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Why does oil stick to steel better than water?

  1. Apr 4, 2008 #1
    I'm new here, so i'm not really sure where to post, but i supposed my question would come under physical chemistry. I am writing an essay on separation techniques in the oil industry, and I came across this machine:


    I was wondering if anyone could tell me why oil sticks to steel better than water?

    Having asked this question in another, less specialised forum, I understand that it is caused by surface tension, But I still don't understand HOW it is caused by surface tension.
  2. jcsd
  3. Apr 4, 2008 #2
    Essentially because oil have a less surface tension than water: paraffin oil 48 mN/m, water 72 mN/m (at 25°C)
  4. Apr 4, 2008 #3
    I understand surface tension. I don't understand how it makes the oil stick to the steel.
  5. Apr 4, 2008 #4

    Andy Resnick

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    That's the reason- the interfacial energy between oil and steel is less than water and steel, so oil will displace water- the energy required to create a unit area of contact between oil and steel is less than the energy required to create a unit area of water and steel.

    If you are asking for a molecular-level description of wetting, well... there isn't one yet.
  6. Apr 4, 2008 #5
    Oil stick to the steel for the same reason water do: Adhesion.


    The balancing between the two different forces adhesion and surface tension determines how much the fluid will stick to the surface.
    P.S. In your first post I assumed you know what is adhesion.
  7. Apr 4, 2008 #6
    OK, so My description of the machine's mechanism will probably be something along the lines of: Oil adheres better to the metal, due to it's lower surface tension. A lower surface tension means it requires less energy to adhere to the surface of the metal, compared to water. the lower energy situation is favoured, therefore only oil wets the surface of the metal.

  8. Apr 4, 2008 #7
    Got it!
  9. Apr 4, 2008 #8

    Andy Resnick

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    I agree, with one small quibble. The term "surface tension" is taken to mean the interfacial energy between a fluid and a vacuum (or at least the liquid and vapor phase of the same liquid). The term "interfacial energy" is a better term to use, since it encompasses fluid-fluid, fluid-solid, and 'surface' interfaces.

    Likewise, it's "wets" rather and "adheres".

    Either way, you understand the concept.
  10. Apr 4, 2008 #9
    Thanks folks!
  11. Apr 5, 2008 #10
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