Anisotropic electrostatic screening of charged colloids in nematic solvents

Abstract

Charged systems where ions, DNA, proteins and/or charged colloidal particles are dispersed in an isotropic fluid, are found in a range of prominent materials and biological systems. However, generally the role of the host material dielectric anisotropy in charged colloidal materials is ignored, despite frequent relevance in defining colloidal self-assembly, biological function and outof-equilibrium behaviour. In this work, we formulate and experimentally demonstrate anisotropic electrostatic interactions in a system of charged colloidal particles in a nematic electrolyte. Experimentally, charged dumpling-shaped near-spherical colloidal particles are used as a model system of charged colloidal particles in a nematic medium, demonstrating anisotropic elastic and electrostatic effective pair interactions for colloidal surface charges tunable from neutral to high. Theoretically, we derive asymptotic expressions for (i) the anisotropic electrostatic potential on the single-particle level and (ii) the effective pair interaction in the system, under the assumption of spherical charged particles in a uniform nematic director field. The analytical expressions for the pair interactions are compared with experiments, demonstrating good qualitative agreement within an experimentally accessible parameter range. More generally, our work extends the traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for charge-stabilised colloidal suspensions in isotropic solvents, to dielectric anisotropic host media with orientational elasticity.

Ionically charged topological defects in nematic fluids

“…For higher screening lengths, the double layer is more elongated (not shown), but then the effects of defects are washed out, see Ref. [33].…”

“…Negative values of g ± correspond to a preference for the nematic phase, which we will not consider here, but can be equally implemented. Finally, typical (isotropic) Debye screening lengths in thermotropics are between 50 − 1000 nm [29,33,68,69], and with doping even values of ∼ 1 nm can be reached [70].…”

“…A known example was the first LCD display that used ionic doping [23]. More recently, charge and ions are shown to affect ion transport [24][25][26][27], surface anchoring [28][29][30], colloidal self-assembly [31][32][33], and surface charge control [34]. Nematic fluids posses orientational order of their building blocks which can exhibit topologically protected patterns, called topological defects.…”