One-dimensional patterns and topological defects in smectic liquid crystal films

Smectic liquid crystals exhibit a remarkable ability to self-organize into a variety of textures, patterns, and arrays extending over large surface areas with microscale periodicity. These structures provide a useful platform for the bottom-up fabrication of microdevices and functional materials, such as microlens arrays and nanoparticle templates. Periodic structures appear in response to a conflict of director alignment created within the smectic film by external fields such as hybrid (planar-homeotropic) anchoring conditions at the film interfaces or an electric field applied normal to the surface anchoring direction. In this article, we review the formation, structure, and applications of one-dimensional (1d) patterns obtained in response to a director bend, typically induced by unidirectional hybrid anchoring conditions in thin films or under applied electric fields. Compared to lattices of focal conics, 1d patterns harbor an unusual array of topological defects, including disclination and dislocation lines, grain boundaries, and curvature walls, running along periodically spaced and bend-free cylindrical stacks of smectic layers. These defects, most of which have been described by Maurice Kleman, appear to drive the structural evolution of 1d patterns as a function of the film thickness. Under unidirectional hybrid anchoring, the defects are oriented perpendicular to the planar anchoring direction and provide highly anisotropic templates for building ordered networks of close-packed nanoparticles. Moreover, the array formation at the transition from the nematic to the smectic phase is intimately connected to the smectic phase frustration by bend distortions, producing an ‘intermediate’ smectic phase with partial penetration of the bend distortion through undercooled nematic domains.