Ultrasonic investigation of viscosity coefficients in the nematic liquid crystal, EBBA

Abstract

The attenuation of ultrasonic compressional waves in the nematic liquid crystal, N-(p-ethoxybenzylidene}-pbutylaniline (EBBA), has been measured as a function of frequency, temperature, and the angle between an aligning magnetic field and the direction of wave propagation. The results are interpreted in terms of theories of nematic liquid crystals based on hydrodynamics and micropolar continuum mechanics. It is shown that the correct expression for the attenuation anisotropy can also be derived from Leslie's theory of anisotropic fluids. It was found that shear viscosity coefficients had the usual exponential dependence on inverse temperature with an activation energy of 10 kcallmole whereas volume viscosity coefficients had a more complex temperature dependence. The attenuation of compressional waves in the unoriented nematic phase is compared with that of the oriented crystal.

Anisotropic wave propagation in nematic liquid crystals

“…we perform a perturbation analysis of (69), split into its real and imaginary parts. At order O(1), we obtain l 0 ¼ 0 and recover the same wave vector and isotropic sound speed given in (39). The real O(h) problem reads…”

“…where the symmetric tensor M is still defined as in (37), provided that k is substituted by k. In the asymptotic limit ωτ → ∞ , the solid-like elastic response analyzed in Sec. 2.5 is recovered: H 1 tends to zero, and (68), ( 69) and (70) tend to (34d), ( 35) and (36), respectively.…”

“…3.1. In particular, it lacks the contribution of the Leslie-Ericksen viscous stress tensor, extended to compressible LCs [36] -which, contrariwise, is the only source of dissipation in the competing second-gradient theory [11,18]. At any rate, the set of two equations stemming from the above hypothesis is solved by the value of τ in (82) and the (reasonably small) dimensionless ratio…”

Viscoelastic nematodynamics

“…For instance, the anisotropy of sound speed and attenuation and their frequency dependence is often described in terms of an elastic material response and relaxation dynamics [2][3][4][5][6]. Structural relaxation processes are also explicitly mentioned in order justify the frequency dependence of the viscosity coefficients [7][8][9][10][11][12]. Recent papers even report the measurement of a viscoelastic response in low molecular weight liquid crystals, either in the nematic or the isotropic phase, when they are subjected to low-frequency mechanical sinusoidal deformations [13,14].…”

Liquid relaxation: A new Parodi-like relation for nematic liquid crystals

“…Along with the original Parodi relation [11] -which we also retrieve-this result lowers to four the number of independent viscosities for a nematic liquid crystal. Both conditions involve only (some of) the six original Leslie coefficients, not the extra three viscosities entering the extension of the Ericksen-Leslie theory to compressible NLCs [12]. Accordingly, only the theory for incompressible NLCs will be presented here, and its predictions tested against experimental data, earlier theoretical predictions, and results from molecular-dynamics (MD) simulations.…”