Piezo-Optic Behavior and the Equation of State of Liquids
Consider the piezo-optic behavior of materials in their transparent region of spectrum. In the case of solids such as alkali-halides , α-quartz , vitreous silica , etc., the relationship between the change in refractive index Δn and pressure becomes slightly nonlinear at high pressures (say above 5 kbar), but the same data exhibit perfect linear relationship between Δn and the Lagrangian strain η in the entire ränge of pressures studied. In the case of liquids such as water and CCl4, as mentioned in a previous article , Δn is grossly nonlinear with pressure, and Δn vs. η is linear only at strains less than 2 or 3%. Motivated by the linearity of Δn vs. η for the solids, one can view the nonlinearity of the Δn vs. η for the liquids as possibly caused by (1) unreliable P-V data used to evaluate η, (2) use of an inappropriate equation of State for the liquids when extrapolating and interpolating literature P-V data to cover the entire 14 kbar pressure ränge, (3) nonapplicability of Lagrangian strain as a strain measure at the very high strains and (4) perhaps intrinsic nonlinearity of Δn with respect to all strain measures for liquids. Investigation into these possibilities was made with high pressure interferometric measurements on a number of liquids under hydrostatic pressure and the results and conclusions are presented here.
KeywordsHydrostatic Pressure Bulk Modulus Strain Measure Volume Strain Lagrangian Strain
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