Coalescence Dynamics of Near-Critical Sulfur Hexaflouride in Microgravity*

Critical fluids have a variety of applications from manufacturing high-tech materials to industrial lubrication and extracting oils from foods. Phase separation of critical fluids cannot be studied on earth due to the increase in compressibility near the critical point and stratification of fluids by density in gravity. We used direct imaging to record snapshots of phase separation that takes place in sulfur hexafluoride (SF6) in weightlessness conditions on the International Space Station (ISS). The system was at liquid-vapor equilibrium slightly below the critical temperature and further cooled down by a 0.2-mK quench that produced a new phase separation. Both full view and microscopic views of the direct observation cell were analyzed to determine the evolution of the radii distributions. In addition, in microscopic view, we measured the diameter of droplets and bubbles in the system throughout multiple coalescence events leading to further support of the coalescence-induced-coalescence model.