Invited Lecture. Complexities in the structure of ferroelectric liquid crystal cells The chevron structure and twisted states

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Abstract
To demonstrate the generality of the chevron structure in ferroelectric liquid crystal (FLC) cells, an optical micrograph is presented, which shows a series of defect lines clearly indicating this structure. The generality is understood as a direct manifestation of the smectic layer undulation playing an important role in the SASC (S*C) phase transition. By establishing the relation between chevrons and asymmetric focal conics, both of which have two senses, ±z, the boundaries between them with opposite senses, i.e. so-called zig-zag defects, have been characterized; the type boundary does not exist as a line but shrinks into a point. The interplay between the chevron structure and the director alignment has been described in detail; in particular, the appearance of unwinding lines in the middle of thick cells, the characterization of internal disclination loops in SSFLC cells, and the law regulating the emission and absorption of internal disclinations by zig-zag defects. With a view to removing zig-zag defects and to increasing the apparent tilt angle, alignment by obliquely evaporated SiO, which may produce cells free from the chevron structure, alignment by rubbing, and three kinds of electric field effects have been assessed. The rubbing may induce a characteristic deformation of the zig-zag defect loop with a teardrop shape as a consequence of reducing the area surrounded by the loop. Finally, further complexities are noted in connection with the polar surface interaction and the material constants which characterize various director and layer deformations.