Gouy Diffractometry in Thermal Diffusion

Abstract

Symmetrical wide‐aperture illumination of binary liquid systems undergoing thermal diffusion produces well‐defined Gouy diffraction patterns, arising from the interference of conjugate rays passing at equal distance above and below the midplane of the diffusion cell. The optical theory of this phenomenon is formulated; the diffraction phenomena are described by incomplete Airy integrals, tabulations of which are given. The theory forms the basis for a new method of measuring Soret coefficients. Applied to experimental data for typical electrolyte and nonelectrolyte systems, Gouy diffractometry gives results of high precision which agree well with older values determined by other methods. The new technique has the advantage of providing two separate estimates for the Soret coefficient, affording a check on the internal consistency of the experiments; further, the presence and approximate magnitude of convective disturbances are automatically revealed.