Lyotropic chromonic liquid crystals (LCLC) have gained increasing attention in the last two decades as an interesting yet, relatively poorly understood class of liquid crystals. They are made up of plank like molecules with a poly-aromatic core and polar peripheral groups. p-p interactions result in the stacking of the constituent molecules and the aggregation process is believed to be isodesmic; that is, addition or removal of molecules to a column is always associated with the same energy cost. The stacks increase in length with increasing concentration and eventually form nematic and columnar phases. Consequently, the aspect ratio of the stacks is a function of both temperature and concentration. Curiously, the twist elastic constant for LCLC is much lower than splay, bend and saddle-splay elastic constants. This results in the emergence of spontaneous twist when confined to different geometries. Twisted structures have been reported previously in disodium cromoglycate (DSCG) tactoids, Sunset Yellow FCF (SSY) droplets and chiral bipolar colloids [1, 2, 3]. More recently, the reflection symmetry breaking of LCLC in a cylindrical geometry has also been reported. Here the director adopts a doubly-twisted configuration even though there is a deformation-free state which corresponds to the axial configuration [4, 5]. The stability of the persistent doubly- twisted configuration was explained by considering the saddle-splay contribution to the free energy.<br/><br/>Unlike conventional nematics, lyotropic liquid crystals are notoriously hard to align. In this talk, we report on the homogeneous planar alignment of Sunset Yellow FCF in a rectangular capillary. Intuitively, an axial director profile with the director pointing along the long axis of the capillary is expected, when a liquid crystal is confined to a rectangular capillary with planar boundary conditions. This corresponds to a deformation free ground state. Remarkably, the in-plane director aligns perpendicular to the long axis of the rectangular capillary resulting in a counterintuitive configuration. In this talk, we provide a rationale for the counter-intuitive director alignment observed in the rectangular capillary, which proceeds through a “twisted” path. We also point to ways to obtain uniform, stable homogenous alignment, and provide an estimate of the anchoring energy, which is higher than for thermotropic liquid crystals, for the alignment technique to provide a monodomain of these LCLCs.

Twisted structures in lyotropic chromonic liquid crystals: Here, there and everywhere

255th National Meeting (2018)

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