Investigation of the Influence of Molecule Deformation on the Dynamics and Collective Excitation Spectra in Nematic Liquid Crystals

The generalization of dynamic equations in uniaxial and biaxial nematic liquid crystals is obtained yielding an approach accounting for the deformation of structural elements of these media. Collective excitation spectra are calculated and their angular dependence is established. The possibility of propagation of one, two, and three acoustic waves in nematics is predicted, and extreme angular characteristics of these waves are elucidated. Results of investigation are compared with available experimental data and satisfactory agreement between them is demonstrated. Low-frequency asymptotics of two-time Green’s functions of uniaxial nematics are calculated. It is found that their structure depends essentially on the character of spatial anisotropy of the considered condensed medium and the shape of molecules. Polarization specific features of acoustic waves in uniaxial nematics are studied. It is shown that the first and second sounds are the superposition of the longitudinal and transverse components. The ratio of amplitudes of these components as a function of angle between the wave vector and the anisotropy axis is established.