Piezo- and elasto-optic properties of liquids under high pressure. I. Refractive index vs pressure and strain

Abstract

The variation of the refractive index of liquid with pressure to 14 kbar has been determined by an optical interferometric method. The liquids studied are carbon tetrachloride, n‐pentane, n‐hexane, n‐heptane, n‐octane, n‐nonane, n‐decane, benzene, chlorobenzene, methyl alcohol, and water at room temperature. Since most of these liquids freeze at room temperature even before 14 kbar is reached, the measurements were carried out up to the freezing point or 14 kbar, whichever pressure is lower. In every case the refractive index increases with pressure with pronounced nonlinearity, particularly at high pressure. This nonlinearity is reduced considerably when the same data on Δn are considered as a function of the volume strain indicating that the nonlinear piezo‐optic behavior of liquids is just a manifestation of nonlinear P–V relationship. Further, it is found that the slight nonlinearity noticed in the Δn−ΔV/V_o relationship could be still further reduced when the variation in refractive index is treated as functions of the Lagrangian and Eulerian strains. In particular, the Eulerian strain ε is found to yield a much larger range of strain over which a simple linear relationship between Δn and ε is obtained irrespective of the nature of the liquid or the equation of state used to compute the strain.