Instabilities and flow-induced structures in lyotropic chromonic liquid crystals

Lyotropic chromonic liquid crystals in the nematic phase are anisotropic fluids. We exploit this intrinsic anisotropy to probe growth morphology transitions that occur in the viscous-fingering instability upon the displacement of the liquid crystal by a less-viscous Newtonian liquid. In isotropic systems, this instability produces complex patterns that are characterized by repeated branching of the evolving structure, which leads to the common morphologies of fractal or dense-branching growth. In anisotropic systems, by contrast, the growth morphology changes to dendritic growth characterized by stable needle-like structures. We show that the morphology transition coincides with the onset of shear-alignment at high shear rates, where the viscous forces from the flow become dominant over the elastic forces from the nematic potential. Below this critical shear rate, the lyotropic chromonic liquid crystal exhibits a tumbling behavior that leads to the formation of novel types of defects and flow structures, including the surprising emergence of chiral domains despite the achiral nature of the material. We discuss the mechanism of the spontaneous mirror symmetry breaking and rationalize the selection of a well-defined period of the chiral domains.