Colloidal nanoparticles in liquid crystals: Bulk properties, biaxiality and untwisting in cholesterics

TL;DR

This paper investigates how colloidal nanoparticles influence the phase behavior and director configurations in liquid crystals, revealing stabilization of biaxial phases and reversible untwisting transitions in cholesterics, with implications for optical tuning.

Contribution

It provides a theoretical analysis of nanoparticle effects on liquid crystal phases, including explicit critical point computations and numerical demonstrations of structural transitions.

Findings

NPs eliminate the isotropic-nematic phase transition

NPs stabilize biaxial phases over certain temperatures

Structural untwisting transitions are reversible and temperature-dependent

Abstract

We study the effects of colloidal nanoparticles (NPs) in liquid crystal samples in the dilute limit, in a Landau--de Gennes theoretical framework. The effects of the suspended NPs are captured by a homogenized energy, as outlined in~\cite{canevari2020design}. For spatially homogeneous samples, we explicitly compute the critical points and minimizers of the modified Landau--de Gennes energy and show that the presence of NP eliminates the first-order isotropic-nematic phase transition, stabilises elusive biaxial phases over some temperature ranges and that the symmetry of the NP boundary conditions or surface treatments dictates the bulk equilibrium phase at high temperatures. We also numerically demonstrate structural transitions from twisted helical director profiles to untwisted director profiles in cholesteric-filled channel geometries, driven by the collective effects of the NPs and increasing temperature. These transitions are reversible upon lowering the temperature in sufficiently large domains, where thermal hysteresis can also be observed. This behaviour opens interesting avenues for tuning the optical properties of confined, nano-doped cholesteric systems.