We measure the azimuthal anchoring strength and easy axis gliding of the nematic 5CB on weakly anisotropic substrate. We study the anchoring evolution at a time scale ranging from a few minutes up to a few months. We separate approximately the elastic and the viscous response and we obtain the gliding velocity as a function of the applied torque. We discuss the observed gliding times and nonlinear gliding viscosity.

Nematic twist-bend phases (N_TB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature N_TB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to N_TB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable N_TB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

The two main classes of liquid-crystal (LC) phases of rodlike molecules are nematics, where the rods align in the same direction (the nematic director $\mathbf{n}$), and smectics, where the rods not only are aligned but also form layers. The electro-optic effects in LC devices that are a backbone in today’s display industry mainly use the Fréedericksz transition, which is the bulk reorientation of a surface-anchored nematic by an electric field. Conventional (uniaxial) smectics do not present a Fréedericksz transition, because, due to their layered structure, the director reorientation would distort the layers, which would cost too much energy. In a worldwide ongoing effort to extend the variety of LC compounds suitable for applications in the display industry, bent-shaped molecules have recently raised much attention, since they present multiple new LC phases with unusual properties. In this paper, we report on a structural and electro-optic study of the LC phases of a bent-shaped dimer. On cooling from the isotropic liquid, this compound shows a usual nematic ($N$), a twist-bend nematic ($N_{\mathrm{TB}}$), and a biaxial smectic-$A$ phase ($\mathrm{Sm}{A}_b$). Quite surprisingly, contrary to usual smectics, $\mathrm{Sm}{A}_b$ presents a remarkable electro-optic response, with low ($<4\mathrm{V}$) voltage threshold, no reorganization of the smectic layers, and low ($<1\mathrm{ms}$) response time (i.e., 30 times faster than the $N$ phase at higher temperature). We interpret this unexpected electro-optic effect as a Fréedericksz transition affecting the secondary director $\mathbf{m}$ of the $\mathrm{Sm}{A}_b$, and we model it by analogy with the usual Fréedericksz transition of the $\mathbf{n}$ director of the uniaxial $N$ phase. Indeed, a Fréedericksz transition affecting only $\mathbf{m}$ in this biaxial fluid smectic does not alter its layered structure and costs little energy. From the point of view of applications, thanks to its low relaxation time, this “biaxial” Fréedericksz transition could be exploited in electro-optic devices that require fast switching.

Nematic liquid crystals possess only orientational order. In liquid crystal cells, their texture is mainly controlled by anchoring conditions and by the values of the Frank elastic constants. Given homogeneous boundary conditions, nematic liquid crystal cells tend to show homogeneous equilibrium textures, even if complex transient states are sometimes observed (under a sudden change in voltage, for example). Any large changes in the elastic constants, however, can significantly impact the cell arrangement and give rise to inhomogeneous textures. In this work, we studied the evolution of the textures of chiral nematic mixtures whose bend elastic constant, K₃₃, becomes very low near the transition to the twist-bend nematic phase. This low value results in the formation of voltage-induced periodic striped textures which are associated with a heliconical instability. We also demonstrated that their wavelength can be tuned by adjusting the voltage.