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Having read the Wikipedia page on superfluids I'm still not sure if stuff like liquid helium at the lambda point actually have surface tension or not. Is superfluidity the same thing? And are there any liquids with no surface tension at room temperature?

Chris Wenham
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  • Superfluids are lacking viscosity, not surface tension. –  Jan 08 '11 at 20:01
  • I don't agree that the surface tension needs to be defined between 2 phases, although it is true. A drop of water is a perfect sphere due to the surface tension. You may say indeed that it is surrounded by air. How about Mercury? If you have a drop of it in vacuum, it will be spherical. Of course, it will evaporate until freezing point, but it will still show surface tension. That is because surface tension is first of all a property of the material (liquid phase). This is why the liquid is liquid, otherwise it would be a gas or a solid. IT has bounding forces between the atoms/molecules and t – catalin Jul 03 '16 at 12:20
  • The surface tension is a function of the interfacial energy and it does depend on what fluids are either side of the interface. The interfacial energy of the mercury/vacuum interface will be different to the mercury/water, mercury/oil or indeed mercury/anything else interface. It is not a property of the material, it is a property of the interface. – John Rennie Jul 03 '16 at 17:03

2 Answers2

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First of all, Marek is right that a surface tension exists only between two different materials (well, I would say between two different phases - for example water and ice). So let's rephrase the question as "Are there two phases with zero surface tension?" and elaborate a little on the answer.

The surface tension is the excess free energy (technically the 'grand potential') associated with the area of the interface between two phases. If the surface tension is positive (it always is), the system minimizes the free energy cost by minimizing the area of contact. This leads, for example, to the spherical shape of a water droplet in coexistence with water vapor. But if the surface tension were zero, we could deform the shape of the water droplet arbitrarily, with no free energy cost, so long as we didn't change its volume! We could even deform it to the point where, from the macroscopic point of view, the water and the water vapor seem to be perfectly mixed. But this actually contradicts the fact that there was phase separation in the first place, because phase separation is an indication that a uniform (single-phase) system can lower its total free energy by splitting into two phases.

So zero surface tension between two phases would be quite an unrealistic situation. There are some situations in physics which are somewhat like having zero surface tension. If you look at the surface tension between water and water vapor as the critical point is approached, the surface tension vanishes. But at the critical point itself there is no distinction between the two phases (and therefore no interface), so we can't say that there is zero surface tension. Another situation where it is tempting to say that surface tension has been made to vanish is when oil and water (which don't mix) are emulsified by adding a surfactant or emulsifier which lowers the surface tension. Eventually, rather than two coexisting phases, we have a single phase with a lot of internal structure (little droplets of oil in water or vice versa). But that is a single phase, not two coexisting phases!

So I don't think that any two coexisting phases have zero surface tension. In particular, liquid helium has a positive surface tension (with air I mean).

Greg P
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  • +1; Still there is zero surface tension at contact of two same phases -- this is of course only a virtual claim about imaginary boundaries, but it is sometimes used. –  Jan 08 '11 at 19:57
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    "If the surface tension is positive (it always is)" @greg Unless I'm mistaken, I thought mercury had a negative surface tension. Which is why in a capillary it is concave at the top (instead of convex as in the case of water). –  Jan 08 '11 at 22:40
  • @Greg: I believe the critical point (second order phase transition) at the top of (say) the liquid/gas boundary is a little special as an additional term in the expansion of the potential is rendered zero, and other effect dominate over short distance scales. That is, the surface tension is effectively zero. – dmckee --- ex-moderator kitten Jan 08 '11 at 23:03
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    @space_cadet: Three materials there. It would be sufficient for the Hg/air boundary to have lower(?) tension than the Hg/glass boundary in contrast to the situation with water, air and glass. (I don't know that this is the case, just noticed that it would be enough...) – dmckee --- ex-moderator kitten Jan 08 '11 at 23:05
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    @space_cade: No, mercury does not have a negative surface tension with air. The shape of the meniscus is determined by the contact angle: http://en.wikipedia.org/wiki/Contact_angle As as dmckee says, this depends on the three materials (mercury, air, glass tube for example) and the surface tension between each pair of them. A negative surface tension leads to the same contradiction I mention in the answer. Even worse, it means that you can lower the free energy arbitrarily by deforming the surface. – Greg P Jan 10 '11 at 15:31
  • @dmckee: As I mention in the answer, yes, the surface tension goes to zero as one approaches the critical point. However, at the critical point itself there is only one phase, since there the distinction between the two phases is lost. In particular, there is no longer any boundary between two phases. – Greg P Jan 10 '11 at 15:32
  • @Greg That wasn't a critisim, rather a starting point for folks who want to read more. To be a little more clear, very near the critical point (and they really mean "very") you get a rapidly changing, lumpy arrangement of the still distinct phases because the surface tension has gotten so close to zero that it is overwhelmed by statistical fluctuations. So it is not truly "zero surface tension", but it is close enough to exhibit the physics that would be associated with such a situation. – dmckee --- ex-moderator kitten Jan 10 '11 at 15:41
  • @dmckee: You are right, it is indeed a good illustration of what things would like if surface tension vanished. – Greg P Jan 10 '11 at 15:45
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Surface tension is not a property of materials but of interfaces between two (or more) materials. It is implicit in its definition that the interface separates two kinds of materials that behave differently (otherwise the interface would be just some imaginary surface inside the one material with no physical meaning) and so there must always be some surface tension that sustains the physical interface.

Marek
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    You've going to have to allow that vacuum is a "material" for the purpose of this definition... – dmckee --- ex-moderator kitten Jan 08 '11 at 00:43
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    @dmckee: sure, vacuum is a well-defined $N \to 0$ limit of air :-) – Marek Jan 08 '11 at 00:51
  • I take a similar issue as dmckee here. You're ignoring the possibility of an interface being created by external fields, gravity for example. I think this idea could still be salvageable. I think more detailed answer may call upon statistical mechanics to doubt the possibility of an instantaneous density change when doing so would leave free energy on the table. – Alan Rominger Jul 30 '12 at 17:57