Complex electric double layers in charged topological colloids

Jeffrey C Everts,Miha Ravnik

doi:10.1038/s41598-018-32550-8

Jeffrey C Everts, Miha Ravnik

Open Access

https://doi.org/10.1038/s41598-018-32550-8

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Abstract

Charged surfaces in contact with liquids containing ions are accompanied in equilibrium by an electric double layer consisting of a layer of electric charge on the surface that is screened by a diffuse ion cloud in the bulk fluid. This screening cloud determines not only the interactions between charged colloidal particles or polyelectrolytes and their self-assembly into ordered structures, but it is also pivotal in understanding energy storage devices, such as electrochemical cells and supercapacitors. However, little is known to what spatial complexity the electric double layers can be designed. Here, we show that electric double layers of non-trivial topology and geometry -including tori, multi-tori and knots- can be realised in charged topological colloidal particles, using numerical modelling within a mean-field Poisson-Boltzmann theory. We show that the complexity of double layers -including geometry and topology- can be tuned by changing the Debye screening length of the medium, or by changing the shape and topology of the (colloidal) particle. More generally, this work is an attempt to introduce concepts of topology in the field of charged colloids, which could lead to novel exciting material design paradigms.

Highlights

Charged surfaces in contact with liquids containing ions are accompanied in equilibrium by an electric double layer consisting of a layer of electric charge on the surface that is screened by a diffuse ion cloud in the bulk fluid
Topological concepts are starting to pave their way into soft condensed matter physics, by e.g. the realization of a mechanical topological insulator[4], the design of topologically protected transport of colloidal particles[5], and understanding the self-assembly of colloidal particles driven by the topological defects in a liquid crystalline host[6,7], leading to a range of topology-conditioned material properties
In this article we demonstrate how electric double layers of complex geometry and topology can be created, and how this couples to a non-trivial particle topology, unlike the topologically simple shapes that are known in the literature, such as rods[27], spheres[28], or ellipsoids[29], which have been studied extensively in complex environments[30,31]

Summary

Introduction

Charged surfaces in contact with liquids containing ions are accompanied in equilibrium by an electric double layer consisting of a layer of electric charge on the surface that is screened by a diffuse ion cloud in the bulk fluid. Charged surfaces that are formed in a liquid medium have a surface charge that stems from cationic and/or anionic surface groups[20], that are screened by the counter and/or co-ions in the medium that form a diffuse ion cloud with a decreasing net charge density in the bulk fluid[21] Such a surface charge with its screening cloud - called the electric double layer - has profound influence on macroscopic thermodynamic properties, which can be of relevance for supercapacitors[22], and further, profoundly affects the self-assembly of charged colloidal particles in bulk[23,24], in external fields[25] and at air-water interfaces[26]. This work is an attempt to bring the concepts of topology to the field of charged and ionic fluids, possibly allowing for new materials and new topology-determined interactions

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