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Old Oct 1st 2011, 01:28 AM   #1
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Post Surface Tension

Hi,
I know that this question might not sound very clever but I have been stuck at this since my physics class yesterday. When we say that the fluid molecules on the free surface of the fluid are being continuously pulled inward due to the force of attraction, don't they actually get pulled inside the liquid. In other words, is the free surface of a fluid dynamic, the molecules being continuously replaced? If so, then which molecules come up? The forces on the molecules inside the fluid are said to be balanced, so they can not rise up, I suppose.
I would be glad to get any help on this matter.
Thanks
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Old Oct 3rd 2011, 12:38 AM   #2
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They do rise up, that's also why you have evaporation. One molecule at the bottom of the vessel gets some velocity upwards, gets hit by other molecules from below it and gets even more speed and if it gets enough velocity, it breaks through the surface tension and this is evaporation!

But coming back to your question, the molecules around the molecule which went down are the ones to take its place, and those that are just below the topmost layer or molecules.
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Old Oct 3rd 2011, 01:19 AM   #3
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Re: Surface Tension

That basically is my question: Why do they rise up? Don't we say that the molecules in the bulk of the fluid are equally attracted on all the sides and are hence balanced. I am unable to reconcile this with the fact that fluid molecules are continuously in motion (which is another established fact). How can one say that the forces are balanced and still the fluid particles are moving?
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Old Oct 3rd 2011, 02:21 AM   #4
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Force is a result of change in momentum. As the molecules move, there is constant change in momentum and hence, constant force.

Or maybe it's the meaning of balanced forces which is not making me see your actual problem...
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