Nematoelasticity of hybrid molecular-colloidal liquid crystals.

Colloidal rods immersed in a thermotropic liquid-crystalline solvent are at the basis of so-called hybrid liquid crystals, which are characterized by tunable nematic fluidity with symmetries ranging from conventional uniaxial nematic or antinematic to orthorhombic [Mundoor et al., Science 360, 768 (2018)SCIEAS0036-807510.1126/science.aap9359]. We provide a theoretical analysis of the elastic moduli of such systems by considering interactions between the individual rods with the embedding solvent through surface-anchoring forces, as well as steric and electrostatic interactions between the rods themselves. For uniaxial systems, the presence of colloidal rods generates a marked increase of the splay elasticity, which we found to be in quantitative agreement with experimental measurements. For orthorhombic hybrid liquid crystals, we provide estimates of all 12 elastic moduli and show that only a small subset of those elastic constants play a relevant role in describing the nematoelastic properties. The complexity and possibilities related to identifying the elastic moduli in experiments are briefly discussed.

Repulsion-attraction switching of nematic colloids formed by liquid crystal dispersions of polygonal prisms.

Self-assembly of colloidal particles due to elastic interactions in nematic liquid crystals promises tunable composite materials and can be guided by exploiting surface functionalization, geometric shape and topology, though these means of controlling self-assembly remain limited. Here, we realize low-symmetry achiral and chiral elastic colloids in the nematic liquid crystals using colloidal polygonal concave and convex prisms. We show that the controlled pinning of disclinations at the prism edges alters the symmetry of director distortions around the prisms and their orientation with respect to the far-field director. The controlled localization of the disclinations at the prism's edges significantly influences the anisotropy of the diffusion properties of prisms dispersed in liquid crystals and allows one to modify their self-assembly. We show that elastic interactions between polygonal prisms can be switched between repulsive and attractive just by controlled re-pinning the disclinations at different edges using laser tweezers. Our findings demonstrate that elastic interactions between colloidal particles dispersed in nematic liquid crystals are sensitive to the topologically equivalent but geometrically rich controlled configurations of the particle-induced defects.

INFLUENCE OF BETA-BLOCKERS AND IVABRADINE ON LONG-TERM PROGNOSIS OF PATIENTS WITH STABLE ANGINA.

The purpose of the study was to evaluate the influence of beta-blockers and ivabradine on long-term prognosis of patients with stable angina. 90 patients with stable angina that have been involved in present study along with nitrates, antiplatelet agents and statins received bisoprolol (Group I), carvedilol (Group II) and ivabradine (Group III). We analyzed the following indicators: patient adherence to treatment, cases of myocardial infarction and circulatory decompensation, which resulted in necessity of patient's hospitalization during the year of observation, calculating the probability of achieving a key prognostic endpoint by patient using the concept of the odds ratio and determination of important components in the progression of the disease. We concluded that in compared patients groups developed cardiovascular death and unstable angina with almost equal frequency. As for cases of myocardial infarction and the need for revascularization, their number was smaller in patients who in addition to standard treatment received ivabradine. Prognosis of patients with stable angina after 12 months of treatment primarily depends on their adherence to treatment, age, heart rate, presence of harmful habits, and markers of endothelial dysfunction, including endothelin-1 and homocysteine.

Surface alignment, anchoring transitions, optical properties, and topological defects in the nematic phase of thermotropic bent-core liquid crystal A131.

We study optical, structural, and surface anchoring properties of thermotropic nematic bent-core material A131. The focus is on the features associated with orientational order as the material has been reported to exhibit not only the usual uniaxial nematic but also the biaxial nematic phase. We demonstrate that A131 experiences a surface anchoring transition from a perpendicular to tilted alignment when the temperature decreases. The features of the tilted state are consistent with surface-induced birefringence associated with smectic layering near the surface and a molecular tilt that changes along the normal to the substrates. The surface-induced birefringence is reduced to zero by a modest electric field that establishes a uniform uniaxial nematic state. Both refractive and absorptive optical properties of A131 are consistent with the uniaxial order. We found no evidence of the "polycrystalline" biaxial behavior in the cells placed in crossed electric and magnetic fields. We observe stable topological point defects (boojums and hedgehogs) and nonsingular "escaped" disclinations pertinent only to the uniaxial order. Finally, freely suspended films of A131 show uniaxial nematic and smectic textures; a decrease in the film thickness expands the temperature range of stability of smectic textures, supporting the idea of surface-induced smectic layering. Our conclusion is that A131 features only a uniaxial nematic phase and that the apparent biaxiality is caused by subtle surface effects rather than by the bulk biaxial phase.

Undulations of lamellar liquid crystals in cells with finite surface anchoring near and well above the threshold.

We study the undulations instability, also known as the Helfrich-Hurault or layers buckling effect, in a cholesteric liquid crystal confined between two parallel plates and caused by an electric field applied along the normal to layers. The cholesteric pitch is much smaller than the cell thickness but sufficiently large for optical study. The three-dimensional patterns of the undulating layers in the bulk and at the surfaces of the cells are determined by fluorescence confocal polarizing microscopy. We demonstrate that the finite surface anchoring at the bounding plates plays a crucial role in the system behavior both near and well above the undulations threshold. The displacement of the layers immediately above the undulation threshold is much larger than the value expected from the theories that assume an infinitely strong surface anchoring. We describe the experimentally observed features by taking into account the finite surface anchoring at the bounding plates and using Lubensky-de Gennes coarse-grained elastic theory of cholesteric liquid crystals. Fitting the data allows us to determine the polar anchoring coefficient Wp and shows that Wp varies strongly with the type of substrates. As the applied field increases well above the threshold value Ec, the layers profile changes from sinusoidal to the sawtooth one. The periodicity of distortions increases through propagation of edge dislocations in the square lattice of the undulations pattern. At E approximately 1.9Ec a phenomenon is observed: the two-dimensional square lattice of undulations transforms into the one-dimensional periodic stripes. The stripes are formed by two sublattices of defect walls of parabolic shape. The main reason for the structure is again the finite surface anchoring, as the superposition of parabolic walls allows the layers to combine a significant tilt in the bulk of the cell with practically unperturbed orientation of layers near the bounding plates.

Switchable two-dimensional gratings based on field-induced layer undulations in cholesteric liquid crystals.

We propose switchable two-dimensional (2D) diffractive gratings with periodic refractive-index modulation arising from layer undulations in cholesteric liquid crystals. The cholesteric cell can be switched between two states: (1) flat layers of a planar cholesteric texture and (2) a square lattice of periodic director modulation associated with layer undulations that produces 2D diffraction patterns. The intensities of the diffraction maxima can be tuned by changing the applied field. The diffractive properties can be optimized for different wavelengths by appropriately choosing cholesteric pitch, cell thickness, and surface treatment.

Electric-field-induced nematic-cholesteric transition and three-dimensional director structures in homeotropic cells.

We study the phase diagram of director structures in cholesteric liquid crystals of negative dielectric anisotropy in homeotropic cells of thickness d which is smaller than the cholesteric pitch p. The basic control parameters are the frustration ratio d/p and the applied voltage U. Upon increasing U, the direct transition from completely unwound homeotropic structure to the translationally invariant configuration (TIC) with uniform in-plane twist is observed at small d/p < or = 0.5. Cholesteric fingers that can be either isolated or arranged periodically occur at 0.5 < or = d/p<1 and at the intermediate U between the homeotropic unwound and TIC structures. The phase boundaries are also shifted by (1) rubbing of homeotropic substrates that produces small deviations from the vertical alignment; (2) particles that become nucleation centers for cholesteric fingers; (3) voltage driving schemes. A novel reentrant behavior of TIC is observed in the rubbed cells with frustration ratios 0.6 < or = d/p < or = 0.75, which disappears with adding nucleation sites or using modulated voltages. In addition, fluorescence confocal polarizing microscopy (FCPM) allows us to directly and unambiguously determine the three-dimensional director structures. For the cells with strictly vertical alignment, FCPM confirms the director models of the vertical cross sections of four types of fingers previously either obtained by computer simulations or proposed using symmetry considerations. For rubbed homeotropic substrates, only two types of fingers are observed, which tend to align along the rubbing direction. Finally, the new means of control are of importance for potential applications of the cholesteric structures, such as switchable gratings based on periodically arranged fingers and eyewear with tunable transparency based on TIC.