Phase separation of critical binary fluids under microgravity: Comparison with matched-density conditions

A number of phase-separation mechanisms in fluids are hidden in earth-bound experiments due to the influence of gravitationally induced flow. Solutions to this problem can be found by using strictly matched-density systems, such as the partially deuterated cyclohexane and methanol system (${\mathrm{C}}^{\mathrm{*}}$C-M), or by performing experiments in space so as to benefit from the microgravity environment. Both types of studies have been performed here to determine quantitatively to what extent such matched-density systems could recreate, on earth, microgravity conditions; they were restricted to the neighborhood of the liquid-liquid critical point. A 6-min microgravity environment has been provided in the sounding rockets of the TEXUS program: Two experiments have been performed, one (T11) using the nonmatched-density system of fully deuterated cyclohexane and methanol (${\mathrm{C}}^{\mathrm{*}}$-M), and one (T13) using the matched-density system ${\mathrm{C}}^{\mathrm{*}}$C-M. In experiment T13 the phase separation was seen to be characterized by a high-contrast, periodiclike pattern, so that the observations could be performed by a simple optical method. In order to analyze quantitatively the possible differences with the earth-based experiments, the origin of the detected images had to be carefully analyzed. It appeared that they are connected to the interface pattern of the phase-separating fluid. Therefore, after digitization and statistical treatment, it was possible to check the important scaling properties of the phase separation: structure factor, reduced moments, typical wavelength pattern. Experiment T11 was qualitatively different from the earth-bound experiment and can be understood only by an unexpected extreme sensitivity of the growth mechanism to the volume fraction. The T13 results did not evidence any deviation between the microgravity and the earth-based experiments, thereby validating a complete study of the phase separation in the critical region using matched-density systems on earth. Gravity effects on very-long-term periods remain to be estimated, however.