The dynamics of flow between two weakly coupled macroscopic quantum reservoirs can be highly counterintuitive. In both superconductors and superfluids,

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 measured the initial magnetic suppression of the B phase of superfluid ^He at pressures from zero to 29 bars. This suppression, which is asymptotically quadratic in the field strength, is always significantly greater than the weak-coupling prediction, even at zero pressure. We also provide the first values of the .4-phase specific-heat jump for pressures below the polycritical point. Microscopic models of normal liquid ^He do not conform to these experimental constraints. Consequently, it is possible that the conventional identification of the order parameter of superfluid ^HQ-A is incorrect.

Parametric expressions are used to calculate the isothermal susceptibility, specific heat, order parameter, and correlation length along the critical isochore and coexistence curve from the asymptotic region to crossover region. These expressions are based on the minimal-subtraction renormalization scheme within the phi(4) model. Using two adjustable parameters in these expressions, we fit the theory globally to recently obtained experimental measurements of isothermal susceptibility and specific heat along the critical isochore and coexistence curve, and early measurements of coexistence curve and light scattering intensity along the critical isochore of 3He near its liquid-vapor critical point. The theory provides good agreement with these experimental measurements within the reduced temperature range |t|

The turbidity (τ ) measurements of Güttinger and Cannell (Phys Rev A 24:3188-3201, 1981) in the temperature range 28 mK ≤ T − T c ≤ 29 K along the critical isochore of homogeneous xenon are reanalyzed. The singular behaviors of the isothermal compressibility (κ T ) and the correlation length (ξ ) predicted from the master crossover functions are introduced in the turbidity functional form derived by Puglielli and Ford (Phys Rev Lett 25:143-146, 1970). We show that the turbidity data are thus well represented by the Ornstein-Zernike approximant, within 1 % precision. We also introduce a new crossover master model (CMM) of the parametric equation of state for a simple fluid system with no adjustable parameter. The CMM model and the phenomenological crossover parametric model are compared with the turbidity data and the coexisting liquid-gas density difference ( ρ LV ). The excellent agreement observed for τ , κ T , ξ , and ρ LV in a finite temperature range well beyond the Ising-like preasymptotic domain confirms that the Ising-like critical crossover behavior of xenon can be described in conformity with the universal features estimated by the renormalization-group methods. Only 4 critical coordinates of the vapor-liquid Y. Garrabos · C. Lecoutre · S. Marre · R. Guillaument CNRS, ICMCB-ESEME, UPR 9048,