Optical Discrimination between Two Mechanisms in Acoustically Birefringent Liquids

Abstract

Two mechanisms of acoustically induced optical anisotropy in liquids have been advanced. For small molecular size, Peterlin has attributed the effect to molecular alignment by the dynamic velocity gradient of the acoustic field. For large molecules, Oka introduced the “Rayleigh Disc” mechanism. The former leads to an induced optical anisotropy in the liquid having the same spatial and temporal periodicity as the acoustic field. The latter, since it depends quadratically on the acoustic pressure gradient, has half the spatial periodicity of the acoustic field. Thus, the two mechanisms lead to distinctly different optical effects. By studying the polarization of the optical far-field diffraction pattern produced by the ultrasonic field, it is possible to discriminate between mechanisms and study the effect due to either mechanism.