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Physical adsorption

  1. Oct 15, 2015 #1
    Guys I want to ask that on surface of water there are unbalanced forces in downward direction so adsorption takes place so how they balance the forces in downward direction? I drawn a free body diagram but I still got a force to water molecule in downward direction so how that molecule is stable?
     
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  3. Oct 15, 2015 #2

    Drakkith

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    I don't think adsorption is the right process to describe water-water interactions at the surface.
     
  4. Oct 15, 2015 #3

    sophiecentaur

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    Could the answer to satisfy your question just be that there are repulsive forces involved when molecules come close enough (there is equilibrium between the molecules)? Does your FBD include this repulsive force? The force that accounts for what we call surface tension could, perhaps imply that the space between the very topmost layers of molecules could be less due the gravitational force and the force from the air pressure on the surface. But, as water is very hard to compress, the difference in spacing would be negligible.
     
  5. Oct 15, 2015 #4
    If I consider two molecules as one molecule of water and one molecule of gas on surface and then draw their FBD then the force of attraction in upward direction to water molecule cancels the unbalanced force exerted on it by bulk molecule but on gas particle there is a force of attraction in downward direction . okay? That's my description of my FBD now if I'm right then according to you there is also repulsive force?
     
  6. Oct 15, 2015 #5

    sophiecentaur

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    But your FBD must include effects in all directions - one water molecule and a number of other water molecules below and to the side. The forces due to the occasional impacting air molecule can be ignored - that just contributes to a Pressure vector into the surface.
    If there were not, the water would shrink to zero volume.
     
  7. Oct 15, 2015 #6
    I didn't got ur last point that water would shrink to zero volume. And my FBD includes includes forces to surface molecule of water by each molecule of bulk i.e side to side ,etc
     
  8. Oct 15, 2015 #7

    sophiecentaur

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    I was pointing out that it isn't just the attractive forces between molecules and atoms that are relevant. The positive cores of molecules will repel each other and the spacing between the atoms and molecules is where the repulsive and attractive forces balance. It is a Potential Energy 'Well'. I don't think that a FBD can particularly help - except in a fairly arm waving way.
     
  9. Oct 16, 2015 #8
    Ohk now the conclusion is when gas particles are sticked to water molecule and when the water molecule is close enough to another molecule then repulsive forces come into action and will balance the space between them and that's the reason they get stable. Correct?
     
  10. Oct 16, 2015 #9

    sophiecentaur

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    Forget the gas molecules; they don't need to form any bonds with the liquid surface for this simple model - they just produce some 'air pressure' due to continual bombardment.
    A solid or liquid occupies a certain equilibrium volume when the attractive forces balance the repulsive forces. Deform it and it will tend to 'spring back'. The repulsive forces restrict the minimum volume of a liquid and the attractive forces will minimise the surface area (hence spherical drops).
    Have you read around this topic? Q and A will only take you so far in this process of understanding. Try this link.
     
  11. Oct 16, 2015 #10
    Yes i almost got the surface tension thank you but when we keep water in a container then why the upper molecules are not seen to be attracted downwards ?
     
  12. Oct 16, 2015 #11

    sophiecentaur

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    Where did you get the idea that they aren't? Did you look at that link or Google surface tension and see any of the dozens of diagrams that you can find?
     
  13. Oct 16, 2015 #12
    I saw many photos on that topic but in practical life we interpret that water in a glass when kept undisturbed and saw the surface its like no force acting on it
     
  14. Oct 16, 2015 #13

    sophiecentaur

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    Photos only show the same thing that we can see when we look at a glass of water. I am suggesting that you should look at the diagrams (at a simple particle level) which give a very good qualitative explanation of how surface tension works. Did you look at the Hyperphysics link I posted? At PF (and in Science in General), we try to look deeper than practical life but attempt to come up with models, at a lower level, to explain what we see in practical life. What do you find lacking in that link, btw? Do you need it to be more mathematical and at a deeper level? Afaik, the topic has been researched much deeper, if you want to go there, once you have accepted the simpler model.
    If it is stationary, there is no net force acting (including g).
     
  15. Oct 17, 2015 #14
    Yeah I want to see how surface tension works at particle level... I searched for it and i also saw the hyperphysics link but it just explained that there is a downward unbalanced force .
     
  16. Oct 17, 2015 #15

    sophiecentaur

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    I think you should be prepared to do some of the thinking for yourself here. I have already pointed out that molecules are in an equilibrium situation with attractive and repulsive forces balancing each other out. You don't need an external force to keep the end of a spring in place, if it's out in space. The net molecular forces are zero until you start to displace the ends. Same with a liquid surface.
    Draw a diagram like the one in Hyperphysics and add all the forces you can think of on a surface molecule or that have been mentioned on this thread and think a bit about the situation. I really don't think I can contribute any more if you don't.
     
  17. Oct 17, 2015 #16
    Okay
     
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