Influence of thermodynamic state parameters on the anisotropy of the ultrasound absorption coefficient in the nematic liquid crystals

The results of measurements of the anisotropy of the ultrasound coefficient in the frequency range 0.5-8.3 MHz in the nematic phase over a wide range of temperatures and pressures are presented. The effectiveness of the acoustic method, which makes it possible to carry out research in conditions of a fixed volume, is shown. In the region of the nematic liquid crystal – isotropic liquid phase transition, an abnormal increase in the ultrasound absorption coefficient is observed, which is associated with the peculiarities of relaxation processes. An increase in the ultrasound frequency leads to a decrease in the anisotropy of the ultrasound absorption coefficient and the disappearance of the maximum in the phase transition region, which is consistent with the conclusions of the nematic phase hydrodynamics. An inversion temperature was found in the studied nematic liquid crystals, at which the anisotropy of the ultrasound absorption coefficient does not depend on pressure in the studied pressure range of 105÷6×107 Pa. The nature of the dependence of the ultrasound absorption index on the thermodynamic parameters of the state is analyzed. The possibility of using the results of measurements of the anisotropy of the ultrasound absorption coefficient to calculate the dissipative coefficients, the values of which are necessary in the design of liquid crystal technical devices, is shown