Imperfect focal conic domains in A smectics: a textural analysis

Abstract

We have analysed optical microscopy observations of focal conic domains (FCDs) with imperfect ellipses on the basis of the theoretical concepts developed by Kleman et al. (Philos. Mag. 2006, 86, 4439). Two types of imperfect ellipses are observed: with kinks (elementary imperfections resulting from a topological interaction of a disclination with a dislocation, at their point of junction) being either in the ellipse plane (‘mouse’) or perpendicular to it (‘turtle’). The experimental conditions for the observation of imperfect FCDs of both types are described. A model describing the shape of a mouse‐type ellipse is compared with observations. Two experimental observations in nematogenic smectics (i.e. which have a smectic → nematic transition) emphasise the predominant role of kinks in dynamical phenomena involving dislocations and FCDs: (i) a reversible and sudden temperature‐induced transformation between two FCD textures, i.e. FCDs with fewer kink‐carrying distortions (called isometric textures) and the higher temperature FCDs with a proliferation of kinks (called non‐isometric); (ii) a shrinking of the ellipses upon heating, up to their sudden and total disappearance at a temperature well below the phase transition. In contrast, in a non‐nematogenic smectic, the ellipse size does not vary upon heating until the appearance of the isotropic phase.