Talk:Surface tension

hello folks (newbie here).

I put a provocative statement in the main entry, to maybe get some discussion going.

The problem is that surface tension can't simply be explained by a "net inward force" argument, which is absurd. A net inward force would cause molecules on the surface to accelerate inwards: and bulk liquid would collapse, contrary to observation.

One theory states that molecules on the surface _do_ accelerate inwards, and sort of bounce back. In this view, the surface region is constantly in motion, and much as grains of sand on a rattling surface are.

But this can't be right either, because molecules in the next layer (ie just a bit deeper than the surface proper must experience a net force _out_ of the bulk. And this doesn't happen either.

My personal opinion is that surface tension is too complicated to be explained by classical mechanics of the highschool type, and any attempt to do so just confuses matters.

Real water is essentially quantum; even in the classical view, molecules have a dipole moment and it's not clear whether this moment is oriented into the liquid, out of the liquid, or parallel to the surface.

Just my two cents....comments please!

[sorry, wasn't logged in. I've just discovered the four tildes thing.

best

Robinh 21:33, 11 Dec 2003 (UTC) ]

The "net inward force" idea is, as stated above, wrong.

One way to see how surface tension works is to see that molecules at the surface, being in the potential wells of fewer neighbours than molecules inside the liquid, have higher energy. They will tend to move to a position of lower energy. This process will stop when equilibrium has been reached. Then it is seen that the density of molecules in the surface layer is very slightly lower than in the bulk of the liquid, because some of them moved. They are therefore further apart than the equilibrium position, and that means that there is an attractive force parallel to the surface, between any two molecules. They do not move, though, because they are pulled from each side. This is the cause of surface tension: far from there being a "surface film", there is a very slight reduction in the number of molecules, in a very thin layer, only a very few molecules thick.

Derek Locke - February 6th 2004.

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Hi Derek

thanks for your comments. I don't want to be overly confrontational here, but it does seem to me that, if there is a "very slight reduction in the number of molecules" in the surface layer, then these molecules will accelerate towards one another and collapse. And they don't.

(or do they? I admit to being 100% baffled by surface tension!)

Basically, I don't think that the surface layer of a liquid is in any sort of static equilibrium (except possibly statistical equilibrium) so one cannot apply equilibrium analysis to the situation (for example, you say above that "some of them moved". My idea of molecules in a surface layer is that they are all vibrating and rotating about like crazy!). But I could be wrong.

I don't want to be critical of your arguments above, but I would say that using the phrase "tends to move" is not helpful here. Surely the correct concept would be acceleration?

I would also suggest that a force is _by definition_ a gradient of potential energy. Your arguments would suggest that an inwards force does indeed exist!

best wishes

Robinh 22:21, 7 Feb 2004 (UTC)

Hello Robinh, Thank you for your comments. I have calculated the reduction in density at the surface. The whole substance is in equilibrium, though subject to thermal motion. Change in density does not always imply any acceleration. For example, in a beam that sags under gravity, the density is a little greater at the top than the bottom. It is in fact the change in density that produces the forces that allow and equilibrium to be found. In a beam, we don't usually refer to "density": we refer to "strain". So the whole point of the less dense surface of the liquid is that the density variation is just that which is needed to compensate for the fact that the surface molecules have fewer neighbours than inner ones do.

I have not seen this "explanation" elsewhere, but it is the only one I have seen that provides a reason for a tension parallel to the surface.

If an inward force exists, it only does so during the minute time that the system needs to reach equilibrium. After that, there are no outward or inward forces, or there would be a flow.

Best regards, Derek Locke _________

Hi Derek

very interesting comments above. I like your "beam" analogy. I will have to think about this. I guess my difficulty is that I imagine water molecules to be like little gas molecules, unable to interact except during collisions. My image cannot be right if your beam analogy holds up (!).

In the beam, one can only understand the situation by assuming local isotropy and local homogeneity (on a molecular scale). Neither of these assumptions are true in the surface layer of water. This might or might not be important, but it seems to me that we should be clear about whether or not we assume them.

Maybe the molecules in the rarefied regions are moving about faster; would this make sense?

best wishes

Robin

Robinh 22:02, 12 Feb 2004 (UTC)

Hello Robin, I believe that in a liquid, each molecule is always interacting with its nearest neighbours. The mean speed of the molecules depends only on the temperature and not on the local density, though it is possible that "temperature" has little meaning if we consider a region that is too small. This is the case because temperature is a property of bulk matter, or at least of samples big enough that the mean energy is constant at the accuracy with which we can measure. Regards Derek ____________

Question from another newbie,(with only the smallest education in physics):How does one break the surface tension of water without resorting to the use of surfactants? My problem is to remove the water from a polymer surface at no more than 135 degrees Fahrenheit, using forced air, vacuum, or any other economical method. This is to be accomplished on a complex surface on the inside of a cube(excluding the bottom face, which will be attached at a later time).

If you wish to merely remove the water, you can easily convert it to a gas state by reducing pressure (vacuum will do it) - or you can use it's adhesive properties (capilliary action) to get rid of it - by using a tissue or something like that! (20040302)

robin and derek,

           i feel that the molecules on the surface have a velocity only in the direction parallel to the surface.they dont move into the surface.they resist moving down.so it appears that there is tension on the surface.the net downward force is balanced by the liquid's resistance to compression.this resistance to compression is like the normal contact force, there is no fixed value.if we place a small object (small enough such that it doesnt displace any liquid)it will exert more force due to which surface expands(depression)the molecules on the surface still stay on the surface keeping the object on the surface.