Behavior of a Salt Water Solution Near the Critical Point Under Microgravity Conditions

The Supercritical Water Mixture (SCWM) experiment conducted on the International Space Station (ISS) is designed to study a salt-water solution in the vicinity of the critical transition with a test cell filled with Na2SO4 (aq) 0.5-w. This salt is a Type II salt which undergoes a dramatic decrease in solubility past waters critical point. The paper discusses the liquid-vapor-salt distribution in the test cell for subcritical, trans-critical, and supercritical regimes. In 0-g, the vapor-liquid interface manifests itself by the presence of a large single vapor bubble that is flattened between the two windows. In contrast with pure water, the vapor bubble remains centered in the test cell and is stable for temperatures up to Tc - 50 mK. The processes leading to critical transition during heat-up, as marked by the break-up of the vapor bubble, depends on the applied heating rate. For temperature ramps 1 mKmin, the bubble interface begins to breakup with the formation of a large number of smaller bubbles. For larger temperature ramps (10mKmin), temperature gradients arise in the cell pushing the bubble toward a window surface. As the critical threshold is crossed small variations in cell temperature ( 1 mK) cause an observable spatial variation in density. However, this spatial variation is not discernible as the temperature is increased further into the supercritical regime. The salt precipitation in the vicinity of the critical crossing appears to be heterogeneous in nature with precipitates observable on the cell windows. Direct imaging as well as light scattering indicate a hexagonal structure of the precipitated salt crystals in the near critical regime. The salt deposition is not fully reversible as the temperature is reduced due to window corrosion effects which traps some of the precipitated salt. Critical temperatures as high as 1.9 K above the value for pure water (647.25 K) have been measured. Finally, results on the critical transition phenomena in the presence of an imposed temperature gradient, demonstrating the remarkable stability of the central bubble, are also presented.