Critical Point in Space: A Quest for Universality

Abstract

The behavior of matter near a 2nd order phase transition is expected to obey universal features. In particular, fluids, liquid mixtures, polymers, which belong to the same class of universality (the class of fluids) should exhibit the same universal scaling laws for many thermodynamics and kinetic parameters. Critical point slowing down is the most notorious. Such divergence or convergence makes the class of fluids extremely sensitive to even minute external disturbances and especially gravity: on earth the fluid becomes compressed under its own weight. Compensating for these effects by space experiments and/or magnetic forces or isotopic density matching has led to enlarge our vision of universality for phase transition. New phenomena have been discovered by suppressing gravity effects, as the thermal “Piston Effect”, which leads to a paradoxical critical point speeding up and the apparent violation of the 2nd thermodynamic law. Another finding is concerned with the use of critical slowing down and weightlessness to investigate the dynamics of phase separation with no gravity-induced sedimentation. The key role of the coalescence of domains makes valid only two simple growth laws. The latter can be successfully applied to a quite different situation, the evolution laws in the well-known biological problem of sorting of embryonic cells. Due to the extreme sensitivity of fluids near their critical point, the effect of vibration can be investigated in much detail, using only one fluid, which represents the whole class of fluids. The investigation of the above thermal and phase transition problems under vibrations indeed suggests that a periodic excitation can act as a kind of artificial gravity, which induces thermal convection, speeds up phase transition and localizes the liquid and vapor phases perpendicular to it.