Acoustic investigations of dynamics of orientational processes in nematic liquid crystals in conical magnetic fields
The results of experimental and theoretical studies on kinetics of orientational processes in the mixture of p-n-butyl-p-heptanoyloxyazoxybenzene (BHOAB) and p-n-butyl-p-methoxyazobenzene (BMOAB) (with 1:2 component ratio) and in p-n-butoxybenzylidene-p-butylanyline (BBBA) in conical magnetic fields are presented. The method of forming a conical magnetic field is proposed. The influence character of the angle β between the induction vector and the axis of the magnetic field rotation on the phase dependence of the ultrasound absorption coefficient was found. The decrease of the angle β causes both the reduction of the phase shift between the director and the magnetic induction vector, and the anisotropy of the ultrasound absorption coefficient. It also widens the frequency range of the synchronous mode of the director movement. When the angle β value is smaller than 56°, the phase shift between the director and the induction vector does not exceed 45° and the asynchronous mode cannot be observed. The analysis of the director motion in the conical magnetic field was carried out. The compliance of the solution of hydrodynamics equations in the conical magnetic field with the experimental results was shown. Unlike the classical method of rotated field, this method can be applied to liquid crystals study in cases of high viscosity coefficients: for nematics near phase transitions into smectic or crystal phase and also for smectic C.