Measurement of the long-range correlation length of SF6very near the critical point

Results are reported of measurements of the long-range correlation length $\xi$, made along the critical isochore. The measurements were made using two different light-scattering techniques and cover the temperature range $4.0\times {10}^{-6\mathrm{}}\le \frac{T}{T_c}-1\mathrm{}\le 3.1\times {10}^{-2\mathrm{}}$. All of the results are accurately fit by the expression $\xi =2.016\mathrm{}{(\frac{T}{T_c}-1)}^{-0.621\mathrm{}}$ Å. Measurements of the isothermal compressibility ${\kappa }_T$, in the temperature range $1.5\times {10}^{-4\mathrm{}}\le \frac{T}{T_c}-1\mathrm{}\le 3.1\times {10}^{-2\mathrm{}}$ are also reported and yield the result ${\kappa }_T=1.327\mathrm{}\times {10}^{-9\mathrm{}}{(\frac{T}{T_c}-1)}^{-1.223\mathrm{}}$ ${\mathrm{cm}}^2$/dyn. From these measurements the value of the exponent $\eta$ may be deduced as 0.03 ± 0.03. The measured values of $\xi$ are used to interpret existing measurements of the Rayleigh linewidth in terms of mode-mode coupling theories. Experimental results are also presented which indicate that stirring may be used to eliminate gravitationally induced density gradients near the critical point without affecting the fluid's critical properties.