This review discusses the physical properties of nematic, cholesteric, and smectic liquid crystals. Molecular theories of the liquid crystal phases are discussed and the molecular field theories of the phase transitions between the various liquid crystal phases are presented. The elastic theory and hydrodynamics of liquid crystals is developed. A wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties, is discussed.

Physics of liquid crystals

We assume that (1) the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility; (2) the free energy may be expanded in powers of Qαβ and of its gradients. This allows a unified description covering the anomalous magnetic birefringence, the intensity of light scattering, and the properties of the nematic/isotropic interface. For a cholesteric, although the optical rotation is huge in the ordered phase, we predict that it should not be anomalous just above the transition point Tc. We also investigate the dynamics of fluctuations of Qαβ, and discuss the flow birefringence, the frequency width of the Rayleigh scattering, and the attenuation of ultrasonic shear waves, in terms of 3 viscosity coefficients.

Short Range Order Effects in the Isotropic Phase of Nematics and Cholesterics

Theory of Flow Phenomena in Liquid Crystals

Phenomenology of short-range-order effects in the isotropic phase of nematic materials

Equilibrium theory of liquid crystals.

Using a light-beat laser spectrometer, we observe the relaxation of the thermally excited fluctuations of anisotropy in the nematic phase of $p\ensuremath{-}\mathrm{azoxyanisole}$. We identify the two low-frequency purely dissipative modes recently predicted. A first study of the angular dependence of the width of these modes allows a determination of three among the six viscosity coefficients introduced by Leslie.

Quasielastic Rayleigh Scattering in Nematic Liquid Crystals

We optically observe the untwisting and alignment, along an applied magnetic field, of high-pitch cholesteric liquid crystals, mixtures of para-azoxyanisole and cholesteryl esters. The results agree quantitatively with de Gennes's predictions and allow a determination of the twist elastic constant of pure para-azoxyanisole.

M. Veyssie

Papers

Quasielastic Rayleigh Scattering in Nematic Liquid Crystals

Magnetically Induced Cholesteric to Nematic Phase Transition in Liquid Crystals