Nematic Orientation Research Articles

Nematic director reorientation at solid and liquid interfaces under flow: SAXS studies in a microfluidic device.

In this work we investigate the interplay between flow and boundary condition effects on the orientation field of a thermotropic nematic liquid crystal under flow and confinement in a microfluidic device. Two types of experiments were performed using synchrotron small-angle X-ray-scattering (SAXS). In the first, a nematic liquid crystal flows through a square-channel cross section at varying flow rates, while the nematic director orientation projected onto the velocity/velocity gradient plane is measured using a 2D detector. At moderate-to-high flow rates, the nematic director is predominantly aligned in the flow direction, but with a small tilt angle of ∼±11° in the velocity gradient direction. The director tilt angle is constant throughout most of the channel width but switches sign when crossing the center of the channel, in agreement with the Ericksen–Leslie–Parodi (ELP) theory. At low flow rates, boundary conditions begin to dominate, and a flow profile resembling the escaped radial director configuration is observed, where the director is seen to vary more smoothly from the edges (with homeotropic alignment) to the center of the channel. In the second experiment, hydrodynamic focusing is employed to confine the nematic phase into a sheet of liquid sandwiched between two layers of Triton X-100 aqueous solutions. The average nematic director orientation shifts to some extent from the flow direction toward the liquid boundaries, although it remains unclear if one tilt angle is dominant through most of the nematic sheet (with abrupt jumps near the boundaries) or if the tilt angle varies smoothly between two extreme values (∼90 and 0°). The technique presented here could be applied to perform high-throughput measurements for assessing the influence of different surfactants on the orientation of nematic phases and may lead to further improvements in areas such as boundary lubrication and clarifying the nature of defect structures in LC displays.

Switchable beam steering with zenithal bistable liquid-crystal blazed gratings.

Switchable beam steerers based on zenithal bistable liquid crystal (LC) gratings are designed and theoretically investigated. The nematic orientation profiles and the optical transmittance properties of the gratings are rigorously calculated, respectively, via a tensorial formulation of the Landau-de Gennes theory and the full-wave finite-element-method. By proper design of the grating geometry, beam steering with high diffraction efficiency is demonstrated between the two stable LC states. The tolerance of the device performance with respect to material parameters is assessed, evidencing spectral operation windows of more than 50 nm in the visible for a beam steering efficiency higher than 90%.

Experimental observation of front propagation in a negatively diffractive inhomogeneous Kerr cavity

A focusing Kerr Fabry-Pérot cavity operating in a negative diffraction regime exhibits transverse propagating fronts connecting two different nematic liquid crystal molecule average orientations. Under an inhomogeneous spatial pumping beam, these fronts stop to propagate and lead to the formation of a stable localized structure. The trajectory of the front position is derived from the mean-field model. Its hyperbolic tangent analytical expression perfectly fits the experimental data.Received 11 February 2014DOI:https://doi.org/10.1103/PhysRevA.90.011806©2014 American Physical Society

Terahertz characterization of tunable metamaterial based on electrically controlled nematic liquid crystal

The terahertz time domain spectroscopy (THz-TDS) system is used to determine the effects of an AC bias voltage on the tunable response of a metamaterial transducer. The tunability of the metamaterial structure, which is based on the rod-split-square resonator, is demonstrated at terahertz frequencies through electrical control of the nematic liquid crystal orientation. Experimental results show that the metamaterial device can be tuned effectively (with transmittance change of up to 19%) by changing the magnitude of the AC bias voltage from 0 to 300 V. This type of tunable metamaterial could find application in the development of devices operating in the THz frequency region for filtering, modulating, and switching of the electromagnetic signals.

Observation of Transient Alignment-Inversion Walls in Nematics of Phenyl Benzoates in the Presence of a Magnetic Field

Formation of new transient walls by a constant magnetic field at the Fréedericsz critical value has been observed. They are oriented along the initial alignment of the nematic phase of phenyl benzoates and appeared only in relatively thick samples with a thickness between 50 and 100 μm of the cells. The excellent planarity of the liquid crystal orientation is considered to be the most important condition for their presence These magnetic walls are transient as they disappear either after a few seconds for 100 μm thick nematic cells or after parts of a second for thinner (50 μm) nematic cells. Nonregular stable magnetic walls, incorporating disclinations with core, appear immediately after the relaxation of the transient walls, when the planarity of the nematic orientation is not perfect. In thinner nematic cells of 20 μm or less, a Fréedericksz transition has only been observed. The formation of transient magnetic walls can be described by a model taking into account alignment-inversion, twisted along Y regions. The transient walls accompanied by a system of Becke lines relax by going through three-dimensional twist-splay-bend deformations.

Side-polished fiber as a sensor for the determination of nematic liquid crystal orientation

The orientation change of nematic liquid crystal (NLC) has been the essential mechanism for various high sensitive sensors. In this paper, a side-polished fiber (SPF) is proposed to sense the orientation change of NLC. We successfully designed the liquid crystal cell embedded with a SPF that enabled us to mechanically rotate the orientation of NLC on the polished surface of the SPF. Our experimental results showed that the orientation change of NLC can indeed be sensed by monitoring the output optical power transmitted through the SPF. The increment of the transmitted optical power through the SPF was measured as large as 28.10dB with the mechanical rotation from 0° to 90°. Within the angle range from 0° to 30°, the response was almost linear. The average response slope was about 0.359dB/°, which indicated relatively high sensitivity. These experimental results indicate that SPF can be used to determine the orientation change of NLC. In particular, there exists a possibility that a SPF overlaid with NLC on its polished surface can potentially be made into a small transducer of a biosensor.

Microphase separation in comblike liquid-crystalline diblock copolymers

The interplay between liquid crystallinity and microphase separation in comblike liquid-crystalline diblock copolymers is examined via a Brazovskii-type phenomenological model using both analytical and numerical calculations. For symmetric diblock copolymers we determine a critical electric field that is required to tilt the orientation of the constituent liquid crystals of the polymer side chains in the microphase-separated lamellar state. Such electrically induced reorientation of the liquid-crystal molecules can lead to substantially large changes of lamellar periodicity. Our numerical results show that highly aligned polymer lamellar domains can self-assemble when the liquid-crystal ordering precedes microphase separation, and that weak electric fields can be used to direct the self-assembly process due to the dielectric anisotropy of the liquid-crystal side chains. We also find that phase separation of asymmetric diblock copolymers can coexist with a network of liquid-crystal nematic orientations, with domain morphology depending on the details of copolymer and liquid-crystal coupling.

Modelling spreading dynamics of nematic liquid crystals in three spatial dimensions

AbstractWe study spreading dynamics of nematic liquid crystal droplets within the framework of the long-wave approximation. A fourth-order nonlinear parabolic partial differential equation governing the free surface evolution is derived. The influence of elastic distortion energy and of imposed anchoring variations at the substrate are explored through linear stability analysis and scaling arguments, which yield useful insight and predictions for the behaviour of spreading droplets. This behaviour is captured by fully nonlinear time-dependent simulations of three-dimensional droplets spreading in the presence of anchoring variations that model simple defects in the nematic orientation at the substrate.

Torsional mechanical responses in azobenzene functionalized liquid crystalline polymer networks

Soft materials capable of both planar and flexural–torsional responses could enable the development of soft robotic elements that emulate the dexterity and functionality of a multitude of creatures in the animal kingdom. Here, we examine the response of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) specifically focusing on realizing large magnitude flexural–torsional responses observed as out-of-plane twisting or coiling. Towards this end, azo-LCNs were prepared in either the twisted nematic (TN) or hybrid orientations. The characterization of the flexural–torsional photomechanical responses is complimented with examination of thermomechanical properties. The diverse range of tailorable photomechanical responses is shown to be strongly dependent on the alignment of the nematic director to the film geometry and the actinic light intensity.

Plasmonic Variable Optical Attenuator Based on Liquid-Crystal Tunable Stripe Waveguides

A long-range surface plasmon polariton variable optical attenuator based on available nematic liquid crystals and polymers is proposed and theoretically investigated. It is demonstrated that the electro-optic control of the nematic molecular orientation is capable of tuning the level of index asymmetry of an Au stripe waveguide and the key properties of the fundamental long-range plasmonic mode, such as modal profile and propagation losses. By proper structural design and material selection, plasmonic in-line intensity modulators are designed, which exhibit very low power consumption, extinction ratios in excess of 30 dB, and insertion losses as low as 1 dB for a device length in the millimeter range. Such active plasmonic elements are envisaged to be used in interchip photonics bus interconnects.

Computer simulations of nematic drops: Coupling between drop shape and nematic order

We perform Monte Carlo computer simulations of nematic drops in equilibrium with their vapor using a Gay-Berne interaction between the rod-like molecules. To generate the drops, we initially perform NPT simulations close to the nematic-vapor coexistence region, allow the system to equilibrate and subsequently induce a sudden volume expansion, followed with NVT simulations. The resultant drops coexist with their vapor and are generally not spherical but elongated, have the rod-like particles tangentially aligned at the surface and an overall nematic orientation along the main axis of the drop. We find that the drop eccentricity increases with increasing molecular elongation, κ. For small κ the nematic texture in the drop is bipolar with two surface defects, or boojums, maximizing their distance along this same axis. For sufficiently high κ, the shape of the drop becomes singular in the vicinity of the defects, and there is a crossover to an almost homogeneous texture; this reflects a transition from a spheroidal to a spindle-like drop.

Novel Behavior in Shear Flow Orientation of Side-Chain Polymethacrylate Nematic Liquid Crystals

Shear flow orientation of nematic liquid crystals (LCs) has been investigated for two side-chain polymethacrylates having the same methoxyazobenzene mesogens but different spacers of either four (PM4MA) or six (PM6MA) methylene groups. These two polymer nematic LCs exhibited orientation behaviors distinct from each other. PM4MA arranged the nematic director in the velocity direction (the b-orientation), whereas PM6MA pointed the director to the neutral (the a-orientation) or velocity gradient (the c-orientation) directions depending on both shear rate and molar mass. Decreasing shear rate or increasing molar mass gave rise to an orientation transition from a to c. Such anomalous nematic orientation behavior is related to the presence of fluctuating smectic clusters, and explainable if fluctuations, which couple to shear flow to induce the a-orientation, have a lifetime which decreases with increasing backbone chain length so as to be sufficiently short to be averaged out under shear flow.

Tunable bi-functional photonic device based on one-dimensional photonic crystal infiltrated with a bistable liquid-crystal layer

We investigated the optical properties of a one-dimensional photonic crystal infiltrated with a bistable chiral tilted homeotropic nematic liquid crystal as the central defect layer. By modulating the nematic director orientation with applied voltage, the electrical tunability of the defect modes was observed in the transmission spectrum. The composite not only is a general tunable device but also involves the green concept in that it can operate in two stable states at 0 V. Under the parallel-polarizer scheme, the spectral characteristics suggest a potential application for this device as an energy-efficient multichannel optical switch.

Novel mechanical properties in lamellar phases of liquid-crystalline diblock copolymers

Structural properties of flexible nematic diblock copolymers in the lamellar phase are investigated using a mean-field model. We address two complementary questions on the mechanics of the system: 1) How does the nematic order affect the elasticity of the one-dimensional solid? 2) What effect does the block copolymer microstructure has on the orientation of the nematic director? In the limit when the microstructure does not influence the nematic director orientation we predict a soft lamellar compression mode. When the microstructure does influence the nematic director orientation, small compressions lead to conventional elasticity, until a critical strain is reached, where there is then a transition to a softer response. On the other hand, we show that an identifiable lamellar symmetry provides a direction along which the nematic director prefers to align. Our model provides avenues to explore nonlinear properties of flexible diblock copolymers in which the monomers on both sides have mesogenic side groups.

On the control of the nematic orientation on the silicon surface processed by a focused gallium ion beam

Preliminary results of studying the orientation of a 5CB nematic on the single-crystalline silicon surface processed by a 30-keV focused Ga ion beam ar e reported. It is shown that the nematic can be controllably imparted on a homeotropic or inclined orientation depending on the surface irradiation dose. By varying the scanning raster of the ion beam, one can obtain different surface patterns with a micrometer resolution and a desired orientation of the nematic.

Orientational Quenched Disorder of a Nematic Liquid Crystal

By means of direct imaging, we map the surface heterogeneities of the nematic director orientation on a SiOx anchoring layer. The spatial correlations of surface director orientations are well fitted with a compressed exponential with exponent of 1.5 and typical correlation length of few microns. To discuss these results a formal analogy is established between the equation governing the nematic surface torques and the Langevin equation. Based on this analogy we prove that the disorder is spatially correlated orientational quenched disorder. The measured correlation length is discussed in terms of substrate morphology and molecular adsorption.

Importance of the surface viscosity on the relaxation of an imposed deformation in a nematic liquid crystal cell

We analyze the influence of the surface viscosity on the relaxation of the nematic deformation induced by an external field on a nematic cell, when the distorting field is removed. The analysis is performed by assuming that the anchoring energy describing the anisotropic part of the nematic–substrate surface tension can be approximated by the form proposed by Rapini and Papoular, and that the viscous torque at the surface is characterized by a surface viscosity η s . We consider a simple situation where by means of an external action the surface easy direction is modified, and investigate the evolution of the nematic orientation, from the initial state to the final one. The Green function approach is used to obtain the exact solution for the tilt angle profile. The results show that the surface viscosity modifies the relaxation process which becomes slower for increasing values of η s .

Elastic anisotropy at a glance: the optical signature of disclination lines

Nematic liquid crystals often host topological defects in the form of disclination lines. Because of the elastic anisotropy intrinsic to nematics, the tight distortions of the molecular ordering around the defect lines couple to the mean nematic orientation inducing deformations on length scales easily observed in conventional optical microscopy. By a synergy of experiments, numerical computations and theoretical modeling we present a quantitative analysis of this phenomenon, clarifying the key role played by twist deformations. We provide simple analytical tools to predict the amplitude of this deformations and the energy involved in it. The effects here described are potentially essential ingredients to all liquid crystal applications involving topological defects.

Response of prestretched nematic elastomers to external fields

We investigate the response of prestretched nematic side-chain liquid single-crystal elastomers to superimposed external shear, electric, and magnetic fields of small amplitude. The prestretching direction is oriented perpendicular to the initial nematic director orientation, which enforces director reorientation. Furthermore, the shear plane contains the direction of prestretch. In this case, we obtain a strongly decreased effective shear modulus in the vicinity of the onset and the completion of the enforced director rotation. For the same regions, we find that it becomes comparatively easy to reorient the director by external electric and magnetic fields. These results were derived using conventional elasticity theory and its coupling to relative director-network rotations.

Reorientation effect and electrical current in a weakly anchored nematic cell

A nematic cell subjected to a large electric field undergoes a molecular reorientation that affects the electrical current flowing through it. To analytically establish the dependence of the current on the applied voltage, the cell is considered as parallel of resistance, R(t) , and capacitance, C(t) , that are connected with the nematic director profile. This profile is determined in the quasistatic regime in which the nematic orientation follows the time variation in the external field normal to the cell plates without delay. The analysis performed for a weakly anchored cell shows that the current presents a peak when the applied voltage overcomes the threshold voltage for the transition of Fréedericksz at a critical time t* as in the case of strong anchoring. For large voltages, R(t)-->R|| and C(t)-->C||, where parallel refers to the nematic director. We show that, for large enough time t>>t*, it is possible to connect the measured current with the extrapolation length characterizing the sample by means of simple analytical expressions. This connection can be used to experimentally estimate the anchoring energy by means of current measurements.