Andrew, if you are thinking of super-critical fluids, the answer is that they don't exhibit surface tension. The reason is that at any temperature above a critical value, you can't distinguish the liquid phase from the gas. There are no liquid drops and no phases that differ in density. Carbon dioxide in a fire extinguisher at room temperature, 25C, is a liquid-- you can hear it slosh when you shake the pressure cylinder, and the pressure of the gas above it is close to 73 atm. If you heat it to 31C, there is only one phase; it's a supercritical fluid. It can dissolve some substances--caffeine from coffee, for example-- and the density is what you would expect for a liquid, but expands like a gas to fill the hole container. Hope this helps.
MikeDowell
