Anisotropy of the Nematic–smectic a Phase Transition

Abstract

Ohio Wesleyan fniuersify, Detau/are, OH 43015Critical behavior at the transition from the nematic phase to the smectic A phase in liquid crystals has been of interest for over two decades. Discussions about the nature of the transition, its analogy to the superconducting transition, and the degree of anisotropy present in the critical exponents have been extensive during this time. This review article concentrates on the question of anisotropy at the nematic-smectic A transition. The experimental and theoretical work related to this question is summarized and compared While progress has been made, there are many interesting questions related to this transition that remain for investigators to answer.Perhaps there has been no liquid crystal phase transition that has been investigated in so many ways by so many researchers and described so often as "intriguing", "fascinating", "puzzling", "complicated" and "poorly understood" as the transition from the nematic (N) to smectic A (A) phase in thermotropic liquid crystal systems. This transition has been studied theoretically: in mean-field theory, by analogy with other transition, using renormalization group techniques, using dislocation techniques, in 1/jV expansions, numerically, using self-consistent models, from above and from below the transition; and it has been studied experimentally: using lightscattering, x ray scattering, calorimetry, on over two dozen different chemical species exhibiting one of several different types of smectic A phases. After more than twenty years some questions still remain. Central among these is the question of the nature of the anisotropy in the critical phenomena associated with this transition.The nematic phase of rod-like liquid crystals has no long-range positional order of the centers of mass of the molecules, but the long axes of the molecules are oriented in a preferred direction specified by the unit vector, n, known as the director. The degree of alignment along n is a measure of the order in the nematic phase. Because the transition from the isotropic (I) phase to the nematic phase is first order, the order parameter specifying the extent of the alignment changes discontinuously at the NI transition. When the temperature of some liquid crystal systems is further reduced, positional order may form in stages, with the first being the onset of a onedimensional density wave. The wealth of phases possessing this one-dimensional order are known as smectic phases, and the one with the greatest symmetry is the smectic A phase, where the density wavevector and the director are parallel. l Liquid Crystals in the Nineties and Beyond Downloaded from www.worldscientific.com by UNIVERSITY OF BIRMINGHAM LIBRARY -INFORMATION SERVICES on 03/19/15. For personal use only.