Equilibration near the liquid-vapor critical point in microgravity

We used interferometry to measure the density changes associated with the late stage of thermal equilibration near the liquid-vapor critical point of SF₆. This experiment was performed on board the Space Shuttle in the absence of the considerable obscuring effects of gravity. The density changes were analyzed in terms of the exponentially decaying modes predicted by a model for heat diffusion in an isothermal container. The time constants depended only on the temperature and not on the sample's history. In the temperature range from Tc+98mK to Tc+28mK, where Tc=319K is the critical temperature, the values of the thermal diffusivity DT derived from the time constants at each temperature are consistent with the values of DT found by previous dynamic light-scattering measurements. From Tc+9 mK to Tc+1.4mK, where Earth-based measurements of DT are unavailable, the measured values of DT agree with an asymptotically correct extrapolation of DT closer to Tc. These data confirm our understanding of thermal equilibration in a range of T-Tc, where measurements on Earth are severely influenced by gravitationally induced convection and stratification of the sample.