When placed near the critical point of their phase diagram, the fluids turn to be highly compressible and their thermal diffusivity tends toward zero. When heating the wall of a closed container filled with a supercritical fluid, the fluid in the very thin diffusion layer expands strongly and produces, like a piston, compression waves which will quickly and evenly heat the whole fluid, even in the absence of convection. It is thus the fourth heat transfer mode, specific to hyper expandable media, called “piston effect” (after the name given by the French team who discovered the phenomenon).
The coordinates (density, temperature, pressure) of the critical point on the pure body's phase diagram are physical specifications of that body as the vaporisation temperature under atmospheric pressure for example. At a density equal to the critical point's density, with temperatures and pressures above their critical values, the liquid and gaseous phases are not discernible anymore. The fluid is called supercritical fluid. Its density is in the vicinity of the liquid's but it is thousands of times more compressible than gas. Near this point, the thermophysical properties are either divergent (isothermal compressibility, thermal conductivity, specific heat at constant pressure) or evanescent (thermal diffusivity).
On the ground, the hydrostatic pressure stratifies the medium and the critical conditions cannot be evenly obtained anymore within a sample. Supercritical fluids are thus very good candidates for a microgravity utilisation which would eliminate the hydrostatic pressure and allow scientists to observe uniform samples.
The microscopic and statistical physics of supercritical fluids improved a lot since the 70's when Kenneth G. Wilson won the Nobel Prize in Physics for his renormalization group theory. In the 90's the macroscopic and hydrodynamic physics of those objects started to develop, motivated by the access to microgravity and the first experiments on heat transfers. In fact, since the thermal diffusivity of fluids tends towards zero when placed in the vicinity of their critical point, the possibility to experiment in microgravity gave the opportunity to scientists to study how, in the absence of heat transfers by radiation, diffusion or convection, the heat could billow more and more slowly as the critical point is reached.
Numerical simulations performed at CNES soon demonstrated that the heat could billow more and more quickly leading to a “critical speeding up” instead of the “critical slowing down” admitted by the critical phenomena's physics. The responsible mechanism have been identified by numerical simulations: medium's thermal-acoustic heating. When heating the container wall, a very thin fluid layer is heated (evanescent thermal diffusivity). The contained fluid expands strongly (expandability tends towards infinity) and produces, like a piston, very strong compression acoustic waves which heat the medium. The bigger the expandability, the quicker the waves will heat the medium. Unlike what was originally thought, it is not about a critical slowing (a few days calculated on account of the heat diffusion) but the critical point proximity (typically a few seconds for 10 mm to 1 K from the critical point).
Released in Physical Review Rapid Communication on February 1990 , this theoretical prediction was confirmed by a rocket probe experiment during the Texus 25 flight in 1991 , corroborating the existence of a fourth heat transfer mode named “Piston Effect” by the critical phenomena's International community.
After this discovery, many research teams dedicated themselves all around the world to the study of this phenomenon and its consequences according to its hydrodynamics. At the French level, two instruments were dedicated to the hydrodynamics of the supercritical fluids: ALICE I and II (ALICE stands for Analyse des Liquides Critiques dans l'Espace - In-Space critical liquids analysis) which operates on board the Russian station MIR.
Nowadays, the continuation and expansion of the research field to the combustion in supercritical water is performed on board the ISS with the DECLIC (DEvice for the study of Critical LIquids and Crystallization) instrument. Industrial patents have been filed, in particular for a thermal compressor for xenon, by the Air Liquide company, and a Grand Prix of the French Academy of Sciences was awarded to the team who made the discovery in 2000. Now a reflection does exist about the application of the phenomenon to the heat pipes development which could carry low energies on large distances without any dissipation.
The piston effect: a fourth heat transfer mode
The last sequence of this video (in french) shows a cell full of supercritical carbonic dioxide, on the ground (G in the video) and in space (D in the video).
The compression effect resulting from the dilatation of the fluid contained in the heated area (in black) can be measured looking at the number of unfolding fringes with increasing density.
The fact that those fringes remain parallel shows that the density is homogeneous and so that the medium's heating by piston effect is not only extremely fast but also homogeneous, just like the theoretical predictions thought (pressure is homogenised, with a time-scale of a few seconds, by the acoustic waves).
This convection-free experiment in space confirmed the existence of a fourth heat transfer mode in the extremely expandable media.
References cited in the article:
 B. Zappoli, D. Bailly, Y. Garrabos, B. Le Neindre, P. Guenoun and D. Beysens « Anomalous heat transport by the piston effect in supercritical fluids under zero gravity », Phys. Rev. E Rap. Com., 41, 4, 2264-2267
 P. Guenoun, B. khalil, D. Beysens, Y. Garrabos, F. Khamoun and B. Zappoli « Thermal cycle around the critical point of carbon dioxide under reduced gravity »,Phys. Rev. E, 47,3 1531-1540