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We study the response of a fluid in near-critical conditions to a heat pulse, in the absence of gravity effects. The fluid under investigation is CO 2 at critical density. It is enclosed in a thermostated sample cell. We apply a theory that accounts for hydrodynamics and a real equation of state. Comparison with experiments performed under reduced gravity on board the MIR orbital station show quantitative agreement and demonstrate that the dynamics of relaxation is ruled by two typical times, a diffusion time t D and a time t c associated to adiabatic heat transport, the so-called ''Piston effect'' ͑PE͒. Three regions are observed in the fluid. First, a hot boundary layer, developing at the heat source, which shows large coupled density-temperature inhomogeneities. This part relaxes by a diffusive process, whose density and temperature relaxations are slowed down close to the critical point. Second, the bulk fluid, which remains uniform in temperature and density and whose dynamics is accelerated near the critical point and governed by the PE time. At the thermostated walls a slightly cooler boundary layer forms that cools the bulk also by a PE mechanism. The final equilibration in temperature and density of the fluid is governed by the diffusion time t D , which corresponds to the slowest mechanism. Comparison with a one-dimensional model for temperature relaxation is performed showing good agreement with experimental temperature measurements. A brief comparison is given with the situation in the presence of gravity.

We report the study of behenic acid Langmuir monolayers spread over chloride salts solutions of cadmium, lead, magnesium, or manganese. These monolayers were investigated by means of surface pressure−area isotherms and grazing incidence X-ray diffraction (GIXD). The effect of the concentration of cations in the subphase on the structure of the monolayers was probed at room temperature and for three different subphase pHs (5.5, 7.5, 10.5). A threshold in subphase concentration is detected for the formation of a superlattice structure corresponding to an inorganic organized layer in addition to the ordered behenic acid monolayer. This threshold is shown to strongly depend on the cation and the subphase pH. Above the threshold, the superstructure is independent of both the cation concentration and the pH. Below the threshold, the ions are disordered but induce a condensing effect on the fatty acid molecules, which is more or less pronounced depending on the ions. Moreover, the combination of isotherms and GIXD allows us to show that the existence of superstructures can be predicted from the shape of the isotherms. Indeed, a good agreement is obtained between the thresholds determined by the two experimental techniques.

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It is possible to adjust the density of cyclohexane by adding a small amount of deuterated cyclohexane. Then the mixture deuterated cyclohexane-cyclohexane-methanol can be made isopycnic.We have determined the critical properties of this system and of the systems cyclohexane-methanol and deuterated cyclohexane-methanol: coexistence curve, correlation length, and osmotic compressibility via refractive index and turbidity measurements. For that purpose, a detailed discussion of the validity of the volume additivity and of the Lorentz-Lorenz formula has been made. The main result is that the isopycnic system acts as a binary mixture for the phase transition properties as shown by the P exponent and the amplitude combinations (Rt+R, ' ) which exhibit the universal values. Some aspects of microgravity conditions can then be created. We have studied in the isopycnic system the phase separation at critical concentration; new features are found after a thermal quench: macroscopic spinodal decomposition structures during the phase separation process and macroscopic wetting layers in the final equilibrium state.