Plastic behavior and polymorphism of smectic liquid crystals

Abstract

Smectic systems are characterized by a lamellar ordering of more or less liquid layers. Mechanically, one expects the layers to present an elastic reaction to normal compression and a viscous reaction to a parallel shear. Rheological experiments are described in various smectic phases, which give the stress/strain relationship for small amplitude oscillations, in various geometries. Non-linear effects associated with texture instabilities are mechanically visible, in the high frequency elastic regime. In smectic A, for layer compression at low frequency, a plastic relaxation probably due to the easy climb of edge dislocation, prevents the observation of the intrisic viscoelastic relaxation associated with permeation. In smectic B, for shear parallel to the layers, above a low frequency Newtonian fluid-like behavior, one observes an elastic regime with a very low C44 ~ 106 cgs. This explains why the B phase appears practically as a lamellar phase, while presenting for X rays a 3D positional ordering. No data up to now can support recent predictions on intrinsic anomalous elastic or viscous behavior of smectic systems, related to large fluctuation of the one dimensional layer ordering.