2-EXPONENTIAL DECAY OF DYNAMIC LIGHT-SCATTERING IN NEAR-CRITICAL FLUID MIXTURES

Two hydrodynamic relaxation modes associated with mass diffusion and t hermal diffusion are present in binary fluids. In near-critical binary fluids a coupling between the two modes results in two characteristic relaxation times, neither of which is associated with pure mass diffu sion or pure thermal diffusion. Instead, the relaxation times are inve rsely proportional to two effective diffusivities D-1 and D-2, which c an he detected experimentally by dynamic light scattering. The physica l meaning of D-1 and D-2 changes as one considers states in the vicini ty of different points on the critical locus: in the infinite-dilution limit the diffusivity D-1 of the slow mode is associated with the the rmal diffusivity and the diffusivity D-2 of the fast mode with the mut ual mass diffusion coefficient, while in the ''incompressible'' liquid -mixture limit D-1 is associated with the mass diffusion coefficient a nd D-2 with the thermal diffusivity. In addition we have determined th e intensities (amplitudes) of these relaxation modes, which can also b e measured with light scattering. We discuss the conditions at which a two-exponential decay of the dynamic correlation function can be meas ured. As an example we consider mixtures of methane and ethane near th e vapor-liquid critical line where the two exponential decays indeed h ave been observed. [S1063-651X(98)05402-6].