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Optical studies of critical phenomena in fluids Fameli, Nicola

Abstract

Accurate optical techniques were employed to study the behaviour of the binary liquid mixture n-heptane+nitrobenzene (C₇H₁₆ + C₆H₅NO₂) and of the pure fluid 1,1-difluoroethylene (C₂H₂F₂) near their respective critical regions. It is found that, in the region of reduced temperature, t = (T[sub c] — T)/T[sub c] < 3 x 10 ⁻⁴ , the order parameter ΔΦ[sub H] for the binary mixture follows a power law ΔΦ[sub H]« t[sup β], with a leading exponent β = 0.367 ± 0.006, which is higher than the predicted theoretical value of β = 0.326 ± 0.002. A careful study of the effect of refractive index gradients caused by the Earth's gravitational field, potential variation of the optical thickness of the sample cells due to wetting of the cell walls by one the phases, and the long thermal equilibration times of binary mixtures has failed to resolve the disagreement between β measured on n-heptane+nitrobenzene and its theoretical value. A significant feature of this investigation is the novel application of an optical technique (the image plane technique) for studying critical phenomena in transparent binary liquids. The Lorentz-Lorenz function, £, of 1,1-difluoroethylene was measured in order to determine the fluid's density from its refractive index. The quantity £. is found to vary by about 1.4% with density of the fluid, p, with a gentle maximum at a density slightly higher than the critical density, p[sub c]. The density measurements were then used to construct the coexistence curve for this material. The coexistence curve of C₂H₂F₂ was measured with the 'Lorentz-Lorenz' experiment and with a new apparatus combining two complementary optical methods, the prism cell technique and the focal plane technique, into one. The measurements carried out on C₂H₂F₂ in this thesis serve as a test of this apparatus. The order parameter data of C₂H₂F₂ are described accurately by a scaling power law in terms of the reduced temperature t, with the critical exponents at their theoretical values. The coexistence curve data^ taken simultaneously with the two combined techniques agree well with each over the whole range of temperature investigated.

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