Dispersion and absorption of sound in nematics

Abstract

The unified hydrodynamic theory for systems with a liquid crystalline phase is generalized to include a frequency dependence of the elastic and dissipative parameters of the system. Application is made to nematics for which the frequency dependent anisotropies of the velocity and absorption of sound are calculated. It is shown that the relaxation approximation for the dispersion leads to reasonable results.

Acoustic investigations of relaxation processes in regions of polymorphic transformations of nematics

“…The temperature dependence of the total velocity anisotropy Aclc = Ac(0)/c and attenuation anisotropy ACX(O)/f2 are The frequency dependence of the total anisotropy of the acoustical parameters in the nematic phase of NPOB were analysed in the frame work of generalized hydrodynamic theory [10]. However, unlike the theory [10], we supposed that both relaxation processes contributed to each elasticity tensor component. In this case the expressions for frequency dependences of the velocity and the attenuation anisotropies can be represented as :…”

“…The procedure used was the following. Using the theoretical result [10], together with the above mentioned proposal, and neglecting the effects of the N-A transition, the coefficients of angular dependence of Ac(0) /c can be expressed as : Equations (7) and (8) were supposed true in the whole nematic region. Consequently, using the experimental results and the data of table I, the calculation of AE(i) -AE(') was performed (Fig.…”

Acoustic Investigations of Dynamics of Orientational Processes in Nematic Liquid Crystals in Conical Magnetic Fields

“…≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡ Значение угла β не оказывает влияния на характер температурной зависимости параметра Δα/f 2 , однако изменение угла β от 90° до 30° приводит к уменьшению анизотропии коэффициента поглощения ультразвука. Во всем температурном интервале нематической фазы зависимость анизотропии коэффициента поглощения ультразвука на квадрат частоты от угла θ между волновым вектором и директором описывается уравнением [6] . Жидк.…”

Ultrasonic and Viscoelastic Properties of Small-Volume Mesogen Samples at the Phase Transition

“…It was referred to as the influence of an acoustic relaxation. Formally, this influence can be taken into account within the framework of generalized hydrodynamics by introduction of the complex modulus of elasticity [13]. The imaginary part of the given modulus determines the relaxation contributions to the absorption coefficient of ultrasound, and the real part to the velocity of ultrasound.…”