Re: Does the surface tension of a liquid vary with the ambient pressure

There are a few different things you could mean by this question, so I'll 
talk about a couple of them and hope that it encompasses what you need to 
know.

Strictly speaking, the vapor/liquid surface tension of a fluid is defined 
for the pure liquid in equilibrium with its pure vapor.  So, there is not 
really a question of varying pressure; the pressure is always the vapor 
pressure at whatever the temperature is (so for water at 100 degrees C, it 
would be standard atmospheric pressure).  So, in this strict sense, it 
does not make sense to talk about varying pressure independently.

However, in practice one often talks about the surface tension between a 
liquid and *air*.  For most practical purposes, this turns out to be about 
the same as the "real" definition.  This is because physically the surface 
tension is primarily determined by the properties of the liquid, and it 
doesn't really matter much whether the vapor on the other side of the 
surface is that liquid's vapor, or air, or any fluid with low density.

So, it is not unreasonable to ask about the effect on surface tension of 
increasing the pressure of air above a liquid.  And the answer to that 
question is that the surface tension would vary with the pressure, but 
that variation would be very small.  This is for the reason mentioned 
above -- as long as the liquid is much more dense the surface tension is 
determined mainly by its properties, and there will be very little 
difference in the liquid phase whether the vapor is air at 1 atm pressure 
or 2 atm.  If you applied *lots* of pressure you would eventually get a 
bigger effect, both because the vapor density would get closer to that of 
the liquid and because the amount of air dissolved in the liquid would 
increase to the point where it had a greater impact on the properties of 
the liquid.

So, in this not-quite-strict sense mentioned above, pressure can affect 
surface tension but it will typically be a small effect.  The 
*temperature* typically has a bigger effect.  This again goes back to the 
fact that the liquid properties mainly determine the surface tension, and 
for liquids (at least well below their critical temperature) temperature 
has a much bigger effect on liquid properties than pressure does.

Finally, I will say that all of this is for vapor/liquid surface tension, 
which is all I am familiar with.  If your concern is mainly about solid-
liquid surface tension (contact angles, wetting, etc.), you will need to 
resubmit and be more specific about that, and maybe they can find somebody 
else who knows about that area.