Chiral domains in a flowing achiral nematic liquid crystal

An achiral nematic liquid in a microfluidic cell could be expected to form achiral structures in a pressure-driven flow. Surprisingly, we find the spontaneous emergence of chiral structures when an achiral lyotropic chromonic liquid crystal in the nematic phase relaxes from a high velocity flow to a steady-state lower velocity flow. The chirality results from a periodic double-twist deformation of the liquid crystal and leads to striking stripe patterns vertical to the flow direction. We show that the double-twist deformation is triggered at regions of biaxial-splay deformation which becomes unstable. The transition from this biaxial-splay deformation to the double-twist deformation can be rationalized by the low Frank–Oseen elastic energy stored in double-twist deformations compared to that stored in biaxial-splay deformations. We show that the size of the chiral structures can be tuned by the local flow velocity and is set by the competition between the elastic torque and the viscous torque acting on the structures.