Flow-induced periodic chiral structures in an achiral nematic liquid crystal

Supramolecular chirality typically originates from either chiral molecular building blocks or external chiral stimuli. Generating chirality in achiral systems in the absence of a chiral input, however, is non-trivial and necessitates spontaneous mirror symmetry breaking. Achiral nematic lyotropic chromonic liquid crystals have been reported to break mirror symmetry under strong surface or geometric constraints. Here we describe a previously unrecognised mechanism for creating chiral structures by subjecting the material to a pressure-driven flow in a microfluidic cell. The chirality arises from a periodic double-twist configuration of the liquid crystal and manifests as a striking stripe pattern. We show that the mirror symmetry breaking is triggered at regions of flow-induced biaxial-splay configurations of the director field, which are unstable to small perturbations and evolve into lower energy structures. The simplicity of this unique pathway to mirror symmetry breaking can shed light on the requirements for forming macroscopic chiral structures.超分子手性通常源自手性分子构筑基元或外部手性刺激。然而，在无手性输入的非手性体系中产生手性并非易事，这需要发生自发镜像对称性破缺。已有研究表明，非手性向列型溶致色团液晶（lyotropic chromonic liquid crystals）可在强表面约束或几何约束下打破镜像对称性。本文报道了一种此前未被发现的构筑手性结构的机制：将该材料置于微流控池（microfluidic cell）内，使其经受压力驱动流。该手性源于液晶的周期性双扭曲构型（double-twist configuration），并呈现为醒目的条纹图案。我们发现，镜像对称性破缺起始于指向矢场（director field）发生流动诱导双轴展曲构型的区域，这类区域对微小扰动具有不稳定性，会演变为低能结构。这种独特的镜像对称性破缺途径十分简洁，可为宏观手性结构的形成条件提供新的认知。