about the suggestion that a substance expands due to ST, which must occur if the molecules are further apart?
     
    Last edited: Jun 29, 2012
  21. Jun 30, 2012 #20
    Your definition of surface tension is the definition of surface tension that I also know.
    It can be measured by finding the force (using a balance) to lift a microscope slide from the liquid surface.
    I have N&P 4th edition. There are other references in my other posts.
     
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