Observation of shear-orientation coupling near isotropic-to-nematic phase transition

In addition to the Brillouin scattering component by thermal phonons carrying molecular translation, the fluctuation of molecular rotation appears in the light scattering spectrum due to a quasi-elastic component in case that a molecule has anisotropy in its shape. Two degrees of freedom, the translation and the rotation, interact with each other in some soft condensed matters consisting of rod-like molecules. A typical example of the coupling is seen in liquid crystals, even in the isotropic phase. Under the strong shear-orientation coupling, the shear wave phonon, which does not normally propagate in fluids, appears in the spectrum of the depolarized (VH) scattering. The spectrum has the shape of a Lorentzian curve with another Lorentzian peak subtracted, and the depth of the inner dip to the outer spectral height directly gives the coupling constant. We measured the coupling constants in the 4-cyano-4/-pentylbiphenyl (5CB) and the 4-cyano-4/-hexylbiphenyl (6! CB) with our optical beating method used for the Brillouin scattering. The obtained values of the coupling constant showed clear dependence on temperature and its behavior was interpreted as critical decreasing towards the virtual transition point of the isotropic-nematic phase transition.