Liquid crystal ‘blue phases’ with a wide temperature range

Abstract

Liquid crystal 'blue phases' are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5-2 degrees C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 degrees C down to 16 degrees C. We prove this with optical texture analysis, selective reflection spectroscopy, Kössel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.

The limits of flexoelectricity in liquid crystals

“…7,17 These reduce the driving voltage for certain electro-optic devices, 7 and produce unusually stable blue phases. 15,18 It is important to ask how far the development can continue; what are the maximum flexoelectric coefficients practically achievable? Helfrich 19 has shown theoretically-using phenomenological and molecularstatistical approaches-that the magnitude of the flexoelectric coefficients should be ≤∼ 10 pC/m, indicating the maximum has almost been reached.…”

Giant flexoelectric effect through interfacial strain relaxation

“…Owing to its intrinsic and universal existence in every dielectric material, flexoelectricity has garnered a wide range of scientific interest and potential applications. Particularly in flexible systems, such as liquid crystals [12,13], low-dimensional crystals (e.g. graphene or carbon nanotubes) [14] and biological molecular membranes [15] or hairs, the flexoelectric effect can be quite significant.…”

Large Colloids in Cholesteric Liquid Crystals

“…Two phases in which the disclinations exhibit cubic symmetry have been identified experimentally (BPI and BPII) while a third (BPIII) is thought to be amorphous [16]. Recently, there have been significant advances in the technology of blue phases where polymer-stabilised formulations allow BPs to the used in a practically useful temperature range [8,19]. This holds out the promise of useful application in, e.g., photonics [5,6].…”