Electrostatically controlled surface boundary conditions in nematic liquid crystals and colloids

Abstract

Differing from isotropic fluids, liquid crystals exhibit highly anisotropic interactions with surfaces, which define boundary conditions for the alignment of constituent rod-like molecules at interfaces with colloidal inclusions and confining substrates. We show that surface alignment of the nematic molecules can be controlled by harnessing the competing aligning effects of surface functionalization and electric field arising from surface charging and bulk counterions. The control of ionic content in the bulk and at surfaces allows for tuning orientations of shape-anisotropic particles like platelets within an aligned nematic host and for changing the orientation of director relative to confining substrates. The ensuing anisotropic elastic and electrostatic interactions enable colloidal crystals with reconfigurable symmetries and orientations of inclusions.

Colloidal interactions and unusual crystallization versus de-mixing of elastic multipoles formed by gold mesoflowers

“…hexadecapoles) forming unusual crystals that can be tuned by packing density and strength of electrostatic repulsions. While here we focused only on the analysis of two-dimensional crystal lattices for illustrative purposes, even more complex behavior is expected in three dimensions, where even elastic quadrupoles can form triclinic crystal lattices 39 . As the symmetries of elastic potentials of high-order multipoles, which form the basis of crystals, become intrinsically incompatible with crystalline lattices, various forms of frustration can arise and lead to complex crystallization behavior that can be potentially tuned through changing concentration of counterions and surface charging.…”

“…Yukawa potential corresponding to the screened electrostatic interactions, which can be tuned by adding counterions through doping LCs with salt and other additives 39 :…”

“…9a,b, this tendency can be inconsistent with the efficiency of two-dimensional packing at high colloidal concentrations. Colloidal particles are electrostatically charged and mutually repel with screened Coulomb electrostatic potential, where the range and the strength of this potential can be tuned 39 , which provides additional knobs to control the self-assembly of nematic colloids. We have performed illustrative molecular dynamic simulations of self-assembly behavior of hexadecapole colloids with strong, weakly screened repulsive Yukawa potential.…”

Anisotropic electrostatic screening of charged colloids in nematic solvents

“…1G) and the strength of the electric field. The displacement of particles was tracked using video microscopy, which allows us to estimate the effective charge Ze, from the balance of the Stokes viscous drag force and the electric force (40)(41)(42). To probe only the electrostatic pair interactions between charged colloidal dumplings, we measured their pair interactions in the isotropic phase of 5CB, where the contribution due to LC elastic forces is eliminated.…”

“…Multipole expansions have been used to describe elasticity-mediated colloidal interactions in LCs, drawing parallels to electrostatic interactions (37)(38)(39). In general, elasticitymediated interactions in LCs are accompanied by screened electrostatic and dispersive (London-van der Waals) (40) interactions; however, the previous studies of such colloidal systems with LC hosts were done for highly anisotropic rod-and disc-shaped particles, so that the role of the anisotropy of colloidal particles and that of the LC medium were not separated because of the particle's shape anisotropy and so far explored while probing phase behavior and self-assembly of colloidal superstructures (41,42).…”