A minimal model of solitons in nematic liquid crystals

TL;DR

This paper introduces a minimal tensor-based model explaining the formation and dynamics of solitons in nematic liquid crystals, highlighting the roles of surface imperfections, flexoelectricity, dielectric contrast, and electric fields.

Contribution

It presents a new minimal theoretical model that predicts soliton structures and behaviors in nematic liquid crystals, aligning with experimental observations.

Findings

Formation of butterfly-shaped director structures in the model

Detachment and propagation of soliton bullets with increasing electric field

Consistency of model predictions with experimental results

Abstract

Solitons in liquid crystals have generated considerable interest. Several hypotheses of varying complexity have been advanced to explain how they emerge, and a consensus has not emerged yet about the underlying forces responsible for their formation or their structure. In this work, we present a minimal model for soliton structures in achiral nematic liquid crystals, which reveals the key requirements needed to generate traveling solitons in the absence of added charges. These include a surface imperfection or inhomogeneity capable of producing a twist, flexoelectricity, dielectric contrast, and an applied AC electric field that can couple to the director's orientation. Our proposed model is based on a tensorial representation of a confined liquid crystal, and it predicts the formation of "butterfly" structures, quadrupolar in character, in regions of a slit channel where the director is twisted by the surface imperfection. As the applied electric field is increased, solitons (or "bullets") become detached from the wings of the butterfly, which then rapidly propagate throughout the system. The main observations that emerge from the model, including the formation and structure of butterflies, bullets, and stripes, as well as the role of surface imperfections and the strength of the applied field, are consistent with our own experimental findings presented here for nematic LCs confined between two chemically treated parallel plates.