Splay-bend elasticity of a nematic liquid crystal with T-shaped molecules

Mixing twist-bend and ferroelectric nematic liquid crystals

Twist-bend (N_{tb}) and ferroelectric (N_{F}) nematic liquid crystals exhibit several novel effects and new physical properties. Here, we report experimental studies on the phase diagram and some physical properties of binary mixtures of CB9CB and RM734 mesogens. Both N-N_{tb} and N-N_{F} phase transition temperatures and the corresponding enthalpies decrease significantly and, eventually, these transitions disappear at some intermediate compositions, stabilizing wide nematic phase (N). Temperature-dependent birefringence several degrees above the N-N_{tb} phase transition shows strong director tilt fluctuations. The critical range of the fluctuations increases with the nematic range and the critical exponent is consistent with the mean field. The spontaneous polarization of RM734 decreases drastically with the addition of CB9CB mesogen. The temperature dependence of the splay elastic constant of the mixtures' high-temperature nematic (N) phase strikingly differs from that of the pristine CB9CB and RM734 mesogens. The study shows that a small inclusion of either compound has a substantial effect on the phase diagram and physical properties.

Microscopic modelling of nematic elastic constants beyond Straley theory

Recent findings on various classes of nematics, whose microscopic structure differs from the prototypical rod-like shape, evidence unusual elastic properties, which challenge existing theories. Here we develop a theoretical and numerical methodology for the calculation of Frank elastic constants, accounting for the coupling between the molecular shape and each specific deformation mode. This is done in the framework of Onsager-Straley's second-virial theory, using a non-local form of the orientational distribution function. The comparison between two benchmark systems, a straight and a bent rod, allows us to illustrate the distinct features of this approach, which include additional order parameters induced by the deformation and, related to this, an ideal contribution to the deformation free energy. Then, using a simple system that can be seen as a minimalist model of liquid crystal trimers, we discuss the impact of different molecular conformations on elastic constants.

Whispering gallery mode lasing in mesomorphic liquid crystal microdroplets

In recent years, investigation on the non-display applications of liquid crystals has increased considerably. One of the emerging applications is whispering gallery mode (WGM) lasing. Here, we report experimental studies on the morphology and WGM lasing in nematic (N), smectic-A (SmA) and smectic-C (SmC) microdroplets dispersed in a highly transparent and low refractive index perfluopolymer. The mesomorphic microdroplets, obtained by varying the temperature, exhibit radial director configuration. The SmA microdroplets are found to be highly stable and robust against mechanical stress compared to the N and SmC microdroplets. We study lasing properties such as intensity, threshold pump energy and linewidth, and show that overall the SmA microdroplets are superior to the N and SmC microdroplets. The experimental results are discussed based on the orientation of the dye molecules, director fluctuations and tilting at the interface.

Room-Temperature Oligomeric Discotic Nematic Liquid Crystals over a Wide Temperature Range: Structure-Property Relationships

The design and synthesis of three room-temperature discotic nematic (N_D ) liquid crystals (LCs) is presented. The LC consists of an azobenzene core attached to which are four pentaalkynylbenzene (PA) units through flexible alkyl spacers. The presence of a short azo linking group provides more disorder in the system, thereby reducing the packing efficiency among the PA units and resulting into the formation of a room-temperature N_D phase over a wide temperature range. Dielectric constant and birefringence studies were performed to gain further insights into the physical properties of the mesophase.

The nematic phases of bent-core liquid crystals

Over the last ten years, the nematic phases of liquid crystals formed from bent-core structures have provoked considerable research because of their remarkable properties. This Minireview summarises some recent measurements of the physical properties of these systems, as well as describing some new data. We concentrate on oxadiazole-based materials as exemplars of this class of nematogens, but also describe some other bent-core systems. The influence of molecular structure on the stability of the nematic phase is described, together with progress in reducing the nematic transition temperatures by modifications to the molecular structure. The physical properties of bent-core nematic materials have proven difficult to study, but patterns are emerging regarding their optical and dielectric properties. Recent breakthroughs in understanding the elastic and flexoelectric behaviour are summarised. Finally, some exemplars of unusual electric field behaviour are described.

Critical behavior of a nonpolar smectogen from high-resolution birefringence measurements

We report high-sensitivity and high-temperature resolution experimental data for the temperature dependence of the optical birefringence of a nonpolar monolayer smectogen 4-butyloxyphenyl-4'-decyloxybenzoate (10[over ¯].O.4[over ¯]) liquid crystal by using a rotating-analyzer technique. The birefringence data cover nematic and smectic-A phases of the 10[over ¯].O.4[over ¯] compound. The birefringence data are used to probe the temperature behavior of the nematic order parameter S(T) in the vicinity of both the nematic-isotropic (N-I) and the nematic-smectic-A (N-SmA) transitions. For the N-I transition, from the data sufficiently far away from the smectic-A phase, the average value of the critical exponent β describing the limiting behavior of S(T) is found to be 0.2507±0.0010, which is in accordance with the so-called tricritical hypothesis, which predicts β=0.25 and excludes higher theoretical values. The critical behavior of S(T) at the N-I transition is discussed in detail by comparing our results with the latest reports in the literature and we conclude that by comparing with the previously reported results, the isotropic internal field assumption by the Vuks-Chandrasekhar-Madhusudana model is adequate to extract the critical behavior of S(T) from the optical birefringence data. We observe that there is no discontinuous behavior in the optical birefringence, signaling the second-order nature of the N-SmA transition. The effect of the coupling between the nematic and smectic-A order parameters on the optical birefringence near the N-SmA transition is also discussed. In a temperature range of about 4K above and below the N-SmA transition, the pretransitional evidence for the N-SmA coupling have been detected. From the analysis of the optical birefringence data above and below the N-SmA transition by means of various fitting expressions we test the validity of the scaling relation λ=1-α between the critical exponent λ describing the limiting behavior of the nematic order parameter and the specific heat capacity exponent α. We then show that the temperature derivative of the nematic order parameter S(T) near T(NA) exhibits the same power-law divergence as the specific heat capacity with an effective critical exponent of 0.2303±0.0035.