Propagation of ultrasound near phase transitions in the liquid crystal working environment of technical devices

Abstract

The results of measurements of the velocity and absorption coefficient of ultrasound in a nematic phase oriented by a magnetic field are presented. Within the framework of the hydrodynamics of the nematic phase, the coefficients of the bulk and shear viscosity of the nematic phase are calculated. The temperature dependence of the shear viscosity coefficients is exponential. The nature of the sharp increase in the bulk viscosity coefficients with an increase in the temperature of the nematic phase in the region of the phase transition nematic liquid crystal - isotropic phase is analyzed. The analysis of the temperature dependence of the ultrasound velocity in the nematic phase, carried out under the assumption of the existence of two processes responsible for the propagation of ultrasound, allowed us to identify the critical component and calculate the relaxation time of the critical contribution. In the vicinity to the phase transition, the dependence on the temperature of the relaxation time of the critical contribution increases sharply with a divergence index equal to 1.03. The relationship between the critical contribution to the absorption and the speed of ultrasound has been established. An array of data necessary for the calculation of liquid crystal technical devices has been obtained.