This review provides an overview of the progress in using the low-gravity environment of space to explore critical phenomena and test modern theoretical predictions. Gravity-induced variations in the hydrostatic pressure and the resulting density gradients adversely affect ground-based measurements near fluid critical points. Performing measurements in a low-gravity environment can significantly reduce these difficulties. A number of significant experiments have been performed in low-Earth orbit. Experiments near the lambda transition in liquid helium explored the regime of large correlation lengths and tested the theoretical predictions to a level of precision that could not be obtained on Earth. Other studies have validated theoretical predictions for the divergence in the viscosity as well as the unexpected critical speeding up of the thermal equilibrium process in pure fluids near the liquid-gas critical point. We describe the scientific content of previously flown low-gravity investigations of critical phenomena as well as those in the development stage, and associated ground-based work.

We have applied the method of Gor'kov for deriving the acoustic radiation potential on a sphere in an arbitrary sound field. Generalized potential and force expressions are derived for arbitrary standing wave modes in rectangular, cylindrical, and spherical geometries for the case where the sphere radius is much smaller than the wavelength {kR < 1). Criteria for determining radiation potential minima are derived and examples of characteristic spatial radiation potential profiles are presented. Single modes that can sustain stable positioning are discussed for each geometry. The localizing force strengths for representative standing wave modes in the three geometries are also compared. In this paper, we consider the positioning of samples due to acoustic forces only. However, the method developed here is general and can be extended to include gravity or other external forces.

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The "linear-model" parametric equation of state of Schofield, Litster, and Ho is used to analyze the effect of gravity near the gas-liquid critical point of a fluid. Detailed results are presented on the density distribution as a function of height, the constant-volume specific heat, and the low-frequency sound velocity, for arbitrary points in the (p, T) plane near the critical point. The influence of gravity on the determination of critical exponents is also considered. It is concluded that even for the thinnest practical samples, gravity corrections may have a significant effect on the exponents. The present theory permits gravity corrections to be made in a self-consistent way, with high accuracy.

The complex dielectric constants of liquids methane and ethane were measured at 90 K and 14.1 GHz, close to the frequency of the Cassini RADAR. The liquid ethane loss tangent is far greater than that of liquid methane, facilitating discrimination by remote sensing. The results suggest a methane-dominated composition for the northern sea, Ligeia Mare, on the basis of a recent loss tangent determination using Cassini RADAR altimetry. This contrasts a previous far higher loss tangent for the southern lake, Ontario Lacus, which is inconsistent with simple mixtures of methane and ethane. The apparent nonequilibrium methane-to-ethane ratio of Ligeia Mare can be explained by poor admixture of periodically cycled methane with a deeper ethane-rich alkanofer system, consistent with obliquity-driven volatile cycling, sequestration of ethane from the hydrocarbon cycle by incorporation into crustal clathrate hydrates, or periodic flushing of Ligeia Mare into adjacent Kraken Mare by fresh rainfall.