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Topological Colloidal Double Layers

Periodic Reporting for period 1 - TCDL (Topological Colloidal Double Layers)

Reporting period: 2018-04-01 to 2020-03-31

"In the project Topological Colloidal Double Layers (TCDL) we addressed the problem how a complex geometry or topology can influence the distribution of ions in a wide variety of systems, spanning charge-stabilised colloidal dispersions, but also biological systems such as charged proteins or DNA. We encounter ions everywhere in daily life, where the simplest example is table salt or sodiumchloride, which dissociates in Na+ and Cl-, and other ions that form an integral part of the human body where they are involved in many cellular processes, but also in electronic devices. Our research, is therefore very topical, and leads as a far-reach goal, to a better general understanding of how ions can be used to manipulate electrical charges around large-scale structures (such as proteins), but also in devices. Moreover, new materials can be discovered, that are based on these complex-shaped ion distributions, having immediate societal and technological benefits.

The overall objectives of the TCDL project were to find out how ionic charges, that can be as simple as table salt, can influence electrical fields and how they distribute themselves around charged complex-shaped particles (such as colloidal particles or biological charged ""big"" molecules such as DNA). Moreover, this may lead to many novel interesting inter-particle interactions and to the formation of new large-scale structures, such as colloidal crystals, which possible photonic applications."
Work performed during the project TCDL is concerned with the development of an underlying theory to describe ionic charges (salt) around complex-shaped topological charged particles and/or in nematic liquid crystals. The main results include the calculation of charge distributions around complex-shaped particles and liquid crystal topological defects (as found in, for example, liquid crystal displays, LCDs). But also, we highlighted the influence of external liquid flow. These theoretical findings are expected to lead to new, exciting experiments. More precisely, we made the following discoveries:

We showed how ions distributions can be tuned when coupled to the underlying particle shape topology and geometry, altering their interaction and perhaps also transport properties (see accompanying figure below).
[see: J. C. Everts and M. Ravnik, Complex electric double layers in charged topological colloids, Sci. Rep. 8, 14119 (2018)].

Ion distributions in a nematic host can significantly alter the surface align of liquid-crystalline molecules along an external wall, which might be useful to tune particle self-assembly.
[see: J. C. Everts and M. Ravnik, Charge-, salt- and flexoelectricity-driven anchoring control in nematics, submitted. Preprint at: arXiv:2003.02914 (2020)].

External flow can induce an inhomogenous surface charge distribution in a flat plate, leading to possible new microfluidic applications.
[see B. L. Werkhoven, J. C. Everts, S. Samin, and R. van Roij, Flow-induced surface charge heterogeneity in electrokinetics due to Stern-layer conductance coupled to reaction kinetics, Phys. Rev. Lett. 120, 264502 (2018)].

Charged colloidal spheres in a nematic electrolyte interact with each other not only with an anisotropic elastic interaction, but the dielectric anisotropy also makes the screened electrostatic interaction anisotropic even for isotropic particle shapes.
[see: J. C. Everts, B. Senyuk, H. Mundoor, M. Ravnik, I. I. Smalyukh, Anisotropic electrostatic and elastic interactions in charged colloid spheres (working title), in preparation]

Ions in nematic hosts can be captured by topological defects due to selective solvation, whereas flexoelectricity can lead to ionic charge separation around the defect core.
[see: J. C. Everts and M. Ravnik, Ionically-charged topological defects in nematic fluids (working title), in preparation].

Using the geometry of the double layer, one can map any charged particle to a singular charge distribution, making it possible for an analytical formulation of the electrostatic potential and effective pair interaction for a wide range of complex-shaped particles
[see: J. C. Everts, Effective charge models for screened electrostatic interactions (working title), in preparation].

Topological defects can be used to manipulate surface charge distributions on charge-regulating or conducting flat plates,
[see: M. Ravnik and J. C. Everts, Topological-defect induced surface-charge heterogeneities in nematic electrolytes, submitted].
"First of all, a better theoretical understanding has been achieved on how ions distribute around complex charged external surfaces and in complex media, such as liquid crystals. Moreover, we found out that these ion distributions influence the alignment of molecular building blocks on an external surface, which has direct electronic applications. Other findings of how ionic charges ""talk"" to complex geometrical and topological media, are for example, the discovery that liquid crystal topological defects can act as charge carriers and even as miniature liquid-like capacitors, in contrast to the usual capacitors where the components are based on solid materials. Finally, a better general understanding of particle interactions has been obtained during this project, highlighting the role of a complex particle shape, but also of a liquid crystal as a medium where the ions reside, compared to more daily-life liquids, such as water."
Influence of a topological particle on the surrounding ion distributions by changing the salt amount