# Surface Tension

Hello everyone,

I would like to challange any competent physicist to tell me in simple terms what surface tension really is. There seems to be an abundant number of websites, not to mention a few books, that are simply too vague and unnecessarily difficult at attempting to explain this topic.

When explicitly focusing on how the "film" or "skin" coating is formed in the surface area of a water liquid due to the cohesion or attraction of molecules, how does it really form?

Some physicists employ the use of "the attraction forces between balk molecules and surface area molecules creates a tension". Does this tell me how the surface tension is formed? can it ever be rationalized by simply stating there's a tension? tension in what sense?

Others go further at saying "All of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression."

So, the molecules are contracted inwards or to the centre of the liquid, causing an imbalance in forces, and in order to balance them, you have to compress the liquid? So what happens if you don't compress the liquid? Will the water magically reduce in volume due to the resultant force?

Or is it, it can only be balanced by the liquid not compressing at all? ("resistance")

## Answers and Replies

Andrew Mason
Homework Helper
Surface tension of water has to do with a water molecule being a polar molecule that can form hydrogen bonds with other water molecules.

Consider a pair of water molecules in a volume of water that are held together by the weak hydrogen bonds . If there are other water molecules in all directions, there is a greater likelihood that they will be pulled apart. For molecules on the water-air surface, there are water molecules below but none above, so the surface molecules tend to hold together better than those surrounded completely by other water molecules.

With non-polar molecules, the surface tension is much less. But there are Vanderwaal's forces between molecules that provide some surface tension.

AM

For the pupose of this disscussion assume that all molecules or atoms in a liquid attract each other equally.
consider some water in a cup, molecules that are anywhere other than at the surface have other molecules all around it (including above and below) so the molecule experiences force in each and every direction and all these forces cancel each other out thus no net force (this is called bulk/balk molecules)

Now consider molecules at the surface, these molecules have other molecules on its sides and below it but not on top, so these molecules experience a force in all directions except upwards, thus a net downward force.
Now each of the molecules at the surface have this downward force and this force is what is called surface tension.

Some physicists employ the use of "the attraction forces between balk molecules and surface area molecules creates a tension". Does this tell me how the surface tension is formed?
yes it does (although not accurately), consider the above
the surface molecules are at the surface while the rest are bulk molecules. however only the molecules just below the surface contribute to the force/tension and not all the bulk molecules.

can it ever be rationalized by simply stating there's a tension? tension in what sense?
yes it can
tension in the sense that the surface layer of molecules have a force/tension downwards

Others go further at saying "All of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression."

So, the molecules are contracted inwards or to the centre of the liquid, causing an imbalance in forces, and in order to balance them, you have to compress the liquid?

first of all let me say that you do not have to "compress the liquid" in fact liquids can't be compressed(at least not significantly).

the molecules are not contracted to the center, take note again that only molecules at the surface experince a force. This force is directed towards the rest of the liquid.

So what happens if you don't compress the liquid? Will the water magically reduce in volume due to the resultant force?

Or is it, it can only be balanced by the liquid not compressing at all? ("resistance")
The water will never reduce magically because
- only surface molecules experience force
- water cannot be compressed
- the force is balanced by "resistance" (liquid not compressing)

I hope I have answered some of your questions

kalesh said:
yes it can
tension in the sense that the surface layer of molecules have a force/tension downwards

It certainly can't if you don't rigorously mention the word downwards! How vague is a statement telling me that there's a tension without including the word downwards?

kalesh said:
the molecules are not contracted to the center, take note again that only molecules at the surface experince a force. This force is directed towards the rest of the liquid.

When I stated the molecules were attracted inwards, I obviously meant the surface molecules.

kalesh said:
The water will never reduce magically because
- only surface molecules experience force
- water cannot be compressed
- the force is balanced by "resistance" (liquid not compressing)

You don't actually think I believed the water molecules reduced in volume magically, did you? It's called sarcasm!

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Andrew Mason said:
Consider a pair of water molecules in a volume of water that are held together by the weak hydrogen bonds . If there are other water molecules in all directions, there is a greater likelihood that they will be pulled apart. For molecules on the water-air surface, there are water molecules below but none above, so the surface molecules tend to hold together better than those surrounded completely by other water molecules.

With non-polar molecules, the surface tension is much less. But there are Vanderwaal's forces between molecules that provide some surface tension.

That's a very succinct and unequivocal concept take that I fully agree with. The only inclusion I would mention is that there are less water molecules on the surface of the liquid because of the simple fact that water is not instantly transfered to a gas state when vaporisation occurs. The transition happens in steps and therefore makes it more effective when the surface molecules are somewhat broken down in number at the surface of the liquid.

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Just a new line of thought, I am not quite sure but I have ever read a definition of surface tension. Whenever there is a fluid interface, surface tension can be defined as Work done require per any small change in Area :

$$\Delta\,W=\sigma\,\Delta\,A$$

and $$\sigma\$$ is the surface tension constant specific to the interface. It sounds some sense to me, since it translate to a bigger tension when more energy is needed to stretch an area. Though what happening in the surface is not told by this formula :surprised