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Surface energy and adhesion

  1. Oct 5, 2012 #1
    Consider comparasion between same states and same type of materials. also consider the substrates to have same surface quality.

    a substance with higher cohesive force would have a higher surface energy. this should be because of the stronger cohesive forces between molecules as the molecule on topmost layer is unsatisfied, now if we consider water & say mercury , Hg has a higher surface energy. so it will not wet the surface ?

    So will it be a good adherent?

    Also, can we generalise that higher surface energy is only for obtuse contact angles? Is there an exception?
     
  2. jcsd
  3. Oct 5, 2012 #2
    I believe wetting angles depend not only on the surface energy of the fluid but also the identity of the substrate.

    For example, mercury has much stronger cohesive forces than water, but if the substrate (say, cellulose or other sorts of hydrophilic polymers) weakly interacts with mercury, the mercury will have a high contact angle. But water will have a low angle with cellulose.

    With brass, the water will have a larger contact angle, but the mercury will have lower.

    Likewise, fluorocarbon polymers have almost 180 degree contact angles with water and are extremely hydrophobic but what about fluorocarbon solvents? But the binding energy of water is much higher than fluorocarbon solvents.
     
  4. Oct 5, 2012 #3

    Borek

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    Staff: Mentor

    Contact angle is a function of all three interfacial energies (see Young's equation), so you can't take only into account and use it for predictions (unless you compare cases where all other things are kept equal - which is not always possible).
     
  5. Oct 5, 2012 #4
    I would like to ask a question. What's the third interface energy? I looked on Wikipedia at "contact angle" and it did mention the fluid-fluid (fluid-air) interface but why is that important?
     
  6. Oct 5, 2012 #5

    Borek

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    You have three phases - and three interfaces. At the point they meet all three forces acting tangentially to the surfaces must sum to zero.
     
  7. Oct 5, 2012 #6
    Chill factor -
    As far as cellulose is concerned, the hydroxyl bonds, i'd think will play a sure role. so they'll interact with water. i don't think surface energy will influence, bcoz i've never heard professors talking in terms of surface energy.
    they just say that like substrates, there will be interaction.

    But, i'd like to know for 2 completely different substrates? like water & mercury.

    As far as ur question is concerned, i'd have to admit my physics is poor bcoz i think 2 much and don't get all answers.
     
  8. Oct 5, 2012 #7
    Borek -
    i do get all of this vector treatment and stuff theoretically.
    take glass water and glass mercury ( 2 chemically different substrates).
    is it only the surface tension which decides the wetting of substrate?

    something like after one value for surface tension for any given substrate, there will be no wetting and below there will be wetting or partial wetting whatever?
     
  9. Oct 5, 2012 #8
    Chill factor - POLYMERS is the root of all questions. check my next thread.
     
  10. Oct 5, 2012 #9

    Borek

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    Staff: Mentor

    Wetting is not a true/false property. Mercury does wet the glass surface, but in much lesser degree than the water does. You can define some contact angle as a limiting case, but I have never seen it done this way.
     
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