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 (LCLC) 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 demonstrate that this occurrence of chiral structures can be rationalized by the disparate elastic constants of the LCLC; the peculiarly low twist elastic constant compared to the bend and splay elastic constants of LCLCs allows for twist deformations, which lead to spontaneous symmetry breaking. We show that the period of the chiral structures depends on the local flow velocity and is set by the competition between the elastic torque and viscous torque acting on the structures.