Orientation of chromonic liquid crystals by topographic linear channels: multi-stable alignment and tactoid structure

We study the alignment behaviour of the chromonic liquid crystal phases of sunset yellow (SSY)/water, disodium cromoglycate (DSCG)/water and their mixtures when confined in cells by polymer films topographically imprinted with linear channels of 250 nm width, depth and spacing. A variety of novel alignment effects are generated by contact with such a patterned surface, as follows. Nematic DSCG and nematic SSY at low concentration and their nematic mixtures orient with the long axes of stacked chromonic aggregates on average parallel to the channels, that is, with the molecular planes normal to the channel axis. These oriented nematics exhibit isotropic/nematic tactoids aligned with their major axes along the channels. Two geometries of the tactoids, elliptic cylinder and rectangular cuboid with hemi cylindrical ends are observed. Additionally, a transition of DSCG tactoids from a three-dimensional (3D) director configuration to a two-dimensional one is observed when the tactoids on one surface of a cell touch the other surface of the cell. In SSY solutions of sufficiently high concentration, multi-stable alignment is found, including preferential in-plane orientation of the director parallel to, perpendicular to and 45° rotated from the channels.