Near-critical point hydrodynamics and microgravity

Near their critical point, fluids exhibit anomalous behavior of thermodynamic parameters (divergence of specific heat, compressibility and expansion coefficients) and transport coefficients (heat conductivity, thermal diffusivity). Weightlessness ("microgravity") environment permits to go very close to the critical point, thus allowing key tests of the Renormalization Group theory to be made. It also results in a very particular hydrodynamics of dense and hyper-compressible gases, where weightlessness experiments play a key role. For instance, a very fast thermalization effect ("Piston effect") is evidenced, where a thermal boundary layer expands and adiabatically heats the whole fluid, leading in some cases to an (apparent) violation of the laws of thermodynamics. Another one is concerned with the use of critical slowing down and microgravity 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; they can be successfully applied to a quite different situation, the evolution laws in the well-known biological problem of sorting of the embryonic cells. Other situations are concerned with the effect of vibrations. The investigation of the above thermal and phase transition problems suggest 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. Some of these phenomena still persist at higher temperature and pressure. Fluids in such supercritical conditions are very appealing to the industry as non-polluting solvents or hosts of chemical reactions with high yield.