Metastable structures in electrospun microfibers with a long-pitch chiral nematic liquid crystal core

Exploiting geometric frustration in liquid crystals is a promising route for directing self-assembly, enabling the creation of complex, hierarchically structured materials such as optical devices, smart fibers, and soft actuators. This work introduces a novel platform for studying geometric frustration: core-sheath microfibers, fabricated via coaxial electrospinning, where a chiral nematic liquid crystal (CLC) core's pitch is on the order of the polymer sheath's inner diameter. These fibers provide a unique testbed to probe the competition between CLC topology and geometric confinement, resulting in discrete, metastable configurations. We identify three distinct configurations in our fibers - radially twisted (RT), eccentric radially twisted (ERT) and axially twisted (AT) - and demonstrate that the ratio of fiber diameter to CLC pitch dictates which structure forms. By examining temperature-dependent structural transitions, we reveal that the sequential transitions from RT to ERT and ERT to AT are driven by a weakening of azimuthal and polar surface anchoring, respectively. We use the observed structures and geometries to determine the helical twisting power (HTP) of CB15 in 5CB to be 5.8 μm. Finally, we report coexistence configurations, where RT, ERT, and AT structures can be observed concurrently within the same fiber or adjacent fibers, offering new possibilities for creating complex architectures.