Light-scattering study on the shear-orientation coupling of liquids near isotropic-to-nematic phase transition

Microscopic interaction between the translation and rotation of rodlike molecules yields the shear-orientation coupling in the macroscopic view. The depolarized scattering component arising from orientational fluctuations gives a constant in the strength of the coupling, which is related to the three viscosity parameters introduced by de Gennes. A high-resolution system of light scattering with an improved optical beating method was utilized for measurements in the 4-cyano-4'-alkylbiphenyl nCB series: propyl (3CB), butyl (4CB), pentyl (5CB), hexyl (6CB), heptyl (7CB), and octyl (8CB). All the coupling constants obtained in these samples showed clear dependence on temperature, and their behaviors were interpreted in terms of a critical phenomenon near T*, the virtual isotropic-nematic phase transition point. The values of T* were also given by the light scattering experiment, and the critical exponent of shear-orientation coupling was determined to be 0.31. Furthermore, the characteristic value of the coupling constant was correlated with the aspect ratio of these rodlike molecules, and two clear facts were found: a stronger coupling in longer molecules; and an odd-even effect with respect to the number of C-C bonds in the alkyl chain.