Molecular deformation and flow birefringence in simple liquids,

A simple molecular deformation theory of flow birefringence is derived for low molecular weight liquids composed of short semi-flexible chain molecules, such as the n-alkane homologues. A hydrodynamic interaction between the molecule and the rest of the liquid is used to show that simple shear flow of such a liquid perturbs the isotropic equilibrium between the different molecular rotational isomeric states which are known to exist in the stationary liquid. The result of this perturbation is a molecular deformation which depends on the orientation of the molecule in the liquid, thus leading to an angular dépendance of the molecular optical polarizability anisotropy, and hence a birefringence in the liquid. The theory applied to the n-alkane n-tetradecane shows that an appreciable contribution to its measured flow' birefringence may arise from molecular deformation, the temperature coefficient of this contribution being 1.67 times greater than that predicted by flow birefringence theories based on molecular orientation. Comparison between theory and recent experimental work is favourable, and the extent to which molecular deformation contributes to the observed Maxwell effect is estimated