A Laser Light Scattering Study of Transport and Critical Phenomena

Abstract

The intensity and Rayleigh linewidth have been measured as a function of temperature and scattering angle for light scattered by concentration fluctuations near the critical point of the binary liquid system 2,6-lutidine-water. From the intensity data it is found that γ = 1.26 ± 0.02 and ν = 0.61 ± 0.07. From the linewidth data the mutual diffusion coefficients were calculated as a function of temperature. It is found that the diffusion coefficient decreases as the critical point is approached. The behavior of the linewidth as a function of k∈ was compared with the Kawasaki theory without the nonlocal viscosity and vertex corrections. General agreement with some systematic deviations is observed. The shear viscosity anomaly in the same system was also studied in detail by measuring the shear viscosities as a function of temperature near the critical point. Results of analyses indicate that the viscosity is at most weakly divergent, with an exponent o ≃ ±0.001. Light scattering techniques have been employed to measure the mutual diffusion coefficient D as a function of concentration in ten binary mixtures and the thermal diffusivity χ in nine pure liquids and one binary mixture. The diffusion coefficient was also measured at one or two concentrations for four binary mixtures. The values obtained are in excellent agreement with the available literature data determined by more classical methods. Under most circumstances light scattering is found to offer a fast and accurate way of determining χ and D. The turbidity τ and the decay rate Γ of the density fluctuations have been measured as a function of temperature on the critical isochore of ethane near the critical point. From the turbidity data absolute values of isothermal compressibilities and correlation lengths were calculated. The isothermal compressibility K T and the correlation length ∈ are found to behave as: K T = 1.24 ± 0.11 x 10 -3 (ΔT/T c ) -1.225 ± 0.02 atm -1 ∈ = 1.64 ± 0.20 (ΔT/T c ) -0.664 ± 0.02 A. From the Γ data thermal diffusivities, thermal conductivities and excess thermal conductivities were calculated as a function of temperature. It is found that the thermal diffusivity does not exhibit a simple power law behavior whereas the excess thermal conductivity does with an exponent of ψ = 0.605 ± 0.02. The singular part of the decay rate Γ s , was compared with the Kawasaki expression with the nonlocal viscosity correction. It is observed that the nonlocal viscosity correction together with the vertex and the correlation function corrections improve the agreement between the theory and the experiment. The results for the isothermal compressibility, the thermal conductivity and the excess thermal conductivity are in very good agreement with the available literature data.