Half-integer Topological Defects Paired via String Micelles in Polar Liquids

TL;DR

This study demonstrates that cationic polymer doping in ferroelectric nematic liquid crystals creates paired half-integer topological defects connected by string micelles, revealing new ways to engineer polar topologies.

Contribution

It introduces a novel method of defect pairing via polymeric micelles in ferroelectric nematic liquids, supported by experimental and theoretical analysis of charge distribution.

Findings

Polymer doping induces paired topological defects connected by string micelles.

String defects exhibit butterfly textures with N"eel-type kink-walls.

Charge double layer model explains defect charge distribution.

Abstract

Ferroelectric nematic (NF) liquid crystals present a compelling platform for exploring topological defects in polar fields, while their structural properties can be significantly altered by ionic doping. In this study, we demonstrate that doping the ferroelectric nematic material RM734 with cationic polymers enable the formation of polymeric micelles that connect pairs of half-integer topological defects. Polarizing optical microscopy reveals that these string defects exhibit butterfly textures, featured with a two-dimensional polarization field divided by N\'eel-type kink-walls into domains exhibiting either uniform polarization or negative splay and bend deformations. Through analysis of electrophoretic motion and direct measurements of polarization divergences, we show that the string micelles are positively charged and their side regions exhibit positive bound charges. To elucidate these observations, we propose a charge double layer model for the string defects: the positive charged cationic polymer chains and densely packed RM734 molecules form a Stern charge layer, while small anionic ions and positive bound charges constitute the charge diffusion layer. Notably, our experiments indicate that only cationic polymer doping effectively induces the formation of these unique string defects. These findings enhance our understanding of ionic doping effects and provide valuable insights for engineering polar topologies in liquid crystal systems.