Spontaneous formation of chiral domains in a flowing achiral nematic liquid crystal

An achiral nematic fluid in a confined geometry 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 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 and saddle splay elastic constant compared to the bend elastic constant of LCLCs allow for twist deformations. We show that the pitch length of the chiral domains depends on the local flow velocity.