Inversion Walls Research Articles

Nematic liquid crystals in lens shape geometry

ABSTRACTSessile nematic droplets provide a possibility to study the combined effect of anchoring at solid and gas interfaces. The combination of various alignments at the two surfaces and external fields results in a variety of director distribution schemes, ranging from a defect-free, almost homogeneous state to configurations with point-, line- and wall defects. This paper reviews recent results obtained on sessile droplets of uniaxial nematic liquid crystals, such as temperature-controlled anchoring transitions, formation and dynamics of Néel wall type metastable inversion walls in magnetic and/or electric fields. The structures of the recently discovered twist-bend and ferroelectric nematic liquid crystals will also be briefly discussed.

Lens-shaped nematic liquid crystal droplets with negative dielectric anisotropy in electric and magnetic fields

ABSTRACT Recently, plano-convex spherical lens-shaped liquid crystal (LC) sessile droplets with positive dielectric anisotropy were studied in magnetic and/or AC electric fields. Here, we present experimental observations in magnetic, AC electric, combined fields and theoretical considerations of plano-convex lens-shaped nematic LC droplets with negative dielectric anisotropy. In purely magnetic field, our results are the same as for positive materials, i.e. an inversion wall forms normal to the field that moves towards the periphery. In contrast to previous observations on nematic LCs with positive dielectric anisotropy, in electric fields applied normal to the base plane, we do not find any wall formation, but only a radial director structure with a central defect line along the field. Above a threshold electric field, the radial symmetry becomes twisted due to elastic constant anisotropy. Applying a magnetic field perpendicular to the vertical electric field, a twisted inversion wall forms together with a vertical central defect line. When the electric field is applied parallel to the base plane of the droplet, a homeotropic central region forms along the electric field. When this field is applied together with a magnetic field applied in the same direction, the homeotropic central region becomes perpendicular to the applied field.

Electric field induced buckling of inversion walls in lens-shape liquid crystal droplets

Lens-shaped liquid crystal samples have the potential for their use in tunable multifocal optical lenses. In previous observations (Phys. Rev. Research, 2, 023261 (2020) and J. Mol. Liq., 334, 116085 (2021)) plano-convex spherical lens-shaped liquid crystal sessile droplets were studied either in magnetic or AC electric fields, and the formation of an inversion wall (a one-dimensional soliton) moving toward the periphery was observed and explained.Here we present experimental observations of the structure of plano-convex lens-shaped nematic liquid crystal droplets subjected to simultaneous competing magnetic and electric fields. It is found that above a threshold electric field the initially straight defect wall buckles to form a zig-zag shape. This phenomenon is compared to those observed in sandwich cells with uniform thickness and the differences in their theoretical description are discussed.

Lens shape liquid crystals in electric fields

Experimental studies of the structure of plano-convex lens shaped liquid crystal droplets subjected to electric fields are presented. Similar to previous observations by Salamon et al. (2020) in DC magnetic fields, we found the formation of an inversion wall normal to AC electric fields. While at low frequencies the direction of the wall is stationary, at higher frequencies it turns toward the external electric field. In both cases, the defect wall is also swept toward the periphery of the drop, where it eventually disappears. The linear displacement of the electric field-induced defect wall could be described by an exponential time dependence without any fitting parameter. This combined with threshold for director deformation enables to determine both the bend elastic constant and the rotational viscosity using much less substance than existing techniques. The rotation of the defect wall at high frequencies is a result of the antiparallel orientation of the effective moment vector and the electric field due to the lower dielectric constant and higher electric conductivity of the defect wall than of those of the rest of the liquid crystal droplet. Uniform electric field-induced generation, rotation and linear movement of defect walls is a unique phenomenon in soft matters.

Research progress of the investigation of intrinsic and extrinsic origin of piezoelectric materials by X-ray diffraction

Ferroelectric/piezoelectric perovskites are an important class of functional material and have broad application prospects in commercial, industrial, military and other areas because of their high dielectric constants, high piezoelectric coefficients, and high electromechanical coupling coefficients. Their structures, applications, and physical mechanisms have been intensively studied in condensed matter physics and material science. The piezoelectric properties of ferroelectric materials mainly originate from the intrinsic field-induced lattice distortion and extrinsic domain inversion and domain wall motion. Therefore, the understanding of and the distinguishing between these mechanisms are important for ascertaining the origin of the high-piezoelectric properties and developing new functional materials. In this article, we review the research progress of technical means and methodology of analyzing the changes of crystal lattices and magnetic domains of materials under the action of an externally applied electric field through the high-energy synchrotron X-ray diffraction experiments. The techniques and analysis methods involved in the review cover the time-resolved X-ray diffraction, single/double-peak analysis, full-pattern refinement, center-of-mass calculation, and field-induced phase transformation analysis, which are used to study the intrinsic and extrinsic contributions to sample’s macroscopic properties. It is expected to provide the research methods, which fulfill the individual experimental requirements, and the technical support for the mechanism analysis of various piezoelectric materials through the introduction and review of various methods.

Peculiarities of Defective Formations in Heterophase Regions of the Thermotropic Phase Transitions in Liquid Crystals

AbstractA unique feature of phase transitions in liquid crystalline materials is the availability of heterophase regions. In such regions, the simultaneous coexistence of high temperature and low temperature phases takes place. Thermal mobility and temperature gradient in the heterophase regions lead to the appearance of various types of defective formations in the aligned and non‐aligned textures of liquid crystalline materials. In the present work, the morphologic, structural, and optical peculiarities of defective formations and non‐homogeneities, which appear in the heterophase regions of phase transitions, are investigated. The singular points, disclination, inversion walls, droplets, and confocal formations are objects of the investigations herein. The heterophase regions of the direct and reverse nematic ↔ isotropic liquid and smectic A ↔ isotropic liquid, smectic C ↔ isotropic liquid, and smectic C* ↔ isotropic liquid thermotropic phase transitions are created by the original method, that is, the capillary temperature wedge method. Crystallo‐optics and crystallo‐physics methods are used in this work.

Coupling phenomenon of anti-connected hybrid alignment nematics in thin cell: From submicron to nanoscale

Within Landau–de Gennes theory, we modeled a system consisting of two anti-connected hybrid alignment nematics (HAN). Our numerical simulation results indicated the occurrence of four states denoted as I, II, III and IV. The states undergo a structure transition as a function of the cell gap d. For d [Formula: see text] d0, where d0is the maximum value for the structure transition to occur, state I and II are bistable, whereas state III and IV are metastable and quadruply degenerate. We found that d0= 15[Formula: see text], where [Formula: see text] is the biaxial characteristic coherence length. When d[Formula: see text] [Formula: see text] d [Formula: see text] d0, where d[Formula: see text] = 7[Formula: see text] and d[Formula: see text] is the critical cell gap for order reconstruction (OR), continuous inversion walls appear both in state III and IV. We first found that the structure transition from inversion walls to defect walls occurs with decreasing d from the submicron to nanoscale. During this process, quadruply degenerate states in state III and IV transformed into doubly degenerate states. Meanwhile, states I and II remained bistable. When d = d[Formula: see text], all states reached a new type of OR pattern with two biaxial walls cross-linked with each other by a novel approach; i.e., from defect walls to OR.

Field effects on inversion walls in nematic films: A computer simulation study

Monte Carlo simulations of lattice spin models are employed to investigate the effects of an external field on the creation and evolution of inversion walls in nematic films with conical boundary conditions. The simulations, based on the well known Lebwohl-Lasher potential, allow to produce simulated polarized microscopy images which are followed during their evolution when the field is applied.

Theoretical analysis of the influence of flexoelectric effect on the defect site in nematic inversion walls**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374087, 11274088, and 11304074), the Natural Science Foundation of Hebei Province, China (Grant Nos. A2014202123 and A2016202282), the Research Project of Hebei Education Department, China (Grant Nos. QN2014130 and QN2015260), and the Key

Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-current electric field normal to the plane of the substrate is applied to the parallel aligned nematic liquid crystal cell with weak anchoring, the rotation of ±1 defects in the narrow inversion walls can be exhibited. The free energy of liquid crystal molecules around the +1 and –1 defect sites in the nematic inversion walls under the electric field was formulated and the electric-field-driven structural changes at the defect site characterized by polar and azimuthal angles of the local director were simulated. The results reveal that the deviation of azimuthal angle induced by flexoelectric effect are consistent with the switching of extinction brushes at the +1 and −1 defects obtained in the experiment conducted by Kumar et al.

对向列相液晶反转壁中+1缺陷处挠曲电效应的理论分析

Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-current electric field normal to the plane of the substrate is applied to the parallel aligned nematic liquid crystal cell with weak anchoring, the rotation of ±1 defects in the narrow inversion walls can be exhibited. The free energy of liquid crystal molecules around the +1 and-1 defect sites in the nematic inversion walls under the electric field was formulated and the electric-field-driven structural changes at the defect site characterized by polar and azimuthal angles of the local director were simulated. The results reveal that the deviation of azimuthal angle induced by flexoelectric effect are consistent with the switching of extinction brushes at the +1 and-1 defects obtained in the experiment conducted by Kumar et al.

Leaning-type polar smectic-C phase in a freely suspended bent-core liquid crystal film.

A rich variety of responsive behavior occurs in complex structured fluids due to their lower symmetries. On the other hand, fluid disorder tends to increase the symmetry of liquid crystal mesophases. Here, we report direct evidence for the existence of a mesophase with CS symmetry. The observations are based on optical studies of director inversion walls in freely suspended films in electric fields under obliquely incident light. This phase is distinguished by the polarization lying in the molecular tilt plane in freely suspended films. Such a low-symmetry polar fluid phase has been long predicted to occur in bent-core liquid crystals. The stability of this phase is attributed to the bent shape of the mesogens and dominating dispersion interactions.

Polar structure of disclination loops in nematic liquid crystals probed by second-harmonic-light scattering.

Angle-resolved, second-harmonic-light scattering (SHLS) measurements are reported for three different classes of thermotropic nematic liquid crystals (NLCs): polar and nonpolar rodlike compounds and a bent-core compound. Results revealing well-defined scattering peaks are interpreted in terms of the electric polarization induced by distortions of the nematic orientational field ("flexopolarity") associated with inversion wall defects, nonsingular disclinations, analogous to Neel walls in ferromagnets, that often exhibit a closed loop morphology in NLCs. Analysis of the SHLS patterns based on this model provides a "proof-of-concept" for a potentially useful method to probe the flexopolar properties of NLCs.

Properties of the static NMR response of a confined thin nematic film of 5CB-d2under crossed electric and magnetic fields: Theory and experiments

This work describes an investigation of the static (or quasistatic) nuclear magnetic resonance (NMR) response in a nematic liquid crystal confined between two planar conducting plates and subject to a magnetic field and an electric field produced by a difference of voltage applied on the plates. Deuterium NMR spectroscopy of 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) under these conditions has revealed a voltage dependent inhomogeneous director distribution for a particular narrow range of voltages and for a fixed magnetic field (that of the spectrometer). In the ideal setup the two plates are assumed to be rigorously parallel, so that a difference of voltage applied on the plates leads to a constant electric field normal to them. When the magnetic field is parallel to the plates (orthogonal geometry) there exists a threshold value of the electric field for which the effect of both fields exactly compensate; moreover, for stronger electric field the director aligns with the electric field while for weaker electric field the director aligns with the magnetic field. If there is a lack of parallelism between the two plates, the electric field becomes inhomogeneous so that it may be larger than the threshold value in some region of the sample and smaller in the remaining part of the sample. In that case the director will adopt essentially two orientations within the sample, namely, parallel or perpendicular to the magnetic field, and the position of the frontier between the two domains depends on the voltage. This feature is clearly shown by deuterium NMR spectra that exhibit a transfer of intensity between two quadrupolar doublets with increase in the applied voltage. The coexistence of two director populations occurs for a range of voltages that depends on the degree of nonparallelism; accordingly, an estimation of this range by NMR yields an experimental estimation of the lack of parallelism. A tiny tilt of the magnetic field (nonorthogonal geometry) entrains a notably different behavior since a single doublet with voltage dependent splitting is observed in this case. In a first stage (simple model) of this work, the main features observed for the orthogonal and nonorthogonal geometries are interpreted within the framework of Leslie-Ericksen theory by employing the concept of a single effective field replacing the two real fields. However, the spectra reveal an additional director distribution, especially for the orthogonal geometry, that cannot be interpreted by this simple approach. In a second stage (advanced model), these less clear features have been investigated by numerical simulations of a two-dimensional model which includes the effects of inversion walls and of the high relative dielectric anisotropy of 5CB.

Transitions between paraelectric and ferroelectric phases of bent-core smectic liquid crystals in the bulk and in thin freely suspended films

We report on the contrasting phase behavior of a bent-core liquid crystal with a large opening angle between the mesogenic units in the bulk and in freely suspended films. Second-harmonic generation experiments and direct observation of director inversion walls in films in an applied electric field reveal that the nonpolar smectic C phase observed in bulk samples becomes a ferroelectric "banana" phase in films, showing that a mesogen with a small steric moment can give a phase with polar order in freely suspended films even when the corresponding bulk phase is paraelectric.

Computer Simulations of Inversion Walls in Nematic Films

Monte Carlo simulations of lattice spin models are employed to investigate the creation of inversion walls in nematic films with specific boundary conditions. The simulations, based on the well known Lebwohl-Lasher potential, allow to produce simulated polarized microscopy images which are followed during their evolution and analysed for different film thicknesses.

Theory and numerical simulation of field-induced director dynamics in confined nematics investigated by nuclear magnetic resonance

We investigate the director reorientation in a nematic liquid crystal confined between two parallel plates and subjected to both a magnetic and an electric field. The permanent magnetic field is used first to align the director and, subsequently, to allow nuclear magnetic resonance (NMR) observation of the director response following the application of the electric field. For sufficiently strong electric fields, the director reorients and aligns parallel to the electric field. In this work we focus on the geometry where the electric and magnetic fields are orthogonal to each other. In this configuration the state of the system, immediately after applying the orthogonal electric field, is steady and unstable, at least in theory, since the director is assumed to be everywhere parallel to the magnetic field. In practice the real state of the system always deviates slightly from the perfect unstable steady state, which induces the start of the director reorientation toward the electric field. The nature and the characteristics of the initial deviation partially determine the reorientation process. In the classical approach, the small deviations from the ideal state are assumed to be due to thermal fluctuations, but this approach fails to account for some recent experimental results. For this reason we were led to investigate slightly non-uniform initial director configurations that are stable under the sole effect of the magnetic field but are sufficient to break the ideal unstable steady state created with the application of the orthogonal electric field. Such non-uniformities must be local or distributed over very small sample volumes, since their effects on the equilibrium NMR spectrum (before the application of the electric field) are not usually observed. In other words, we consider the presence of inversion walls in the bulk of the sample and local misalignments of the director on the boundary plates and investigate the effects of such non-uniformities on the response of the nematic to the application of the electric field, as observed by NMR. The model we propose, including such effects in parallel with thermal fluctuations, is able to account for the recently observed features of the field-induced director dynamics.

Synthesis and properties of novel thermotropic liquid crystalline copoly(ester-imide)s

Four novel copoly(ester-imide)s based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride, bis(trimellitic acid anhydride) phenyl ester and di-p-aminophenyl ester of dicarboxylic acids were synthesized via two-step method in order to investigate whether two imide mesogenic units with different conformation and polarity could control the formation of LC-phase. Polarizes light microscopy (PLM) and differential scanning calorimetry (DSC) have shown that three polymers formed the nematic phase with “thread schlieren texture” and a polymer formed the nematic phase with “Inversion walls texture”.

Peculiarities of inversion walls and singular points in oriented textures of nematic mesophase

AbstractIn this work the nature and peculiarities of the inversion walls and singular points, which can spontaneously arise or can be stimulated in the homeotropic and planar oriented textures of the thermotropic nematogens, have been investigated for large temperature interval. The interaction force and mean approach velocity of the singular points, having opposite optical sign, have been calculated. The optical mappings and sketches of the inversion walls and singularities, taking place in the oriented textures of the nematogens, are presented. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Physical Mechanism for Bistable Polar Switching of a Nematic Liquid Crystal in a Boundary Layer Configuration

We describe a physical mechanism for bistable polar switching of a nematic liquid crystal (LC) in a boundary layer (BL) configuration. The transition from a symmetrical horizontal state to one of two asymmetrical states originates from the flexoelectric coupling with an external electric field. It is found that the renormalization of the anchoring energy accounts for the appearance of the bistable polar switching in the BL configuration. Numerical simulations are carried out to understand how the flexoelectricity and the anchoring strength affect the nature of the polar switching in the BL configuration of a nematic LC cell. The bistability associated with an inversion wall disappears in a certain range of the flexoelectric coefficient.

Field-induced texture transitions in a bent-core nematic liquid crystal

It is demonstrated in electro-optic experiments that an external electric field of the order of 10;{5} V/m induces persisting texture transitions in a nematic phase formed by bent-core mesogens. The field-induced metastable state is identified by its optical and electric properties. After the field is switched off, the original and induced states can coexist in domains for about one hour in planar sandwich cells. During this time, the induced domains gradually shrink but they can be stabilized in moderate electric fields. The occurrence of similar domains in homeotropic cells suggests that the transition into a metastable biaxial state is observed. In the field-free planar ground state, the formation of inversion walls is observed inside the metastable domains.