Flow-Birefringence in Gases an Example of the Kinetic Theory Based on the Boltzmann-Equation for Rotating Particles

Abstract

Flow birefringence is an example for the nonequilibrium alignment phenomena which occur in polyatomic gases. In this paper the attention is focussed on gases of linear molecules. Firstly, it is pointed out that the anisotropic part of the electric permeability tensor which is responsible for birefringence is proportional to the (2nd rank) tensor polarization of the rotational molecular angular momentum. Then, the collision-induced tensor polarization set up by a viscous flow is calculated from transport-relaxation equations which, in turn, are derived from the Waldmann-Snider equation, a Boltzmann equation for rotating molecules. Thus the flow birefringence is expressed in terms of properties of single molecules and of their binary scattering amplitude. A relation between flow-birefringence and the Senftleben-Beenakker effect on the viscosity is established. Furthermore, the phenomenon reciprocal to flow birefringence is considered. If a deviation of the tensor polarization from its equilibrium value is maintained externally, the collision-induced coupling between tensor polarization and the friction pressure tensor gives rise to an anisotropy in velocity space. Finally,it is briefly discussed that effects similar to those observed in polyatomic gases should also occur with atomic vapors, provided that the atoms have a nonvanishing electron-orbital angular momentum.