V. G. Nazarenko

Ordered Droplet Structures at the Liquid Crystal Surface and Elastic-Capillary Colloidal Interactions

We demonstrate a variety of ordered patterns, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC). The surface placement introduces a new type of particle interaction as compared to particles entirely in the LC bulk. Namely, director deformations caused by the particles lead to distortions of the interface and thus to capillary attraction. The elastic-capillary coupling is strong enough to remain relevant even at the micron-scale when its buoyancy-capillary counterpart becomes irrelevant.

Effect of collective molecular reorientations on Brownian motion of colloids in nematic liquid crystal.

Confusing Colloids in Liquid Crystals In a simple fluid, particle diffusion such as the motion of colloidal particles shows a change in the mean squared displacement that is proportional with time. Within a nematic liquid crystal, diffusion of the molecules may show anisotropic behavior. Turiv et al. (p. 1351; see the Perspective by Abbott) asked what happens to colloidal particles in a nematic liquid crystal. At short times, anomalous diffusion was observed with motion both slower and faster than the long-term behavior, indicative of a complex coupling between the diffusive motion of the colloidal particles and the motion of the liquid crystal molecules. Colloidal particles in a nematic liquid crystal experience anomalous and anisotropic Brownian motion. [Also see Perspective by Abbott] In the simplest realization of Brownian motion, a colloidal sphere moves randomly in an isotropic fluid; its mean squared displacement (MSD) grows linearly with time τ. Brownian motion in an orientationally ordered fluid—a nematic—is anisotropic, with the MSD being larger along the axis of molecular orientation, called the director. We found that at short time scales, the anisotropic diffusion in a nematic becomes anomalous, with the MSD growing slower or faster than τ; these states are respectively termed subdiffusion and superdiffusion. The anomalous diffusion occurs at time scales that correspond to the relaxation times of director deformations around the sphere. Once the nematic melts, the diffusion becomes normal and isotropic. Our experiment shows that the deformations and fluctuations of long-range orientational order profoundly influence diffusive regimes.

Surface Alignment and Anchoring Transitions in Nematic Lyotropic Chromonic Liquid Crystal

The surface alignment of lyotropic chromonic liquid crystals can not only be planar (tangential) but also homeotropic, with self-assembled aggregates perpendicular to the substrate, as demonstrated by mapping optical retardation and by three-dimensional imaging of the director field. With time, the homeotropic nematic undergoes a transition into a tangential state. The anchoring transition is discontinuous and can be described by a double-well anchoring potential with two minima corresponding to tangential and homeotropic orientation.

Design of 2D Binary Colloidal Crystals in a Nematic Liquid Crystal

In this paper, we examine directed self-assembly in a 2D binary system of dipolar and quadrupolar colloidal particles with normal surface boundary conditions, dispersed in the nematic liquid crystal. Using the laser tweezers, we assembled a large variety of stable 2D colloidal crystal structures. In all analyzed structures, the particles, their surface treatment and the cell conditions were the same, which gives us the ability to systematically follow the evolution of colloidal assembly when many particles are present. We present an analogy between molecular self-assembly and organization of colloidal microspheres in liquid crystalline medium to extend the strategy for designing colloidal crystalline structures of different level of complexity.

Lyotropic chromonic liquid crystal semiconductors for water-solution processable organic electronics

We propose lyotropic chromonic liquid crystals (LCLCs) as a distinct class of materials for organic electronics. In water, the chromonic molecules stack on top of each other into elongated aggregates that form orientationally ordered phases. The aligned aggregated structure is preserved when the material is deposited onto a substrate and dried. The dried LCLC films show a strongly anisotropic electric conductivity of semiconductor type. The field-effect carrier mobility measured along the molecular aggregates in unoptimized films of LCLC V20 is 0.03 cm^2 V^(-1) s^(-1). Easy processibility, low cost, and high mobility demonstrate the potential of LCLCs for microelectronic applications.

Hypothesis of Dye Aggregation in a Nematic Liquid Crystal: From Experiment to a Model of the Enhanced Light-Director Interaction

We report experimental and theoretical arguments showing that the effect of strong enhancement of the light-induced director reorientation in a nematic liquid crystal doped with anthraquinone dye molecules (Jánossy effect) can be related to their aggregation. The torque on the director is due to dependence of the entropy of the aggregate subsystem on the electric field vector relative to the director.

Nano-Architecture of Self-Assembled Monolayer and Multilayer Stacks of Lyotropic Chromonic Liquid Crystalline Dyes

ABSTRACT The mesomorphic state of the lyotropic chromonic liquid crystal (LCLC) materials allows one to prepare mono- and multimolecular thin films with a distinct feature of a long-range orientational order in the plane of the film that makes the dried LCLC films drastically different from the classic Langmuir-Blodgett surfactant films. The thin (1–10 nanometers) LCLC films are deposited by the electrostatic layer-by-layer technique, with alternating monolayers of LCLC and oppositely charged polyions. In this work, we report on a new scheme of electrostatic layer-by-layer deposition, in which the passive polyion layers are eliminated: The film is deposited by alternating positively and negatively charged homologs of a LCLC material with the same molecular core structure. The assembled dried films demonstrate long-range orientational order as evidenced by the measurements of absorption and dichroic ratio as a function of the angle between the deposition direction and polarization of probing light.

Chromonic nematic phase and scalar order parameter of indanthrone derivative with ionic additives

We investigate influence of different ionic additives on the phase behaviour and scalar order parameter of lyotropic chromonic nematic liquid crystals formed by the molecules representing derivatives of indanthrone. KI, (NH4)2SO4 and NaCl salts increase biphasic nematic region on the temperature-concentration phase diagram, whereas the scalar orientational order parameter is hardly sensitive to their presence. We suggest that these changes are attributed to increase in the aggregate length and polydispersity of the latter, while the interaction between aggregates remains unaffected. The Na2SO4 salt lowers anomalously the measured value of the scalar order parameter to 0.42, for which the nematic phase should not exist. We address this anomaly to modification of the shape of chromonic aggregates due to growing of side chains within the rod-like aggregates. The NH4Cl salt does not affect the phase diagram and the scalar order parameter of Blue 250 chromonematic. PACS: 78.20.Ci, 87.15.Mi, 78.20.Fm, 42.79.Kr, 61.30.Gd. UDC: 535.34

Aggregation of Anthraquinone Dye Molecules in a Nematic Liquid Crystal

We report an experimental evidence that molecules of anthraquinone dye aggregate in an anisotropic solvent, a nematic liquid crystal (NLC). We observe strong nonlinear dependencies of the dielectric permittivity and anisotropy of light absorption on the dye concentration c. Moreover, the data suggest that the aggregates are linear, but, in contrast to the isotropic case, their formation in a NLC undergoes two different concentration regimes with a well pronounced crossover. These effects are explained by the interplay between the aggregates’ lengths and their orientational ordering due to the interaction with the nematic director.

Alignment techniques for preparation of protein-containing surfactant nematic cells

We report on first successful alignment of a surfactant lyotropic nematic between rubbed glass substrates either clean or spin-coated with a polymer layer. Worsening of the alignment either at small number of rubbings (less than 60), at long time delay before filling the cell or when washing the rubbed substrates in deionized water, suggest that the rubbinginduced electrostatic charging of the substrate surface is the main alignment mechanism. Prepared surfonematic cells have good optical quality allowing us to perform optical characterization of the studied surfonematic. The surfonematic is optically negative with the weak light wavelengths dispersion. The absolute value of the birefringence is low: -(6x10-4). For the first time we report that hemoglobin molecules can be dissolved in the surfonematic matrix. Hemoglobin dissolved in well-aligned surfonematic matrix display polarization spectra which differ from its spectrum in water: 1) wide absorption band in the spectral range 540-640nm for hemoglobin in the surfonematic is different from that for water solutions of hemoglobin; 2) polarization spectra of hemoglobin in aligned surfonematic display weak dichroism leading to the estimation of the orientation order parameter for hemoglobin molecules to be about 10-2. These spectral features indicate that hemoglobin residues form anisotropic complexes with positively charged surfactant residues. At the studied concentration these complexes are weakly aligned by the surfonematic matrix but are not birefringent and do not affect the orientational order of the surfonematic matrix.