Structures In Nematic Liquid Crystals Research Articles

Structural, Optical, and Mechanical Insights into Cellulose Nanocrystal Chiral Nematic Film Engineering by Two Assembly Techniques.

Iridescent cellulose nanocrystal (CNC) films with chiral nematic nanostructures exhibit great potential in optical devices, sensors, painting, and anticounterfeiting applications. CNCs can assemble into a chiral nematic liquid crystal structure by evaporation-assisted self-assembly (EISA) and vacuum-assisted self-assembly (VASA) techniques. However, there is a lack of comprehensive examinations of their structure-property correlations, which are essential for fabricating materials with unique properties. In this work, we gained insights into the optical, mechanical, and structural differences of CNC films engineered using the two techniques. In contrast to the random self-assembly at the liquid-air interface in EISA, the continuous external pressure in the VASA process forces CNCs to assemble at the filter-liquid interface. This results in fewer defects in the interfaces between tactoids and highly ordered cholesteric phases. Owing to the distinct CNC assembly behaviors, the films prepared by these two methods show great differences in the nanostructure, microstructure, and macroscopic morphology. Consequently, the highly ordered cholesteric structure gives VASA-CNC films a more uniform structural color and enhanced mechanical performance. These fundamental understandings of the relationship of structure-property nanoengineering through various assembly techniques are essential for designing and constructing high-performance chiral iridescent CNC materials.

Iridescent transparent passive radiative cooling enabled by cellulose nanocrystal assemblies

Recently, passive daytime radiative cooling (PDRC) materials that reflect sunlight and transmit heat through atmospheric transparent windows to outer space without any energy consumption have attracted broad interest. However, most PDRC materials have the complicated structures, white or mirror-like appearance with poor transparency. Here, we reported a transparent iridescent PDRC coating material constructed by the evaporation-induced self-assembly (EISA) of cellulose nanocrystals (CNCs). The iridescent PDRC coating not only shows high light transparency (75 %) because the right-handed circularly polarized light can transmit the unique left-handed chiral nematic liquid crystal structure, but also shows high solar reflection efficiency (95.2 %) and infrared emission efficiency (96.5 %) that endows the coating with high radiative cooling efficiency (119 W/m2). More importantly, the PDRC coating can apply on polymethyl methacrylate (PMMA), polyethylene terephthalate (PET) film and textiles, and the substrate color has no negative impact for cooling. With the excellent radiative cooling efficiency, high transparency, facile preparation process and general applicability, the CNCs-based PDRC coating opens a new pathway toward the potential application of PDRC for environmental sustainability purpose.

Electrically Induced Photonic Structures in Cholesteric and Nematic Liquid Crystals

Electrically Induced Photonic Structures in Cholesteric and Nematic Liquid Crystals

Recent Progress on Photonic Cellulose Nanocrystal Films for Sensing Applications

Abstract: Cellulose nanocrystals (CNCs) have triggered considerable research interest in the last few years owing to their unique optical, biodegradation, and mechanical behavior. Herein, recent progress on the sensing application of photonic CNC films is summarized and discussed based on the analyses of the latest studies. We briefly introduce the three approaches for pre-paring CNCs: mechanical treatment, acid hydrolysis, and enzymatic hydrolysis, recapitulating their differences in preparation and properties. Then, when the aqueous suspension of cellu-lose nanocrystals (CNCs) reaches a specific concentration, it will self-assemble to form a left-handed nematic liquid crystal structure, and this structure can be maintained in films after water evaporation, which has strong photonic crystal properties. The periodic layered structure in the film interferes and diffracts with light, showing a rainbow color. Photonic CNC com-posites that combine CNCs and functional materials have good properties and broad prospects. Finally, we highlight the advanced applications of photonic CNC films, including mechanical sensing, thermal sensing, and humidity sensing. The prospects and ongoing challenges of pho-tonic CNC films were summarized.

Molecules structure and viscosity relationship of nematic liquid crystal and BPNN-QSAR model

Nematic liquid crystal materials designed for optics, microwave communication tuning, etc. need high response speed, which is related to the rotational viscosity and the birefringent index of the liquid crystal. In order to achieve a wide tuning range of phase modulation, the nematic liquid crystals often employ large π-electron conjugated systems and large polar groups to enhance the birefringence and dielectric anisotropy of the liquid crystal molecule, which, however, increases the viscosity of the liquid crystal material, deteriorating the response speed of the microwave device. Herein, we explore the viscosity of the nematic liquid crystal from the perspective of liquid crystal compound structure by testing the viscosity of our designed and synthesized forty-two different nematic liquid crystals by using a rotating rheometer at 25 ℃. To the best of our knowledge, the BPNN-QSAR quantitative structure-activity model between nematic liquid crystal molecular structure and viscosity is established for the first time. The correlation coefficient between the predicted value and the experimental value is <i>q</i><sup>2</sup> = 0.607 > 0.5, indicating that the model can be used to predict the viscosity performances of liquid crystal compounds. Besides, the molecular structure descriptors affecting the viscosity properties are explored. Based on the practical application and this model, seven liquid crystal molecules of two series with large birefringent index are designed and tested. The viscosity predicted by the BPNN model is smaller than that of the molecules of the same type and matches with the measured viscosity.

Minimization of the threshold voltage parameter of the co-doped ZnO doped liquid crystals by machine learning algorithms

This study aims to examine the influence of the co-doped semiconductor nanostructure (Al-Cu):ZnO on the electro-optical properties of the E7 coded pure nematic liquid crystal structures and minimize the threshold voltage of pure E7 liquid crystal. To determine the ideal concentration ratios of the materials for the minimum threshold voltage, we employed different machine learning algorithms. In this context, we first produced twelve composite structures through lab experimentation with different concentrations and created an experimental dataset for the machine learning algorithms. Next, the ideal concentration ratios were estimated using the AdaBoost algorithm, which has an R^2 of 96% on the experimental dataset. Finally, additional composite structures having the estimated concentration ratios were produced. The results show that, with the help of the employed machine learning algorithms, the threshold voltage of pure E7 liquid crystal was reduced by 19% via the (Al-Cu):ZnO doping.

Monte Carlo simulations of chiral and achiral nematic droplets: thermal quenches and role of the elastic constants

ABSTRACTWe used Monte Carlo simulations with an elastically anisotropic pairwise potential to simulate the equilibrium structure of chiral and achiral nematic liquid crystals in spherical confinement under perpendicular and tangential strong anchoring. The stable structures are obtained by a quenching process, where the temperature is a parameter of the system. We calculate and map the distribution of elastic energy corresponding to splay, twist, and bend. Furthermore, we study the effect of changing the ratio of the elastic constants for achiral and low chirality droplets and found a diverse scenario of possible structures. The method described here allows the introduction of temperature and elastic anisotropy naturally within its framework. It is simpler to implement and faster to execute than the Landau-de Gennes (LdG) method used for simulating liquid crystal droplets.

New quasiperiodic structures in nematic liquid crystals.

Liquid crystal molecules tend to align with each other, often forming regions of opposite alignment that meet at a boundary-topological defects. These often offer information on configuration of the liquid crystal molecules with competing constraints on their order. Here, we experimentally demonstrate a mechanism to generate topological defects in the form of spatially oscillatory domain walls in nematic liquid crystals. We initially orient the molecules perpendicular to the substrate (i.e. homeotropic alignment) and when a horizontal electric field is applied, domain walls that change their shape with time emerge. These walls form at low frequencies of the applied electric field and remain stable as the frequency increases. If the initial biasing field is at larger frequencies (kHz regime), the domain walls still form, but are not oscillatory. We develop a general theory to predict the three-dimensional liquid crystal director evolution in any two-dimensional varying field. This theory gives the time dependence for the domain walls and confirms that both the oscillatory and straight walls are stable.

The Nematic Chiral Liquid Crystal Structure of the Cardiac Myoarchitecture: Disclinations and Topological Singularities.

This is our second article devoted to the cardiac myoarchitecture considered as a nematic chiral liquid crystal (NCLC). While the first article focused on the myoarchitecture of the left ventricle (LV), this new article extends to the whole ventricular mass and introduces the concept of disclinations and topological singularities, which characterize the differences and relationships between the left and right ventricles (RV). At the level of the ventricular apices, we constantly observed a vortex shape at the LV apex, corresponding, in the terminology of liquid crystals, to a "+1 disclination"; we never observed this at the RV apex. At the level of the interventricular septum (IVS), we identified "-1/2 disclinations" at the anterior and posterior parts. During the perinatal period, there was a significant difference in their distribution, with more "-1/2 disclinations" in the posterior part of the IVS. After birth, concomitant to major physiological changes, the number of "-1/2 disclinations" significantly decreased, both in the anterior and posterior parts of the IVS. Finally, the description of the disclinations must be considered in any attempt to segment the whole ventricular mass, in biomechanical studies, and, more generally, for the characterization of myocardial remodeling.

Structure and optical properties of twist-bend nematic liquid crystals doped with chiral dopants.

Twist-bend nematic liquid crystals (N_{TB} LCs), although consisting of achiral molecules, possess a spontaneous conic helix. They have been intensively studied and utilized in many applications in recent years. Herein we add chiral molecules to N_{TB} LCs and study their effects on the structure of the conic helix. We observe that the system is in the regular chiral nematic phase at high temperature and is still in the twist-bend nematic phase at low temperature. The addition of the chiral molecules does not induce a twist of the conic helical axis. The main effect of the chiral molecules is increasing the cone angle of the conic helix. We show that the structural chirality parameters in the chiral nematic phase and the twist-bend chiral nematic phase can be calculated from the same intrinsic chirality parameter, which only depends on the molecular structure and concentrations of the chiral molecule. We also observe a pretransitional phenomenon that the helical pitch of the chiral nematic phase increases dramatically when temperature is decreased toward the chiral nematic to twist-bend nematic phase transition temperature.

The preparation of cellulose nanocrystal/1,3‐butylene glycol composite structural color films and humidity‐responsive

AbstractCellulose nanocrystals (CNCs) can self‐assemble in an aqueous solution to form a chiral nematic liquid crystal structure, which is maintained in the subsequent solid iridescent film and presents a unique structural color. In this study, in order to explore the effect of ultrasound on the optical properties of CNC, CNC films with a chiral nematic helix structure and different structural colors were prepared by ultrasonic‐assisted dispersion and natural static evaporation. Results showed that prolonging the ultrasonic duration significantly increased layer pitches and red‐shifted the color of CNC films, likely due to increased steric hindrance and a larger electric double layer. On this basis, CNC/1,3‐butylene glycol (BG) composite films were prepared with uniform and structural color. Results showed that BG was incorporated into CNCs most likely through hydrogen bonds. Moreover, the presence of BG increased the layer pitches and red‐shifted the structural color of CNC/BG composite films, which indicates that structural color of the film has a good stability response to humidity. On the whole, our work provides a new point of view in understanding the role of ultrasound and small‐molecule additives in the preparation of CNC‐based films.

Tomographic measurement of dielectric tensors at optical frequency.

The dielectric tensor is a physical descriptor of fundamental light-matter interactions, characterizing anisotropic materials with principal refractive indices and optic axes. Despite its importance in scientific and industrial applications ranging from material science to soft matter physics, the direct measurement of the three-dimensional dielectric tensor has been limited by the vectorial and inhomogeneous nature of light scattering from anisotropic materials. Here, we present a dielectric tensor tomographic approach to directly measure dielectric tensors of anisotropic structures including the spatial variations of principal refractive indices and directors. The anisotropic structure is illuminated with a polarized plane wave with various angles and polarization states. Then, the scattered fields are holographically measured and converted into vectorial diffracted field components. Finally, by inversely solving a vectorial wave equation, the three-dimensional dielectric tensor is reconstructed. Using this approach, we demonstrate quantitative tomographic measurements of various nematic liquid-crystal structures and their fast three-dimensional non-equilibrium dynamics.

Design rules for 2D field mediated assembly of different shaped colloids into diverse microstructures.

Assembling different shaped particles into ordered microstructures is an open challenge in creating multifunctional particle-based materials and devices. Here, we report the two-dimensional (2D) AC electric field mediated assembly of different shaped colloidal particles into amorphous, liquid crystalline, and crystalline microstructures. Particle shapes investigated include disks, ellipses, squares, and rectangles, which show how systematic variations in anisotropy and corner curvature determine the number and type of resulting microstructures. AC electric fields induce dipolar interactions to control particle positional and orientational order. Microstructural states are determined via particle tracking to compute order parameters, which agree with computer simulations and show how particle packing and dipolar interactions together produce each structure. Results demonstrate how choice of particle shape and field conditions enable kinetically viable routes to assemble nematic, tetratic, and smectic liquid crystal structures as well as crystals with stretched 4- and 6-fold symmetry. Results show it is possible to assemble all corresponding hard particle phases, but also show how dipolar interactions influence and produce additional microstructures. Our findings provide design rules for the assembly of diverse microstructures of different shaped particles in AC electric fields, which could enable scalable and reconfigurable particle-based materials, displays, and printing technologies.

Coarse-grained theory for motion of solitons and skyrmions in liquid crystals.

Recent experiments have found that applied electric fields can induce motion of skyrmions in chiral nematic liquid crystals. To understand the magnitude and direction of the induced motion, we develop a coarse-grained approach to describe dynamics of skyrmions, similar to our group's previous work on the dynamics of disclinations. In this approach, we represent a localized excitation in terms of a few macroscopic degrees of freedom, including the position of the excitation and the orientation of the background director. We then derive the Rayleigh dissipation function, and hence the equations of motion, in terms of these macroscopic variables. We demonstrate this theoretical approach for 1D motion of a sine-Gordon soliton, and then extend it to 2D motion of a skyrmion. Our results show that skyrmions move in a direction perpendicular to the induced tilt of the background director. When the applied field is removed, skyrmions move in the opposite direction but not with equal magnitude, and hence the overall motion may be rectified.

Hysteresis and Fréedericksz thresholds for twisted states in chiral nematic liquid crystals: Minimum-energy path approach

We study minimum-energy pathways (MEPs) between the branches of metastable helical structures in chiral nematic liquid crystals (CNLCs) subjected to the electric field applied across the cell. By performing stability analysis we have found that, for the branches with non-vanishing half-turn number, the threshold (critical) voltage of the Fréedericksz transition is an increasing function of the free twisting wave number. The curves for the threshold voltage depend on the elastic anisotropy and determine the zero-field critical free twisting number where the director out-of-plane fluctuations destabilize the CNLC helix. For each MEP passing through a first order saddle point we have computed the energy barrier as the energy difference between the saddle-point and the initial structures at different values of the applied field. In our calculations, where the initial approximation for a MEP at the next step was determined by the MEP obtained at the previous step, the electric field dependence of the energy barrier is found to exhibit the hysteresis. This is the hysteresis of electrically driven transition of the saddle-point configuration between the planar and the tilted structures involving out-of-plane director deformations. It turned out that, by contrast to the second-order Fréedericksz transition, this transition is first order and we have studied how it depends on the zenithal anchoring energy strength.

Patterned surface alignment to create complex three-dimensional nematic and chiral nematic liquid crystal structures

ABSTRACT Combining liquid crystals (LCs) with well-designed anchoring patterns at the substrates offers tremendous potential for the development of functional electro-optic devices or stimuli-responsive actuators. Photo-alignment techniques nowadays allow an almost arbitrary control over the surface anchoring and this flexibility is used to design highly efficient flat optical LC components with different functionalities. Part of this research, dealing with nematic and chiral nematic LC between substrates with patterned azimuthal anchoring, is reviewed here. The focus is on understanding the self-assembly of complex structures, steered by an interplay between surface anchoring and LC elasticity. Additional insight into the LC bulk behaviour is obtained by comparing experimental results with numerical simulations of the director configuration. Periodic anchoring patterns with azimuthal rotation at the top and bottom substrate are studied, as well as ring-shaped alignment patterns with a 180° or 360° azimuthal rotation in a confined region in space. Different combinations of anchoring patterns at the top and bottom substrates are investigated and in addition to nematic liquid crystal (NLC), also short and long-pitch chiral nematic liquid crystal (CLC) is considered.

Multiple Twisted Chiral Nematic Structures in Cylindrical Confinement

In this article, we theoretically and numerically study the chirality and saddle-splay elastic constant ( K 24 ) -enabled stability of multiple twist-like nematic liquid crystal (LC) structures in cylindrical confinement. We focus on the so-called radially z-twisted (RZT) and radially twisted (RT) configurations, which simultaneously exhibit twists in different spatial directions. We express the free energies of the structures in terms of dimensionless wave vectors, which characterise the structures and play the roles of order parameters. The impact of different confinement anchoring conditions is explored. A simple Landau-type analysis provides an insight into how different model parameters influence the stability of structures. We determine conditions for which the structures are stable in chiral and also nonchiral LCs. In particular, we find that the RZT structure could exhibit macroscopic chirality inversion upon varying the relevant parameters. This phenomenon could be exploited for the measurement of K 24 .

On the Problem of the Mechanism of Binary Acoustic Action on a Homeotropic Structure of Nematic Liquid Crystals

A universal model of the distortion mechanism of a homeotropic nematic liquid crystal layer under joint action of the periodic shear and compression has been verified and experimentally confirmed for the first time. The model takes into account all physical factors responsible for this phenomenon and adequately describes it in a wider frequency range including ultrasonic frequencies than that for which such a description was obtained previously. The relationship between the optical response of the mesophase layer and the amplitudes and frequencies of the binary action is found.

Active distant binocular filter with complementary nematic liquid crystal structures for observation of millisecond stereoscopic images

The fast active distant binocular filter (DBF) with two complementary nematic liquid crystal (NLC) π-structures for observing the frame-sequential stereoscopic images is described. The DBF is characterized by steep front and rear edges of the pulse optical response defined by the fast (about 50 to 200 μs) rise time of the π-structures. Their relatively long decay time (about 2 ms) during relaxation processes does not affect the transition times of the DBF optical response. The corresponding long phase shifts in two π-structures are mutually compensated with use of the developed algorithms of electronic control. The DBF optical characteristics are studied theoretically and experimentally for selection of white light fluxes with duration in several milliseconds typical for the common stereoscopic system. The minimum duration of the selected view images can be a fraction of a millisecond. When compared stereo glasses with conventional NLC shutters, the described active DBF allows to provide the increased optical efficiency and absence of mechanical stress on the face (head) of the observer to improve the comfort of stereo image observation.

Nematic and Cholesteric Liquid Crystal Structures in Cells with Tangential-Conical Boundary Conditions

Orientational structures formed in nematic and cholesteric layers with tangential-conical boundary conditions have been investigated. LC cells with one substrate specifying the conical surface anchoring and another substrate specifying the tangential one have been considered. The director configurations and topological defects have been identified analyzing the texture patterns obtained by polarizing microscope in comparison with the structures and optical textures calculated by free energy minimization procedure of director field and finite-difference time-domain method, respectively. The domains, periodic structures and two-dimensional defects proper to the LC cells with tangential-conical anchoring have been studied depending on the layer thickness and cholesteric pitch.