Effect of Electric Fields on the Director Field and Shape of Nematic Tactoids

TL;DR

This paper models how electric fields influence the shape and director configuration of nematic tactoids, revealing that significant elongation requires specific director field arrangements and aligning well with experimental observations.

Contribution

It introduces a model for partially bipolar tactoids that accounts for elastic, surface, and Coulomb energies, explaining elongation behavior under electric fields.

Findings

Tactoid elongation is limited unless the director field is bipolar or quasi bipolar.

Significant elongation occurs only if the director field is frozen in a certain configuration.

Model predictions align with experimental data on tactoid behavior under electric fields.

Abstract

Tactoids are spindle shaped-droplets of a uniaxial nematic phase suspended in the co-existing isotropic phase. They are found in dispersions of a wide variety of elongated colloidal particles, including actin, fd virus, carbon nanotubes, vanadium peroxide and chitin nanocrystals. Recent experiments on tactoids of chitin nanocrystals in water show that electric fields can very strongly elongate tactoids even though the dielectric properties of the co-existing isotropic and nematic phases differ only subtly. We develop a model for partially bipolar tactoids, where the degree of bipolarness of the director field is free to adjust to optimize the sum of the elastic, surface and Coulomb energies of the system. By means of a combination of a scaling analysis and a numerical study, we investigate the elongation and director field's behavior of the tactoids as a function of their size, the strength of the electric field, the surface tension, anchoring strength, the elastic constants and the electric susceptibility anisotropy. We find that tactoids cannot elongate significantly due to an external electric field, unless the director field is bipolar or quasi bipolar and is somehow frozen in the field-free configuration. Presuming that this is the case, we find reasonable agreement with experimental data.