Comment on "structure of smectic defect cores: X-ray study of 8CB liquid crystal ultrathin films".
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Comment↗
Biomedical subjects
O D Lavrentovich
Publications and source records attributed to O D Lavrentovich.
Three-dimensional imaging of dielectric patterns in electrohydrodynamic convection of a nematic liquid crystal.
The transition from surface to bulk normal dielectric rolls in a nematic liquid crystal is imaged by fluorescence confocal polarizing microscopy. The three-dimensional director structure and the liquid flow are scanned in both the layer plane and the transverse plane. Two systems of small-scale convective flow are formed, one at each electrode. Strong anchoring makes director oscillations difficult and charges accumulate by the Carr-Helfrich mechanism. The middle region is a structureless convection where the director oscillates with the frequency of the applied voltage. The small-scale flow eventually fills the cell from one electrode to the other as one system of thin and elongated rolls. The true dielectric mode is not a director pattern, rather a surface flow instability.
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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.
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Lyotropic liquid crystal as a real-time detector of microbial immune complexes.
AIMS: To design a simple method for the detection of microbe-immune complexes exploiting the optical and elastic properties of a biocompatible liquid crystalline material. METHODS AND RESULTS: Aqueous solution of disodium cromoglycate (DSCG), a lyotropic chromonic liquid crystal (LCLC), was aligned in a glass cell so as to be optically dark in polarized light. Immune complexes of at least three to four organisms altered the DSCG alignment such that polarized light was subsequently transmitted to reveal the presence of pathogens as optically bright regions around the immune complexes. CONCLUSIONS: This work describes the first method to detect viable micro-organisms in real time using LCLC. SIGNIFICANCE AND IMPACT OF THE STUDY: This technique provides a powerful tool for the detection of microbes in minutes, exploiting the optical and elastic properties of LC.
Antibodies, Bacterial↗
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.
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Optical characterization of the nematic lyotropic chromonic liquid crystals: light absorption, birefringence, and scalar order parameter.
We report on the optical properties of the nematic (N) phase formed by lyotropic chromonic liquid crystals (LCLCs) in well aligned planar samples. LCLCs belong to a broad class of materials formed by one-dimensional molecular self-assembly and are similar to other systems such as "living polymers" and "wormlike micelles." We study three water soluble LCLC forming materials: disodium chromoglycate, a derivative of indanthrone called Blue 27, and a derivative of perylene called Violet 20. The individual molecules have a planklike shape and assemble into rodlike aggregates that form the phase once the concentration exceeds about 0.1 M. The uniform surface alignment of the N phase is achieved by buffed polyimide layers. According to the light absorption anisotropy data, the molecular planes are on average perpendicular to the aggregate axes and thus to the nematic director. We determined the birefringence of these materials in the N and biphasic N-isotropic (I) regions and found it to be negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic low-molecular-weight nematic materials. In the absorbing materials Blue 27 and Violet 20, the wavelength dependence of birefringence is nonmonotonic because of the effect of anomalous dispersion near the absorption bands. We describe positive and negative tactoids formed as the nuclei of the new phase in the biphasic N-I region (which is wide in all three materials studied). Finally, we determined the scalar order parameter of the phase of Blue 27 and found it to be relatively high, in the range 0.72-0.79, which puts the finding into the domain of general validity of the Onsager model. However, the observed temperature dependence of the scalar order parameter points to the importance of factors not accounted for in the athermal Onsager model, such as interaggregate interactions and the temperature dependence of the aggregate length.
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Elasticity-mediated self-organization and colloidal interactions of solid spheres with tangential anchoring in a nematic liquid crystal.
Using laser tweezers, we study colloidal interactions of solid microspheres in the nematic bulk caused by elastic distortions around the particles with tangential surface anchoring. The interactions overcome the Brownian motion when the interparticle separation r-->p is less than 3 particle diameters. The particles attract when the angle theta between r-->p and the uniform far-field director n0 is between 0 degrees and approximately 70 degrees and repel when 75 degrees <or approximately theta<or=90 degrees. The particles aggregate in chains directed at approximately 30 degrees to n0 and, at higher concentrations, form complex kinetically trapped structures.
Colloids↗
Electrorotation of colloidal particles in liquid crystals.
We present the first observations of dc electric-field-induced rotational motion of finite particles in liquid crystals. We show that the electrorotation is essentially identical to the well-known Quincke rotation, which in liquid crystals triggers an additional translational motion at higher fields. In the smectic phase the translational motion is confined to the two-dimensional geometry of smectic layers, in contrast to the isotropic and nematic phases, where the particles can move in all three dimensions. We demonstrate that by a proper analysis of the electrorotation, one can determine the in-plane viscosity of smectic liquid crystals. This method needs only a small amount of material, does not require uniform alignment over large areas, and enables probing rheological properties locally.
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Dielectric torque and orientation dynamics of liquid crystals with dielectric dispersion.
We demonstrate that the finite rate of dielectric relaxation in liquid crystals which has been ignored previously causes profound effects in the fast dielectric reorientation of the director. We propose a theory of dielectric response in which the electric displacement depends not only on the present (as in the standard theory) but also on the past values of electric field and director. We design an experiment with a dual-frequency nematic in which the standard "instantaneous" model and our model predict effects of opposite signs; the experimental data support the latter model.
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Selective imaging of 3D director fields and study of defects in biaxial smectic A liquid crystals.
We report on the selective imaging of different director fields in a biaxial smectic A (SmAb) liquid crystal using Fluorescence Confocal Polarizing Microscopy (FCPM) and Polarizing Microscopy (PM). The patterns of two directors, namely the director n(a) perpendicular to the lamellae and the director n(b) in their planes are visualized by doping the liquid crystal with two fluorescent dyes with different orientation of the transition dipoles with respect to the lamellar matrix. The properties of defects such as disclinations and focal conic domains (FCDs) are consistent with the non-polar D2h-symmetry of the SmA(b) mesophase in the studied mixture of bent-core and rod-like molecules: (1) majority of defects in the director n(b) are half-integer "+/-1/2" disclinations; (2) the integer-strength "+/-1" defects tend to split into the "+/-1/2" disclinations. We compare the vertical cross-sections of the "+/-1" disclinations in the field in SmA(b) and uniaxial nematic samples. In SmA(b), the "+/-1" disclinations do not escape into the third dimension, while in the nematic samples with Schlieren textures they do despite the surface anchoring at the plates; the experimentally determined director field around the escaped disclination capped by a pair of surface point defects--boojums matches the one predicted recently [C. Chiccoli et al., Phys. Rev. E 66, 030701 (2002)]. The FCD structure in SmA(b) is similar to that in SmA and SmC in terms of the normal to the layers but differs significantly in terms of the director n(b) field parallel to the smectic layers. The FCDs in SmA(b) can be associated with topologically non-trivial configurations of n(b) in the surrounding matrix that are equivalent to the disclination lines.
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Real-time microbe detection based on director distortions around growing immune complexes in lyotropic chromonic liquid crystals.
We describe director distortions in the nematic liquid crystal (LC) caused by a spherical particle with tangential surface orientation of the director and show that light transmittance through the distorted region is a steep function of the particle's size. The effect allows us to propose a real-time microbial sensor based on a nontoxic lyotropic chromonic LC (LCLC) that detects and amplifies the presence of immune complexes. A cassette is filled with LCLC, antibody, and antigen-bearing particles. Small and isolated particles cause no macroscopic distortions of the LCLC. Upon antibody-antigen binding, the growing immune complexes distort the director and cause detectable optical transmittance between crossed polarizers.
Antigen-Antibody Complex↗
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.
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Pretransitional fluctuations in the isotropic phase of a lyotropic chromonic liquid crystal.
We have studied isotropic-to-nematic pretransitional fluctuations in an aqueous solution of disodium cromoglycate (cromolyn) by static and dynamic light scattering. Cromolyn is a representative of lyotropic chromonic liquid crystals with building units being elongated rods formed by aggregates of disk-like molecules. By combining light-scattering and viscosity measurements we have determined the correlation length and relaxation time of the orientational order-parameter fluctuations and estimated the size of the cromolyn aggregates. The pretransitional behavior of light scattering does not completely follow the classic Landau-de Gennes model. This feature is most probably associated with the variable length of cromolyn aggregates. We have observed a dramatic increase of the shear viscosity near the transition to the nematic phase, the fact which correlates with the idea of growing supramolecular aggregates. The steep temperature dependence of the viscosity is accompanied by a practically temperature-independent translational diffusion coefficient.
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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.
Adsorption↗
Comment on "self-organized periodic photonic structure in a nonchiral liquid crystal".
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Comment↗
Anchoring-mediated interaction of edge dislocations with bounding surfaces in confined cholesteric liquid crystals.
We employ the fluorescent confocal polarizing microscopy to image edge dislocations in cholesteric liquid crystals. Surface anchoring at the bounding plates determines the structure and behavior of defects. Two types of plates set in-plane director orientation but differ in the type of associated anchoring potentials. Plates with strong polar anchoring and nonzero azimuthal anchoring repel the dislocations, while plates with weak polar anchoring and no azimuthal anchoring allow the dislocations to escape through the boundary. To explain the results, we propose a coarse-grained model of cholesteric anchoring.
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Three-dimensional director structures of defects in Grandjean-Cano wedges of cholesteric liquid crystals studied by fluorescence confocal polarizing microscopy.
We use a nondestructive technique of fluorescence confocal polarizing microscopy to visualize three-dimensional director patterns of defects in Grandjean-Cano wedges filled with a cholesteric liquid crystal of pitch p=5 microm. Strong surface anchoring of the director causes a stable lattice of dislocations in the bulk. Optical slicing in the vertical cross sections of the wedges allows us to establish the detailed structure of dislocations and their kinks. Dislocations of Burgers vector b=p/2 are located in the thin part of the sample, very close to the bisector plane. Their cores are split into a pair of tau(-1/2) and lambda(+1/2) disclinations. Pairs of lambda(-1/2) and tau(+1/2) disclinations are observed when the b=p/2 dislocation forms a kink. The kinks along the b=p/2 dislocations change the level of dislocations by +/-p/4 and +/-p/2; these kinks are confined to the glide plane and are very long, (5-10) p. Above some critical thickness h(c) of the wedge sample, the dislocations are of Burgers vector b=p. They are often found away from the bisector plane. The core of b=p dislocations is split into a pair of nonsingular lambda(-1/2) and lambda(+1/2) disclinations. The kinks along the b=p dislocation are of a typical size p and form cusps in the direction perpendicular to the glide plane. At the cusp, lambda(-1/2) and lambda(+1/2) disclinations interchange ends. Other defect structures inlude "Lehmann clusters," i.e., dislocations of zero Burgers vector formed by two lambda(-1/2) and two lambda(+1/2) disclinations and dislocations of nonzero Burgers vector with a core split into more than two disclinations. We employ the coarse-grained Lubensky-de Gennes model of the cholesteric phase to describe some of the observed features. We calculate the elastic energy of a dislocation away from the core, estimate the energy of the core split into disclinations of different types, study the effect of finite sample thickness on the dislocations energy, and calculate the Peach-Koehler elastic forces that occur when a dislocation is shifted from its equilibrium position. Balance of the dilation/compression energy in the wedge and the energy of dislocations defines the value of h(c) and allows to estimate the core energy of the dislocations. Finally, we consider the Peierls-Nabarro mechanisms hindering glide of dislocations across the cholesteric layers. Because of the split disclination character of the core, glide is difficult as compared to climb, especially for b=p dislocations.
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Topological defects in schlieren textures of biaxial and uniaxial nematics.
Monte Carlo and theoretical studies of thin 3D films of biaxial and uniaxial nematics with tangential boundary conditions show distinct differences in structure and evolution of topological defects. In the uniaxial films, defects of strength k=+/-1 are point defects that bear no bulk singularity and disappear by annihilation with each other. In the biaxial films, k=+/-1 defects are true singular bulk disclinations that split into pairs of k=+/-1/2 lines; the latter disappear by annihilation processes of the type +1/2-1/2=0. These observed differences are of relevance for the current debate on the existence of biaxial phases.
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