Universal relaxation behavior of classical liquid crystals at hypersonic frequencies

Abstract

Within this work we present comparative Brillouin and ultrasonic investigations of p-methoxybenzylidene p-(n-butylaniline), p-azoxyanisol, and 4-cyano-4'-pentyl-biphenyl performed in the vicinity of the nematic-isotropic transition. We confirm the existence of a universal hypersonic relaxation mechanism of weak activation energy and a related significant splitting of the two principal longitudinal acoustic modes within the nematic phase. Brillouin refractometry is proposed as a method to determine the magnitude of the nematic order parameter simultaneously with the second-order elastic stiffness constants. The origin of the hypersonic anisotropy is discussed in terms of thermal relaxations and the evolution of the nematic order parameter.