Study of the Phase Boundary for C₆F₆ and SF₆ under Microgravity

Abstract

Two groups of experimental data obtained in the vicinity of the critical point are discussed. Group I describes the level h_t of the meniscus separating the two phases of the substance in the cell. The measurements were performed for SF₆ under the condition (g = 9.8 m s^–2) during an experiment conducted in a space laboratory. Group II includes data on the density of liquid and vapor measured for C₆F₆ along the saturation curve under terrestrial condition. 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 h_t level (experiment 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 τ = (T – T_c)/T_c = 2 × 10^–6–0.01. An approach is considered that takes into account the influence under microgravity (g = g_M ⪡ 9.8 m s^–2) on the height h (experiment 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 I and II in a wide temperature range and correspond to the scaling theory of critical phenomena.