Fast thermalization in supercritical fluids

Heat transport in supercritical fluids in the absence of gravity is shown to occur in a novel and fast manner. Interferometric observations of the heat transport in supercritical ${\mathrm{CO}}_2$ at different densities and temperatures around the critical point show two interrelated mechanisms. A diffusing thermal boundary layer and fast adiabatic density increase in the bulk fluid via propagation of pressure waves have been evidenced. The border of the expanding thermal layer acts as a piston. The growth of the thermal boundary layer can be described using a simple scaled function and the bulk density increase is shown to be an adiabatic process. Near the critical point, the diffusing boundary layer can become unstable.