crystals Article The Beauty of Twist-Bend Nematic Phase: Fast Switching Domains, First Order Fréedericksz Transition and a Hierarchy of Structures Vitaly P. Panov 1,2 , Jang-Kun Song 2 , Georg H. Mehl 3 and Jagdish K. Vij 1, *   Citation: Panov,V.P.; Song, J.-K.; Mehl, G.H.; Vij, J.K. The Beauty of Twist-Bend Nematic Phase: Fast Switching Domains, First Order Fréedericksz Transition and a Hierarchy of Structures. Crystals 2021, 11, 621. https://doi.org/10.3390/ cryst11060621 Academic Editor: Charles Rosenblatt Received: 29 April 2021 Accepted: 25 May 2021 Published: 31 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Electronic and Electrical Engineering, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland; panovv@mee.tcd.ie 2 School of Electronic & Electrical Engineering, Sungkyunkwan University, Suwon 440-746, Korea; jk.song@skku.edu 3 Department of Chemistry, University of Hull, Hull HU6 7RX, UK; g.h.mehl@hull.ac.uk * Correspondence: jvij@tcd.ie Abstract: The twist-bend nematic phase (N TB ) exhibits a complicated hierarchy of structures respon- sible for several intriguing properties presented here. These are: the observation of a fast electrooptic response, the exhibition of a large electroclinic effect, and the observation of an unusual pattern of the temperature dependence of birefringence of bent-shaped bimesogens in parallel-rubbed planar- aligned cells. These unusual effects inspired the use of highly sophisticated techniques that led to the discovery of the twist-bend nematic phase. Results of the optical retardation of a parallel-rubbed planar-aligned cell show that the ‘heliconical angle’ (the angle the local director makes with the optical axis) starts increasing in the high temperature N phase, it exhibits a jump at the N–N TB transition temperature and continues to increase in magnitude with a further reduction in temper- ature. The liquid crystalline parallel-rubbed planar-aligned and twist-aligned cells in this phase exhibit fascinating phenomena such as a demonstration of the beautiful stripes and dependence of their periodicity on temperature. The Fréedericksz transition in the N TB phase is found to be of the first order both in rubbed planar and homeotropic-aligned cells, in contrast to the second order transition exhibited by a conventional nematic phase. This transition shows a significant hysteresis as well as an abrupt change in the orientation of the director as a function of the applied electric field. Hierarchical structures are revealed using the technique of polymer templating the structure of the liquid crystalline phase of interest, and imaging of the resulting structure by scanning electron microscopy. Keywords: twist-bend nematic phase; nematic liquid crystals; stripe textures; Fréedericksz transition; optical retardation; birefringence 1. Introduction Nematic liquid crystals are used ubiquitously in the multibillion display industry. Over the years, the device architecture and the materials have improved to the extent that the switching times of liquid crystals currently can be reduced to 1.0 ms or less [1] without the use of over-drive circuitry. This offers a reasonable speed of switching for most current display applications, including the future development of high-resolution fast displays based on the colour sequential display mode. Furthermore, nematic liquid crystal materials can be designed to approach an ideal dielectric. This is achieved by reducing its DC conductivity and dielectric loss for frequencies where the display will operate. The liquid crystalline nematic phase offers higher energy efficiency as it consumes extremely low levels of power when compared to other display technologies. Currently, the main area of improvement is to address the brightness of a display, such as in mobile phones. Their nematic liquid crystals (NLCs) are continually being replaced by organic Crystals 2021, 11, 621. https://doi.org/10.3390/cryst11060621 https://www.mdpi.com/journal/crystals