How do you measure surface tension?

In the world of industrial cleaning technology we talk about surface tension a lot! So much so, in fact, that it is hard to enter into any discussion of cleaning without having the subject of surface tension arise.  In cleaning chemistry, for example, we are always looking for lower surface tension to promote penetration of small surface features and blind holes.  Surface tension has a major effect on ultrasonic cavitation and implosion.  A less well-known fact is that surface tension has a significant effect on the droplet size and pattern produced by spray nozzles.  So, if this thing called surface tension is as important as it appears, we should be able to put numbers on it and measure it, right?  Absolutely!  But there are also some difficulties.

In preparing to write this blog, I decided (as always) to do a little research.  My thought was that this must be a simple thing with a simple answer – – well, not so much!  There are several general methods of measuring surface tension with several different implementations of each.  The most common method appears to be lowering a thin plate, rod, wire shape or tube into the liquid to wet it and then, using a balance or other weighing device, lifting the immersed item while measuring the weight exerted on it by the surface tension of the liquid.  The maximum force corrected for the weight of the item, buoyancy and other factors including the adhesion of the liquid being measured to the item immersed can be used to measure surface tension.

But what if the liquid does not wet the surface?  The technique then becomes one of measuring the force that must be exerted downward to break the surface tension which is a completely different problem.

Another method uses the droplet size that can be generated at the end of a hypodermic needle with one of several end configurations as an indicator of surface tension.  Higher surface tension liquids, of course, produce larger droplets.  This method is complicated by the extension of the droplet as it breaks free, the density of the liquid and several other factors.

Finally, there are methods that use the size bubble produced as a gas is introduced into a liquid through a small tube to determine surface tension.  Smaller bubbles indicate lower surface tension.

What I expected to be a simple challenge turned out to be majorly complex involving a number of different techniques and a LOT of math – – much more than I can cover in this blog.  What I did get out of all of this was that there seems to be no single way of measuring surface tension that is applicable in all situations.  The other thing is that it is very easy to confuse surface tension and wettability.  Using mercury as an example, mercury has a very high surface tension but will not wet glass yet it easily wets copper despite its high surface tension.

–  FJF  –