Eur. Phys. J. B (2017) 90: 22
https://doi.org/10.1140/epjb/e2017-70512-3

Regular Article

Nonequilibrium fluctuations from a nematic under a thermal gradient and a gravity field^*

II. Rayleigh scattering

¹ Maestría en Ciencias de la Complejidad, Universidad Autónoma de la Ciudad de México, 03100 México, Mexico
² Instituto de Física, Universidad Nacional Autónoma de México. Apdo. Postal. 20-364, 01000 CDMX, México, Mexico
³ FENOMEC and Fellow of SNI, México, Mexico

^a e-mail: jofcap@hotmail.com

Received: 31 August 2016
Received in final form: 2 December 2016
Published online: 1 February 2017

Abstract

The fluctuating hydrodynamic treatment developed in the previous article for a nematic liquid crystal under the influence of a thermal gradient α and a uniform gravity field g, is used to calculate its Rayleigh light scattering spectrum. We find that the dissipative thermal gradient force enhances the Rayleigh-line intensity which varies as k^-4 with the fluctuations of the wave number k. The Rayleigh line consists of three central Lorentzians, two of which are determined by the visco-heat modes coupling the entropy and director fluctuations, which is a pure non-equilibrium effect. The third Lorentzian is due only to director fluctuations. We find that the former peaks contain the Rayleigh wings owing to the orientational fluctuations of the aniosotropic molecules. It is also shown that the obtained spectrum reduces to the known equilibrium spectrum of a nematic and to that of a simple fluid. For the particular case in which the decay rates are diffusive, we calculate and plot the amplitudes of non-equilibrium fluctuations of the dynamic structure factor as a function of |α|²/k⁴, and also, the intermediate function in the equilibrium and non-equilibrium states.