Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen

Miscibility Studies of Bismesogen CBnCB Forming Nematic Twist-Bend Phase with Cyanobiphenyls nCB

This work aims to determine how the nematic twist-bend phase (NTB) of bismesogens containing two rigid parts of cyanobiphenyls connected with a linking chain containing n = 7, 9, and 11 methylene groups behaves in mixtures with structurally similar cyanobiphenyls nCB, n = 4-12, 14. The whole phase diagrams are presented for the CB7CB-nCB system. For the other systems, CB9CB-nCB and CB11CB-nCB, only curves corresponding to NTB-N phase transition are presented. Based on the temperature-concentration range of the existence of NTB phase, it was established that an increase in the alkyl chain length of CBnCB causes an increase in the stability of the NTB phase. But surprisingly, an increase in the alkyl chain length of nCB compounds does not change the slope of the NTB-N equilibrium line on phase diagrams. It is slightly bigger when the nCB compound has the same length of alkyl chain as the length of the linking group of a bismesogen. XRD studies were carried out for two mixtures.

Polymerisation of twist-bend nematic textures for electro-optical applications

Polymer-stabilised liquid crystals (PSLCs) have recently been used to maintain the focal conic domains (FCDs) typical of the smectic A phase in the nematic phase for smart window applications. The newly discovered twist-bend nematic phase of bent-shaped dimers also exhibits FCDs due to its pseudo-layered structure. The variety of topological defects in the NTB phase is arguably even greater than in the smectic A phase, but the NTB phase is often metastable and usually crystallises at room temperature, which hinders its use in electro-optical applications. Here we show how different textures (FCDs, rope-like texture, double helices) of the NTB phase can be polymerised and then maintained in the nematic phase, at room temperature. This allows us to combine in PSLCs the optical properties of these defects, the thermal stability of the nematic phase and its reversible response to an electric field. We also show that the polymerised FCDs of the NTB phase could be used in smart glass applications and that the polymerised rope-like texture could be of interest for optical modulators and beam steering. In addition, the polymerisation of double helices could help to better understand their formation and structure in the NTB phase. More fundamentally, our work shows that despite the lack of density modulation, the textures of the NTB phase, thanks to its periodic character, can be exploited in the same way as those of the smectic A phase.

Dipole-dipole correlations in the nematic phases of symmetric cyanobiphenyl dimers and their binary mixtures with 5CB

We report on the temperature dependence of birefringence and of the static dielectric permittivity tensor in a series of binary mixtures between the symmetric, bent-shaped, 1'',9''-bis(4-cyanobiphenyl-4'-yl)nonane (CB9CB) dimer and the monomeric nematogen 5CB. In the studied composition range the mixtures exhibit two nematic phases with distinct birefringence and dielectric features. Birefringence measurements are used to estimate the temperature dependence of the tilt between the axis defining the nanoscale helical modulation of the low temperature nematic phase with the (local) direction of the maximal alignment of the cyanobiphenyl units. Planar as well as magnetically and/or electrically aligned samples are used to measure the perpendicular and parallel components of the dielectric permittivity in both nematic phases. A self-consistent molecular field theory that takes into account flexibility and symmetry of the constituent mesogens is introduced for the calculation of order parameters and intra-molecular orientational dipolar correlations of the flexible dimers as a function of temperature/concentration. Utilising the tilt angle, as calculated from the birefringence measurements, and the predictions of the molecular theory, dielectric permittivity is modelled in the framework of the anisotropic version of the Kirkwood-Fröhlich theory. Using the inter-molecular Kirkwood correlation factors as adjustable parameters, excellent agreement between theory and permittivity measurements across the whole temperature range and composition of the mixtures is obtained. The importance of the orientational, intra- and inter-molecular, dipolar correlations, their relative impact on the static dielectric properties, as well as their connection with the local structure of the nematic phases of bent-shaped bimesogens, is discussed.

Optical and flexoelectric biosensing based on a hybrid-aligned liquid crystal of anomalously small bend elastic constant

Liquid crystal (LC)-based biosensors rely on the response of the LC molecules to perturbation generated by analytes at the interface, leading to the susceptible change in molecular alignment or orientation. The sensitivity of these biosensors is primarily dependent on the LC's material properties and surface anchoring strength. By incorporation of an unconventional mesogenic compound (CB7CB) coupled with the hybrid-alignment cell configuration, this work presents a binary nematic LC for label-free biosensing, manifesting a novel sensing technology that takes advantage of CB7CB-induced flexoelectricity in the transducer. Herein, we prepared LC mixtures by blending a typical rod-like nematic LC (E7) with the bent-core mesogen CB7CB in various weight ratios and studied the effect of the CB7CB content on E7/CB7CB-based biosensing performance in vertically aligned and hybrid-aligned nematic (HAN) cells. Owing to the anomalously small bend elastic constant K₃₃ in CB7CB, the mixture designated CB45 with the highest CB7CB weight percentage (45 wt% in this study) was best applicable to biosensing in HAN cells. When observed under a polarizing optical microscope, CB45 in the HAN geometry showed the capability of detection of as low as 10^-10 g/mL for the protein standard bovine serum albumin (BSA). Moreover, the quantitation of the assay was fulfilled by both dielectric and light transmission measurements of the hybrid-aligned cholesteric CB45/R5011. The limit of detection of 7 × 10^-10 g/mL was achieved by spectrometric analysis. To the best of our knowledge, this work is the first to demonstrate flexoelectric biosensing on the basis of flexoelectric polarization associated with giant flexoelectricity in CB7CB partially constituting the LC transducer.

Structure and Lehmann rotation of drops in a surfactant-doped bent-core liquid crystal

The structure of the nematic (cholesteric) drops that form at the clearing temperature of a mixture of the bent-core molecule CB7CB and the rodlike molecule 8CB doped with a surfactant is optically determined. Using experimental observations and numerical simulations, it is demonstrated that the director field inside these drops is not escaped concentric, as previously proposed, but twisted bipolar. The Lehmann rotation of these drops in the presence of a temperature gradient is described. Their rotation velocity is shown to be proportional to the temperature gradient and to the surface twist angle of the director field and inversely proportional to the drop radius, thus revealing a fundamental scaling law for the Lehmann effect of nematic and cholesteric twisted-bipolar droplets.

In situ measurements of twist and bend elastic constants in the oblique helicoidal cholesteric

Unique electro-optical properties of the oblique helicoidal cholesteric (Ch_{OH}) stem from its heliconical director structure. An applied electric field preserves the single-harmonic modulation of the director while tuning the Ch_{OH} period and the corresponding Bragg-peak wavelength within a broad spectral range. We use the response of Ch_{OH} to the electric field to measure the elastic constants of twist K_{22} and bend K_{33} directly in the cholesteric phase. The temperature dependencies of K_{22} and K_{33} allow us to determine the range of the electric tunability of the Ch_{OH} pitch and the heliconical angle. The data are important for understanding how molecular composition and chirality influence macroscopic elastic properties of the chiral liquid crystals and for the development of Ch_{OH}-based optical devices.

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.

Nematic Liquid Crystals

The discovery of liquid crystals (LCs) is dated to the year 1888, when Friedrich Reinitzer reported his observation of the double melting points of cholesterol benzoate [...]