Ferroelectric nematic liquid crystals are newly discovered polar fluids characterized by microscopic orientational ordering and macroscopic spontaneous polarizations. In this talk, I will present two intriguing topological structures in polar fluids that are dramatically different from those in solid state ferroelectric materials. (1) p walls that delineate regions of opposite polarizations are ubiquitous within ferroelectric materials. I will show that π walls in polar fluids are composed of twin disclinations spaced horizontally, enclosing a subdomain with left- or right-handed p twists. The degenerate geometric configurations of these twin disclinations give rise to Z₂ kinks and antikinks, effectively partitioning subdomains of opposite chirality like Ising chains. (2) Topological defects with half-integer winding numbers are uncommon in polar materials as set by topological restraints. I will show that doping with cationic polymers can significantly change the polar fluids, making the polarization fields constrained in two-dimensional plane like an XY model and facilitating the formation of string micelles connecting a half-integer vortex and antivortex pair.


Biosketch

Qi-Huo Wei graduated from Nanjing University in 1993 with a PhD in condensed matter physics. He was a tenured professor at the Liquid Crystal Institute and Department of Physics in Kent State University in the US and joined Southern University of Science and Technology as a professor in 2020. He was a recipient of Alexander von Humboldt research fellow and the CAREER award of National Science Foundation. Prof. Wei has a broad research interest, including soft matter physics, micro/nanooptics, nanomanufacturing and biomimetics, and has made original contributions in areas related to single file diffusion, nanoparticle plasmonic coupling, Brownian motion of low symmetry particles, command of living matter, and photopatterning of molecular orientations.

Please contact phweb@ust.hk should you have questions about the talk.