Ionically Charged Topological Defects in Nematic Fluids

Jeffrey C Everts,Miha Ravnik

doi:10.1103/physrevx.11.011054

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.

Highlights

The ability to spatially control electric charge has relevance in a range of fields—from charged polymers [1,2], and biological [3] and active matter [4], to colloidal materials [5], complex fluids [6], and even microelectronics [7]
Creating localized 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
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

Summary

INTRODUCTION

The ability to spatially control electric charge has relevance in a range of fields—from charged polymers [1,2], and biological [3] and active matter [4], to colloidal materials [5], complex fluids [6], and even microelectronics [7]. The fluid-fluid interfacial tension is altered [19], and the interface carries an intrinsic capacitance, which can alter the total capacitance of electrochemical cells [20,21] These approaches were shown recently to even drive surface phase transitions between various types of electric double layers containing an antagonistic salt [22]. The central idea of this paper is that efficient charge separation in the bulk of the material, combined with the possibility to manipulate the topological defects of nematic electrolytes, can lead to the capacity to use topological defects as controllable fluidlike microelectronic elements. We show that coupling of ions to the nematic-ordered structure is strong in topological defects and leads to charging of the defect cores, which are screened by bulk electric “multi” (double) layers. The idea of this paper is that topological defects as part of a more general, topological, soft-matter platform could be transformed into soft microelectronic circuits

METHODS AND MATERIAL

ELECTRIC DOUBLE LAYER OF AN ISOTROPIC-NEMATIC INTERFACE

WEDGE DISCLINATIONS

LIQUID-CRYSTAL COLLOIDS

EXPERIMENTAL RELEVANCE

VIII. CONCLUSIONS AND OUTLOOK