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UBC Theses and Dissertations

Optical studies of pure fluids about their critical points Tiong, Pang Kian


Three optical experiments were performed on pure fluids near their critical points. In the first two setups, CH₃F and H₂C:CF₂ were each tested in a temperature-controlled, prism-shaped cell and a thin parallel-windows cell. In the prism cell, a laser beam was additionally deflected by the fluid present. From the deflection data, the refractive index was related to the density to find the Lorentz-Lorenz function. Critical temperature (Tc), density, refractive index and electronic polarizability were found. In the second experiment, a critically-filled, thin parallel-windows cell was placed in one arm of a Mach-Zehnder interoferometer. Fluid density was monitored by changes in the fringe pattern with changing cell temperature. The aim was to improve on the precision of Tc: Tc(CH₃F)=(44∙9O87±O∙OOO2)C; H₂C:CF₂)=(29∙7419±O∙OOO1)C; and, to study the coexistence curve and diameter as close to Tc as possible. The critical behaviour was compared to the theoretical renormalization group calculations. The derived coefficients were tested against a proposed three-body interaction to explain the field-mixing term in the diameter near the critical point. It was found that H₂C:CF₂ behaved as predicted by such an interaction; CH₃F (and CHF₃) did not. The third experiment was a feasibility study to find out if (critical) isotherms could be measured optically in a setup which combined the prism and parallel-windows cells. The aim was to map isotherms in as wide a range of pressure and density as possible and to probe the critical region directly. Pressure was monitored by a precise digital pressure gauge. CH₃F and CHF₃ were tested in this system. It was found that at low densities, the calculated second and third virial coefficients agreed with reference values. However, the data around the critical point were not accurate enough for use to calculate the critical exponent, δ. The calculated value was consistently smaller than the expected value. It was believed that the present setup had thermal isolation problems. Suggestions were made as to the improvements of this isotherm cell setup. Lastly, a joint project with the Department of Ophthalmology, UBC to assemble a vitreous fluorophotometer is discussed in Appendix F. The upgrading of the instrument took up the initial two years of this PhD programme.

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