Theoretical models of modulated nematic phases

• Published in: Soft matter Vol. 19; no. 15; pp. 2675 - 274
• Main Author: Szmigielski, Micha
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 12.04.2023
• Subjects: Crystals; Elastic deformation; Elastic properties; Ferroelectricity; Free energy; Liquid crystals; Magnetic effects; Mathematical models; Phases
• ISSN: 1744-683X; 1744-6848; 1744-6848
• DOI: 10.1039/d2sm01600a

Novel modulated nematic phases, such as twist-bend nematics, splay-bend nematics and splay nematics, are an important subject of research in the field of liquid crystals. In this article fundamental information about the discovery, structure and properties of these phases is presented. Various theoretical models of elastic properties are compared, especially the proposed formulae for the free energy density of modulated nematic phases and the conditions for their stability. The emphasis is put on the variety of material parameters and variables in the mathematical description of the structures. The elastic models are classified according to a few criteria. Flexopolarisation is indicated as a main phenomenon responsible for the formation of modulated nematic phases. The elastic models are used for analysing the deformations of the twist-bend nematic structure in external fields. Dielectric, flexoelectric, ferroelectric and magnetic effects are considered. Two types of distortions are distinguished: microscopic (connected with the deformation of the director distribution) and macroscopic (related to the change of the optic axis direction). This review can be a starting point for further studies, for example computer simulations of modulated phases and design of liquid crystalline devices. This paper presents fundamental information about novel modulated nematic phases, namely twist-bend nematics, splay-bend nematics and splay nematics, and the review of their elasticity models and the deformations occurring in external fields.