Imperfections in focal conic domains: the role of dislocations

TL;DR

This paper investigates imperfections in focal conic domains (FCDs) in smectic A materials, attributing deviations from ideal geometry to interactions with dislocations, especially near phase transitions.

Contribution

It provides a theoretical framework for understanding how dislocations cause imperfections in FCDs, supported by models and experimental correlations.

Findings

FCD imperfections are linked to dislocation interactions.

Dislocation-induced kinks affect FCD geometry.

Models explain experimental observations of FCD distortions.

Abstract

It is usual to think of Focal Conic Domains (FCD) as perfect geometric constructions in which the layers are folded into Dupin cyclides, about an ellipse and a hyperbola that are conjugate. This ideal picture is often far from reality. We have investigated in detail the FCDs in several materials which have a transition from a smectic A (SmA) to a nematic phase. The ellipse and the hyperbola are seldom perfect, and the FCD textures also suffer large transformations (in shape or/and in nature) when approaching the transition to the nematic phase, or appear imperfect on cooling from the nematic phase. We interpret these imperfections as due to the interaction of FCDs with dislocations. We analyze theoretically the general principles subtending the interaction mechanisms between FCDs and finite Burgers vector dislocations, namely the formation of kinks on disclinations, to which dislocations are attached, and we present models relating to some experimental results. Whereas the principles of the interactions are very general, their realizations can differ widely in function of the boundary conditions.