Phase Separation in Fluids in the Absence of Gravity Effects

Abstract

Gravity effects during the phase separation of binary fluids in the critical region have been suppressed by two means: annuling the gravity in a space experiment and/or using a strictly density-matched mixture on earth. Such an isodensity system can be produced by partially deuterating one component in a binary mixture of cyclohexane and methanol. It has been demonstrated that this does not affect the phase transition. Periodic-like patterns can thus be observed. They grow from a microscopic scale up to the final equilibrium stage, determined by the competition between wetting forces, finite volume effects and the remaining gravity influence. These structures are measured as a 2-D section of the 3-D pattern of interfaces between the phase domains. However only interfaces orientated perpendicular to the plane of observation can be detected. This combines with the interconnectivity of the domains to make the visible interface periodicity (L_m) the same as that of domains. After a numerical analysis, the structure factor Ŝ of these interfaces can be obtained, and its scaling properties can be checked. Light-scattering experiments are also reported, so that the scaling properties of the corresponding 3-D structure factor S can be compared to those of Ŝ; especially the equivalence between L_m, as measured from the direct observation or from light-scattering, can be tested, and the difference and similarities between S and Ŝ can be analyzed. Finally emphasis is placed on the possibility of studying quantitatively the phase separation of fluids through this direct observation, and so, even at a microscopic level.