Analysis of the pitch in binary cholesteric liquid crystal mixtures

Surface-field-induced heliconical instability in the cholesteric phase of a mixture of a flexible dimer (CB7CB) and a rodlike molecule (8CB)

In 1968, de Gennes and Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K_{3}<K_{2} where K_{3} (K_{2}) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by Xiang et al. in a cholesteric phase made of a liquid crystal dimer material (CB7CB) in which K_{3} is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface field that acts as an effective electric field E with E≡1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the N_{TB} phase.

Heliconical-fluctuation-induced compensation point in the diluted cholesteric phase of mixtures containing the flexible dimer CB7CB

We show experimentally and theoretically that the heliconical fluctuations that develop in a cholesteric phase (Ch) close to a transition to a chiral twist-bend nematic phase (N_{TB}) may lead to the appearance of a compensation point. At this point, the equilibrium twist of the cholesteric phase vanishes and changes sign. Mixtures of the flexible dimer CB7CB and the rodlike molecules 8CB or 5CB, doped with a small amount of the chiral molecules R811, S2011, CC, or CB15, are used in experiments to determine the conditions for the appearance of a compensation point.

Synthesis and Characterization of Novel Bio-Chiral Dopants Obtained from Bio-Betulin Produced by a Fermentation Process

Cholesteric liquid crystals are frequently produced by the addition of chiral dopants to achiral nematic hosts. We report here the synthesis and performance of chiral dopants obtained from bio-betulin produced by a fermentation process. An important aspect of this work is to point out that the fermentation process used to obtain the starting materials is much easier and cheaper when carried out in large volumes than isolating it from the natural product. The performance of the dopants obtained from bio-betulin is indistinguishable from those obtained from commercially available synthetic betulin.

Cholesteric liquid crystals with a broad light reflection band

The cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective - the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for innovative applications like polarizer-free reflective displays, broadband polarizers, optical data storage media, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.

Analytical description for the chiral nematic state in terms of molecular parameters

An analytical description for helical twisting in chiral liquid crystals is derived, which explores the correlation between the macroscopic properties of a liquid crystal and the parameters of single molecules. This theory is based on a simple model of a spiral-like rigid molecules that possess a limited number of parameters, each having transparent geometrical meaning. Expressions for the macroscopic helical pitch in the chiral nematic phase and the nematic order parameters were obtained in terms of molecular parameters. The theory explains the experimentally observed helical sense inversions induced by a change of temperature or mutual concentration of components in mixtures. It is also shown that the helical sense inversion may be observed in the process of denaturation of polypeptides. Different reasons for helical sense inversions were identified, such as competition between the dispersion attraction and the steric repulsion, distinct mechanisms of the steric packing for different kinds of molecules, and biaxiality of molecules.

Molecular theory of helical sense inversions in chiral nematic liquid crystals

A molecular theory of the helical twisting in chiral liquid crystals is developed, which provides an explanation for the experimentally observed helical sense inversion induced by a change of concentration in binary mixtures of chiral and nonchiral nematic liquid crystals. The theory also describes the sense inversion induced by a change of temperature observed in some single component nematics. The theory present is based on a simple model of a chiral rigid molecule, composed of several equivalent nonchiral sites, which are arranged in the molecule to form a chiral configuration. The macroscopic helical pitch in the chiral nematic phase, twist elastic constant, and nematic order parameters are calculated using the same molecular model. It is shown that the helical sense inversion can be determined by a large biaxiality of chiral molecules. It is also demonstrated that the biaxiality is important in determining the variation of the helical pitch with temperature and concentration.