I ran across an eye-opening report this week, "Nearly perfect fluidity: from cold atomic gases to hot quark gluon plasmas". Not exactly the area that I normally work in although I have mentioned before that subatomic physics can contribute to polymer science. Instead it the abstract that got me to start reading the article.
Liquid viscosity drops with increasing temperature, while gas viscosity increases with temperature. This means that for a fluid, there is a minimum viscosity, probably in a supercritical region. One way to scale viscosity across fluids is with the kinematic viscosity (h/n, h is the viscosity, n is the density) but density is hard to measure/define for the quark gluon plasmas (I'll just have to take the authors word on it) so a better option is divide by the entropy, s. And then using string theory (which we all know is better called "the string hypothesis" since its never been tested), it's proposed that h/s >= h-bar/(4pkb), (kb is Boltzmann's constant). So there we have it - an absolute lower limit for viscosity.
Water at 226 bar and 650 K is still well above the limit, but that doesn't surprise me given the strongly associative nature of H2O molecules. 4He gets much closer at 2.2 bars and 5.1 K, but the champ is the quark gluon plasma at conditions of 880 x 1032 bar and 2 x 1012 K. The rest of the article is mostly in the realm of exotic physics, at which point I returned to my regularly scheduled reading. I'm not going to run a those conditions in any extruders.