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Thermodynamic Aspects of Phase Stability
Published in Mary Anne White, Physical Properties of Materials, 2018
The refractive index of a typical liquid is quite different from that of its vapor, mostly due to differences in density. Therefore, there is a discontinuity in refractive index on passing from liquid to vapor. At and near the critical point, there are large fluctuations in refractive index, leading to a phenomenon known as critical opalescence: The material appears milky and opaque. Use a diagram showing the light path to explain the origin of critical opalescence of a material in which both the liquid and vapor are transparent and colorless.
Phase Transitions
Published in Jeffrey Olafsen, Sturge’s Statistical and Thermal Physics, 2019
Other properties are also analogous. For t=0, corresponding to T=Tc, Equation (15.29) becomes s∝r3, so that the critical isotherm p(Δn) is cubic with zero slope at the critical point, like the critical isotherm B(m) in the Weiss model. Just above the critical point, the isothermal compressibility (∂n∂p)T varies as t−1, like the magnetic susceptibility in the Weiss model. From Equation (6.50), the divergent compressibility implies that density fluctuations are very large at the critical point. The corresponding fluctuations in the refractive index produce strong light scattering, a beautiful phenomenon known as critical opalescence.23 Similarly, the divergent susceptibility of a ferromagnet leads to large fluctuations in the magnetization near Tc (see Problem 15.10). These fluctuations are very important, and are, in fact, large enough to put the whole basis of the theory in doubt, as we shall see in the final section of this chapter.
Minireview: Methodology and instrumentation to evaluate critical pressure and temperature parameters
Published in Instrumentation Science & Technology, 2018
Jianguo Liu, Hailiang Lü, Hailiang Wang
This method has been applied mostly to research in this field, with a relatively high acceptable degree of accuracy. The experimental principle is that by observing the meniscus transformation (critical opalescence), critical parameters can be determined. The critical point or critical temperature–critical pressure is the point at which the meniscus (critical opalescence) disappeared or reappeared. The theories for this approach are simple, but it requires sophisticated test equipment. Selected early and recent test equipment is described and explained in the following sections.