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I understand that water molecules at the surface have a net inward attraction due to the lack of water molecules above them. I've been reading a bunch of articles, and they say that this inward pull causes water to act as if the surface is covered by a thin, elastic material, but what does that mean?

Is surface tension caused only by the attraction between the water molecules on the surface to other water molecules on the surface? If so, why is surface tension said to be caused by the inward pull; if not, then what role does the attraction to the molecules underneath the surface play in surface tension?

If an intruder molecule were attempting to puncture the water surface, how would the water molecules around it behave in order to keep it floating?

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Not sure why people are downvoting this, this isn't a bad question. Might be a duplicate, though, but I haven't bothered to check yet. –  DumpsterDoofus Mar 31 at 3:27
    
I did read through about 4 pages of questions in the surface-tension tag before creating a new one. –  Physics Enthusiast Mar 31 at 3:29
    
Related: physics.stackexchange.com/questions/55833 –  Michiel Mar 31 at 17:14

1 Answer 1

It is not simply the water-air surface tension that allows the insect to walk on water. It is the combination of the legs not being wetted and the surface tension. The legs of water striders are hydrophobic.

Water molecules are strongly attracted to one another. This is due to "hydrogen bonding": a proton in water is shared between two oxygen atoms of two water molecules. Considering only water and air, minimizing the interface surface area is the lowest energy state, because it allows for maximum interaction between water molecules. If the water molecules were attracted to the molecules of the insect legs and wetted them, the legs would sink into the liquid. However, in the context of the legs not being wetted, the attractive forces of the water molecules result in a net upward force on the legs of the insect as the legs deform the surface.

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I think the term "hydrogen bonding" (even in quotes) is not the correct explanation, non-wetting objects can also be held by other liquids with sufficient surface tension, also the ones without hydrogen bridges. The intermolecular attraction due to Van der Waals forces is the primary reason that interfacial area is minimized. –  Michiel Mar 31 at 17:19
    
hydrogen bonding is only the reason intermolecular forces and surface tension are very strong for water. Other intermolecular forces (dipole-dipole, London forces) will also cause surface tension in other liquids as you are saying. –  DavePhD Mar 31 at 17:26

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