Study of the phase boundary for $\rm C_6\rm F_6$ and $\rm SF_6$ under microgravity

Two groups of experimental data obtained in the vicinity of the critical point are discussed. Group $\rm I$ describes the level $h_t$ of the meniscus separating the two phases of the substance in the cell. The measurements were performed for $\rm SF_6$ under conditions $(g = 9.8$ m s$^{–2})$ during an experiment conducted in a space laboratory. Group $\rm II$ includes data on the density of liquid and vapor measured for $\rm C_6\rm F_6$ along the saturation curve under conditions $(g = 9.8$ m s$^{–2})$. In both cases, the studied two-phase sample is located in a horizontal cylindrical cell. In the second experiment, the gravitational effect was also measured along the isotherms as the dependence of the sample density on the height $h$ measured from the bottom of the cell. An equation relating the ht level (experiment $\rm I$) with such functions as the order parameter $f_s$ and the average diameter $f_d$ is derived in this work. The obtained equation describes the initial experimental data at relative temperatures $\tau = (T - T_c)/T_c = 2 \times 10^{–6}$–$0.01$. An approach is considered that takes into account the influence under microgravity $(g = g_M \ll 9.8$ m s$^{–2})$ on the height $h$ (experiment $\rm II$). The dependences that represent $f_s$ and $f_d$ and the density of the liquid and gas phases along the saturation curve of these substances are obtained. These dependences agree satisfactorily with the results of experiments $\rm I$ and $\rm II$ in a wide temperature range and correspond to the scaling theory of critical phenomena.