Ionically charged topological defects in nematic fluids

TL;DR

This paper demonstrates that topological defects in nematic electrolytes can serve as localized charge regions and electric multilayers, offering new ways to manipulate charge profiles without surface constraints.

Contribution

It introduces a theoretical framework showing how topological defects in nematic fluids can act as charge separation zones, expanding the understanding of ionic behavior in anisotropic electrolytes.

Findings

Topological defects can host localized charge and form electric multilayers.

Ions couple effectively with defect cores via ion solvability and flexoelectricity.

Different defect geometries exhibit distinct charge profiles and behaviors.

Abstract

Charge profiles in liquid electrolytes are of crucial importance for applications, such as supercapacitors, fuel cells, batteries, or the self-assembly of particles in colloidal or biological settings. However, creating localised (screened) charge profiles in the bulk of such electrolytes, generally requires the presence of surfaces -- for example, provided by colloidal particles or outer surfaces of the material -- which poses a fundamental constraint on the material design. Here, we show that topological defects in nematic electrolytes can perform as regions for local charge separation, forming charged defect cores and in some geometries even electric multilayers, as opposed to the electric double layers found in isotropic electrolytes. Using a Landau-de Gennes-Poisson-Boltzmann theoretical framework, we show that ions highly effectively couple with the topological defect cores via ion solvability, and with the local director-field distortions of the defects via flexoelectricity. The defect charging is shown for different defect types -- lines, points, and walls -- using geometries of ionically screened flat isotropic-nematic interfaces, radial hedgehog point defects and half-integer wedge disclinations in the bulk and as stabilised by (charged) colloidal particles. More generally, our findings are relevant for possible applications where topological defects act as diffuse ionic capacitors or as ionic charge carriers.