Spinodal decomposition patterns in an isodensity critical binary fluid: Direct-visualization and light-scattering analyses

In order to study the phase-separation process near the critical point of fluid mixtures (spinodal decomposition) without the influence of the Earth's gravity, we have used a carefully density-matched system of deuterated cyclohexane, cyclohexane, and methanol. Such a system is known to behave as a real binary fluid [C. Houessou, P. Guenoun, R. Gastaud, F. Perrot, and D. Beysens, Phys. Rev. A 32, 1818 (1985)], and our own previous microgravity experiments have demonstrated that the gravity influence was negligible during the whole separation. A light-scattering analysis has been performed, which provided the three-dimensional structure factor S mainly in the first stages of the separation. For the late stages a necessary alternative is the study of the images of the separating fluid. The interest of such a direct observation lies in the possibility of extracting not only statistical properties analogous to those obtained with the light-scattering technique, but also of analyzing the morphology of the phase-separation process and the motion of interfaces. The origin of such images is not straightforward. It is shown that they reproduce the section of the pattern of interfaces between phases, in a plane located close to the exit window. Because the domains are interconnected, and because only the interfaces nearly parallel to the light direction are detected, this pattern exhibits the same periodicity as that of the domains. From these images, a numerical treatment using video techniques and computer analysis allows a two-dimensional structure factor \ifmmode \hat{S}\else \^{S}\fi{} to be obtained. The similarities and differences with the corresponding three-dimensional factor S are discussed. The scaling properties of the phase separation---the time invariance of the reduced structure factors F and F^, of the reduced second moments r and r^, the universal behavior of the typical wave vector ${K}_{m}^{\mathrm{*}}$---can be verified in the whole available range, up to the final equilibrium state. Here wetting forces and the residual gravity effects compete to give to the system its equilibrium morphology.