Director Orientation Research Articles

Photoinduced Birefringence and Liquid Crystal Orientation on Polymers with Different Azobenzene Content in the Main Chain.

In this paper, we give an overview of novel main-chain azobenzene-based fluorinated poly(arylene ether)s with different content of azo groups, aiming at providing a better understanding of the link between a number of N═N bonds and the macroscopic response of the material. We discuss chemical synthesis and molecular structure and report on a comprehensive analysis of the polymer properties, thermal behavior, and mechanical strength. We show that a higher content of azobenzene moieties reduces the mechanical strength of the polymer materials. On the other hand, polymers with a higher content of azobenzene demonstrate higher values of induced birefringence due to a larger number of azobenzene in the trans form. The photoisomerization constants of all polymers fall within a very close range. The minor variations are attributed to the number of azobenzene groups in the polymer composition and the conformational arrangements of the polymer chain packing. The developed light-sensitive polymers were employed for dynamic control and manipulation of the liquid crystal orientation by polarization of the incident light. After the double irradiation of the substrates using appropriate photomasks, we made patterned cells that consist of domains with different high-resolution liquid crystal director orientations.

Cascaded chiral birefringent media enabled planar lens with programable chromatic aberration

Wavefront control is the fundamental requirement in optical informatics. Planar optics have drawn intensive attention due to the merits of compactness and light weight. However, it remains a challenge to freely manipulate the dispersion, hindering practical applications, especially in imaging. Here, we propose the concept of frequency-synthesized phase engineering to solve this problem. A phasefront-frequency matrix is properly designed to encode different spatial phases to separate frequencies, thus makes arbitrary dispersion tailoring and even frequency-separated functionalization possible. The periodically rotated director endows cholesteric liquid crystal with a spin and frequency selective reflection. Moreover, via presetting the local initial orientation of liquid crystal, geometric phase is encoded to the reflected light. We verify the proposed strategy by cascading the chiral anisotropic optical media of specifically designed helical pitches and initial director orientations. By this means, planar lenses with RGB achromatic, enhanced chromatic aberration and color routing properties are demonstrated. Inch-sized and high-efficient lenses are fabricated with low crosstalk among colors. It releases the freedom of dispersion control of planar optics, and even enables frequency decoupled phase modulations. This work brings new insights to functional planar optics and may upgrade the performance of existing optical apparatuses.

Defect Modes Generated in a Stack of Spin-Coated Chiral Liquid Crystal Layers

Nematic chiral liquid crystals (CLCs) are characterized by a helical arrangement of nematic LC molecules. A layer of CLC typically exhibits an optical reflection band due to Bragg reflection in the helical structure. When several layers of CLC are spin-coated and polymerized on top of each other without a barrier layer in between, defect modes can form in their reflection spectrum. By comparing experimental results and simulations, we investigate the origin of the defect modes, thereby revealing details on the behavior of the materials at the interfaces during deposition. Simulations show that these defect modes can originate from the migration of chiral dopant leading to a layer with a smaller pitch or from a discontinuity in the director orientation at the interface between two layers.

Directional Adhesion of Monodomain Liquid Crystalline Elastomers.

Pressure-sensitive adhesives (PSAs) are widely employed in consumer goods, health care, and commercial industry. Anisotropic adhesion of PSAs is often desirable to enable high force capacity coupled with facile release and has typically been realized through the introduction of complex surface and/or bulk microstructures while also maintaining high surface conformability. Although effective, microstructure fabrication can add cost and complexity to adhesive fabrication. Here, we explore aligned liquid crystalline elastomers (LCEs) as directional adhesives. Aligned LCEs exhibit direction-dependent stiffness, dissipation, and nonlinear deformation under load. By varying the cross-link content, we study how the bulk mechanical properties of LCEs correlate to their peel strength and peel anisotropy. We demonstrate up to a 9-fold difference in peel force measured when the LCE is peeled parallel vs perpendicular to the alignment axis. Opportunities to spatially localize adhesion are presented in a monolithic LCE patterned with different director orientations.

Optimized configuration for liquid crystal Stokes polarimeters in the presence of fast-axis orientation errors.

We present an optimal configuration for Stokes polarimeters based on liquid crystal variable retarders, with the minimum number of measurements. Due to the inherent variations of the director orientation of the liquid crystal molecules, we propose a configuration that minimizes the sensibility of the polarimeter to fast-axis variations. For the optimization we consider a scheme that maximizes the volume of a tetrahedron inscribed in the Poincare sphere, to address additive and Poisson noise, with one of the vertices invariant to changes in the axis positions. We provide numerical simulations, considering misalignment errors, to analyze the robustness of the configuration. The results show that the proposed configuration helps to maintain the volume enclosed by the tetrahedron with high tolerance to fast-axis orientation errors. The condition number will remain below 3.07 for common misalignment errors and below 1.88 for more controlled liquid crystals. This optimization will improve the performance of liquid crystals polarimeters, with a more robust configuration that also considers misalignment errors, beyond additive and Poisson noise.

Liquid crystal-based label-free low-cost sensing platform: Engineering design based on interfacial interaction and transport phenomena

Typically, the detection methods of chemical species in sensing platform involves the use of labelling agents. Even though it is selective, the process is complex and time consuming. Liquid crystals (LC) are sensitive and rapidly responsive to the presence of any foreign species. When a LC layer interacts with aqueous (ionic surfactant) solution, there is homeotropic alignment of the director orientation due to the anisotropic interactions at the interface. This leads to the dark birefringence patterns observed in polarised microscope under cross polarisation. In the presence of H+ in the aqueous layer, the LC orientation is distorted resulting in a change of the birefringence patterns in the polarised micrographs. We have prepared an optical cell where the aqueous solution can interact with the LC layer confined within micrometre sized grids. The flow arrangement and the associated mass transfer process is optimised for enhanced interaction. The elastic interactions of the LC layer with H+ at the interface distorts the director orientation, and the birefringence pattern is captured with polarised microscope (cross-polarisation). There is a correlation between the polarised micrographs and the concentration of the H+, which forms the basis of detection principle. The detection of H+ has no interference with salts. The present method can be used for recognition of (bio-) chemical reactions releasing H+. The developed optical flow cell is inexpensive, less than $1. The outcome of the present work may be useful for technological adaption and prospective use in point-of-care devices and field test kits.

Statics and dynamics of point boojums, line and modified Saturn ring topological defects in nematic confined geometry.

In this paper we study the static structure and the dynamics of topological defects associated with isotropic droplets in nematic environment. Investigations were made in confined geometry of optical cells when the droplet size was of the order of or larger than the gap of the cell. We observed the coexistence of point boojums and Saturn ring or modified Saturn ring defects. We found transformation of the Saturn ring defect to two localized broad defects at increasing the droplet size. At droplet coalescence antipodes of point and localized broad defects were born and the dynamics of their annihilation with existing defects was investigated. We found strong difference in the process of annihilation of point and localized broad defects. Microscope images of isotropic droplets in nematic environment in a planar cell. The director orientation far from the droplets is in horizontal direction. The photographs were taken with crossed vertical and horizontal polarizers (a) and with a single horizontal polarizer (b). The cell thickness is 100μm. Droplet diameter is less than the cell thickness. 1 and 2 are point boojums, L is the Saturn ring defect.

Multifunctional Al-doped ZnO thin films for vertically aligned liquid crystal devices

The integration of ITO-free transparent conductive layers remains a big challenge for development of next generation technologies. Here, we demonstrate the feasibility of transparent and conductive Aluminum-doped Zinc Oxide (AZO) thin films to operate simultaneously as electrode and alignment layer in Liquid Crystal (LC) device configuration. Controlling the growth process of AZO thin films by Atomic Layer Deposition (ALD) technique results of predominantly (100) crystallographic oriented AZO with very high transparency and excellent conductivity. The exceptionally low surface free energy of (100) oriented AZO, along with the topological modification following the mechanical rubbing treatment were used to elucidate the mechanism of LC molecules alignment on the surface of AZO film. The uniform vertical orientation of the nematic LC director was confirmed by polarized optical microscopy and pretilt angle measurements. The competitive electro-optical performance in terms of phase modulation, threshold and saturation voltages and contrast ratio of the assembled LC device reveals the great potential of AZO thin films as transparent electrode and alignment layer for future LC display applications with versatile functionality.

Non-Local Q-Tensor Approach for Description of Elastic Deformations of Nematic Liquid Crystals at Sub-Micron Scale

Within the framework of phenomenological approach, a generalized concept of a non-local order parameter, that have the form of a traceless correlation tensor function or a tensor integral operator, is introduced. The main relationships, which determine the equilibrium orientational states and the phase transitions of a deformed nematic liquid crystal, are described. The resulting expression for the free energy is quadratic with respect to the gradients of the order parameter tensor. The limiting transition of the non-local order parameter to the local one leads to an expression that complements the classical Oseen–Frank theory of elasticity. The considered model includes eight independent elastic constants. Three of them relate to "bulk" constants analogous to Frank constants, another three ones relate to "surface" constants, and the remaining two constants determine the anisotropic effect of the gradient of the scalar order parameter on the director field. The constants are necessary for the consideration of a liquid crystal state near defects, as well as for the description of orientation effects caused by the inhomogeneity of order parameter. It is shown that in the free energy expression, the "surface" terms contribute to the director orientation in the case of a non-constant scalar order parameter, even in the approximation of rigid boundary conditions.

Cholesteric Liquid Crystals Based Micro‐Fingerprints Generator for Anti‐Counterfeiting Labels

AbstractIn this study, a method is presented to fabricate optical fingerprint‐like patterns. The method relies on the use of commercially available chiral nematic liquid crystals (CNLCs) confined in microspheres. The peculiar optical texture is obtained by applying a high‐frequency voltage to the micrometric objects able to distort the molecular director orientation. The texture can be stabilized by doping the CNLC with photosensitive materials. Each microsphere shows a different fingerprint‐like pattern that is generated in a completely random manner making this procedure suitable to create physical unclonable functions (PUFs) keys. The optical patterns can be stored to create an artificial fingerprints database or they can be used to fabricate electroluminescent labels to be exploited as complex anti‐counterfeiting devices. An authentication software is developed and used to test the robustness of the proposed anti‐counterfeiting system.

Mechanism of Pressure-Sensitive Adhesion in Nematic Elastomers.

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed that this is the cause of their anomalously high-pressure-sensitive adhesion (PSA). Here, we investigate the mechanism behind this enhanced PSA by first preparing thin adhesive tapes with LCE of varying cross-linking densities, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90° peel, lap shear, and probe tack tests. The study confirms that the enhanced PSA is only present in (and due to) the nematic phase of the elastomer, and the strength of bonding takes over 24 h to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in the nematic domains after pressing against the surface. We confirm this mechanism by showing that freshly pressed and annealed tape reaches the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film.

Propagation and attenuation of elastic waves in nematic elastomer hollow cylinders

As an intelligent soft material, nematic elastomers (NEs) possess both rubber elasticity and the unique properties of liquid crystals. Such combination gives NEs unusual mechanical response. In this paper, guided waves in nematic elastomer hollow cylinders are investigated. Based on the viscoelasticity theory, the wave governing equations are derived and solved by the Legendre polynomial series approach (LPSA). This approach can directly obtain eigenvalues/eigenvectors characterizing wave propagation and attenuation or field profiles. The effects of circumferential order and radius-thickness ratio on dispersion and attenuation curves are discussed. Besides, dispersion and attenuation curves with different initial director orientations are presented. It is found that the existence of the director results in mode conversions between adjacent flexural longitudinal modes and flexural torsional modes. The results provide important guidance for dynamic design and device analysis of cylinder-shaped nematic elastomer devices.

Colossal transverse magnetoresistance due to nematic superconducting phase fluctuations in a copper oxide.

Electronic anisotropy ("nematicity") has been detected in cuprate superconductors by various experimental techniques. Using angle-resolved transverse resistance (ARTR) measurements, a very sensitive and background-free technique that can detect 0.5% anisotropy in transport, we have observed it also in La2-xSrxCuO4 (LSCO) for 0.02 ≤ x ≤ 0.25. A central enigma in LSCO is the rotation of the nematic director (orientation of the largest longitudinal resistance) with temperature; this has not been seen before in any material. Here, we address this puzzle by measuring the angle-resolved transverse magnetoresistance (ARTMR) in LSCO. We report the discovery of colossal transverse magnetoresistance (CTMR)-an order-of-magnitude drop in the transverse resistivity in the magnetic field of 6 T. We show that the apparent rotation of the nematic director is caused by anisotropic superconducting fluctuations, which are not aligned with the normal electron fluid, consistent with coexisting bond-aligned and diagonal nematic orders. We quantify this by modeling the (magneto-)conductivity as a sum of normal (Drude) and paraconducting (Aslamazov-Larkin) channels but extended to contain anisotropic Drude and Cooper-pair effective mass tensors. Strikingly, the anisotropy of Cooper-pair stiffness is much larger than that of the normal electrons. It grows dramatically on the underdoped side, where the fluctuations become quasi-one-dimensional. Our analysis is general rather than model dependent. Still, we discuss some candidate microscopic models, including coupled strongly-correlated ladders where the transverse (interladder) phase stiffness is low compared with the longitudinal intraladder stiffness, as well as the anisotropic superconducting fluctuations expected close to the transition to a pair-density wave state.

Light‐Responsive Nematic Colloids and Colloidal Crystals

AbstractRational control over the periodic arrangement of particles by means of external stimuli is a technologically important aspect of colloidal science with important physical underpinnings. Here, robust structural control of particle assemblies in a nematic liquid crystal (NLC) is demonstrated by dissolving trace amounts of light‐responsive azo‐dendrimer molecules, which spontaneously get adsorbed on the particle surface. The azo‐dendrimer molecules in the presence of external UV irradiation undergo conformational change (trans‐cis); as a result, they transmit the mechanical torque to surrounding LC molecules and alter the near‐field director orientation. The director re‐orientation at the surface of the particles causes topological defect transformation, which involves elastic dipoles, quadrupoles, and hexadecapoles. The defect transformation can be emulated in colloidal assemblies toward different purposes such as rotation of chains and restructuring of 2D colloidal crystals. In this study, various topological aspects of light‐activated defect transformation and its application in the collective manipulation of colloidal assemblies are presented.

Thin pyramidal cones in nematic liquid crystals.

The present study investigates the arrangement of hollow pyramidal cone shells and their interactions with degenerate planar anchoring on the inner and outer surfaces of particles within the nematic host. The shell thickness is in order of the nematic coherence length. The numerical behavior of colloids is determined by minimizing the Landau-de Gennes free energy in the presence of the Fournier surface energy and using the finite element method. Colloidal pyramidal cones can orient parallel and perpendicular with the far director orientation. In the parallel alignment, we found the splay director distortion into the pyramid with two boojum defects at the inner and outer tips. The director shows bending distortion without defect patterns when the pyramid is aligned perpendicularly. They induce long-range dipolar interaction and can form nested structures in close contact.

TIC Reorientation under Electric and Magnetic Fields in Homeotropic Samples of Cholesteric LC with Negative Dielectric Anisotropy

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field B→ parallel to the electrodes. The configuration can be either parallel or perpendicular to the magnetic field depending on the values of the sample thickness, pitch, and applied voltage, with two equiprobable orientations in each case. The transition between the parallel and perpendicular orientations has hysteresis, suggesting that it is first order. When B→ is slightly tilted with respect to the electrode plane, the indeterminacy on the TIC orientation is removed when the TIC is directed along B→.

Principle of virtual power and drilling degrees of freedom for dynamic modeling of the behavior of liquid crystal elastomer films

In this work, we aim to model the reorientation process of mesogens in nematic liquid crystal elastomers within the context of dynamics. We consider a continuum model with separate mappings for the deformation of the monolithic material and the orientation of the nematic director, where the latter describes the inclination of the mesogens. We achieve the inextensibility of the nematic director through the introduction of drilling degrees of freedom. We combine this approach with the application of the principle of virtual power and a mixed finite element formulation, in order to formulate distinct momentum and angular momentum balance laws for the two separate mappings. Furthermore, we include in our continuum model a volume load and a surface load associated only with the orientation mapping. We show in the presented three numerical examples that our formulation enables the fulfillment of all momentum and angular momentum balance laws.

Programmable electric-field-induced bending shapes of dielectric liquid crystal elastomer sheets

The nematic director of dielectric liquid crystal elastomers (DLCEs) can be rotated by electric field, which makes the DLCE macroscopically deformed. In this paper, the electric-field-induced bending of DLCE sheets with director orientation gradient in thickness are investigated theoretically. The results reveal that the flat DLCE sheets in response to electric fields can deform to surfaces with zero, positive, or negative Gaussian curvature. Bending shapes of DLCE sheets can be encoded by patterning the nematic director field. Specifically, bending shapes with zero and positive Gaussian curvatures caused by unidirectional and multidirectional bending, respectively, are encoded by the splay–bend and splay–twist–bend director alignments, while the bending shape with negative Gaussian curvature is encoded by the twist or splay–twist–bend​ director alignments. Moreover, for each of these bending shapes, the Gaussian curvature and principal curvatures can also be maximized and optimized by adjusting the initial director alignments. The results imply that it is possible to program more complex bending shapes such as human face, in surface topography engineering, and this work provides a theoretical basis for promoting the application of LCEs as the electrically-triggered actuators and sensors.

Giant flexoelectric effect with liquid crystal dimer CB7CB between concentric cylinders

ABSTRACT Liquid crystal dimer CB7CB (1″,7″-bis(4-cyanobiphenyl-4′-yl)heptane) is confined between two concentric cylinders with the liquid crystal director initially aligned axially along the concentric cylinder. When a non-uniform radial electric field perpendicular to the concentric cylinder axis is applied, the director is gradually changed as the applied voltage increases. We obtain a threshold voltage analytical formula for the angular deformation of the director between two concentric cylinders. In addition, the effect of different flexoelectric coefficients on the deformation of the director between two concentric cylinders under the radial electric field was also investigated. The results show that the flexoelectric coefficients will determine how the director will be deformed. It provides a more accurate theoretical analysis for predicting the coupling effects of dielectric effect and giant flexoelectric effect on the director orientation distribution between two concentric cylinders under a non-uniform radial electric field.

ВЗАИМОДЕЙСТВИЕ И ДИНАМИКА ТОПОЛОГИЧЕСКИХ ДЕФЕКТОВ В ДОМЕННОЙ СТРУКТУРЕ ЗАКРУЧЕННОГО НЕМАТИКА

The dynamics and interaction of topological defects in the domain structure of π/2 twisted nematic liquid crystal are studied. A feature of Williams domains in twisted nematics is that hydrodynamic flows in them, along with the tangential velocity component, also have an axial component, the direction of which is opposite in neighboring domains. The axial velocity component arises as a result of the strong coupling between the initial twisted director orientation n and the hydrodynamic flow velocity. In domain structures of π/2 twisted nematics, as well as in planar oriented ones, edge dislocations with topological charges S =±1 arise. With increasing applied voltage density of dislocations grows. Dislocations move both along the Williams domains ( climb ) and perpendicular to them ( glide ). At a certain rate of the increasing applied voltage U > U c, distortions of the domain structure begin to propagate on both sides of the defect core. This leads to the formation of a special topological defects - dislocations with a “diffused” core, or linear defects, which are oriented perpendicular to the Williams domains and move mainly along them. The arising linear defect has the same topological charge as the initial dislocation. It is shown that their interaction is qualitatively well described by the perturbed sine-Gordon equation.