## Mass Diffusion And Thermal Diffusion

From Eq. (4) we note that the nonequilibrium enhancement strongly depends on the value of the Soret coefficient. Since the information on the Soret coefficient ST available in the literature is limited, it was decided to measure ST of polystyrene-toluene solutions in our laboratory as well [22]. For this purpose an optical beam-deflection method was used. This method was first applied by Giglio and Vendramini [24] to polymer solutions and binary liquid mixtures near a critical mixing point. The method was subsequently improved by Kolodner et al. [25] and by Zhang et al. [14] for measuring mass diffusion and thermal diffusion in liquid mixtures. In this method one observes the deflection of a laser beam propagating horizontally through the solution upon the imposition of a temperature gradient across the liquid layer. The rate of change of the deflection corresponds to the rate by which a concentration gradient is established and it yields the mass diffusion coefficient D, also referred to as collective diffusion coefficient Dc, of a polymer solution. The final deflection measures the final concentration gradient Vw resulting from the imposed temperature gradient VT and, hence, yields the Soret coefficient in accordance with Eq. (1). The experimental measurements are in good agreement with the measurements obtained by Kohler et al. [26] from a forced Rayleigh-scattering method. For details the reader is referred to the paper of Zhang et al. [22].

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