Crystal Reorientation Research Articles

Molecular reorientation of a nematic liquid crystal by thermal expansion

A unique feature of nematic liquid crystals is orientational order of molecules that can be controlled by electromagnetic fields, surface modifications and pressure gradients. Here we demonstrate a new effect in which the orientation of nematic liquid crystal molecules is altered by thermal expansion. Thermal expansion (or contraction) causes the nematic liquid crystal to flow; the flow imposes a realigning torque on the nematic liquid crystal molecules and the optic axis. The optical and mechanical responses activated by a simple temperature change can be used in sensing, photonics, microfluidic, optofluidic and lab-on-a-chip applications as they do not require externally imposed gradients of temperature, pressure, surface realignment, nor electromagnetic fields. The effect has important ramifications for the current search of the biaxial nematic phase as the optical features of thermally induced structural changes in the uniaxial nematic liquid crystal mimic the features expected of the biaxial nematic liquid crystal.

Extreme nonlinear optics of nematic liquid crystals [Invited

Laser induced director axis reorientation and order parameter changes in nematic liquid crystals give rise to extremely nonlinear optical responses characterized by nonlinear index coefficients that can reach values exceeding 103 cm2/Watt. An historical account and critical analyses of the theoretical backgrounds and experimental observations of several exemplary nonlinear optical phenomena are presented. Emphasis is placed on identifying and detailing the critical roles played by unique properties of nematic liquid crystals in these processes, including self-action effects, wave mixing, and switching phenomena for a variety of interaction geometries.

Hybrid Micro‐Macro‐Modeling of Evolving Anisotropies and Length Scales in Finite Plasticity of Polycrystals

AbstractThe paper focuses on an efficient multiscale approach to the description of evolving anisotropies in polycrystals. The predictive description of complex overall effects needs multiscale methods, taking into account basic ingredients of the material microstructure by model‐inherent scale‐bridging techniques. Straightforward homogenization‐based multiscale methods, such as FEM2 type two‐scale bridgings, are too time consuming for the solution of real engineering problems. A computationally manageable approach is provided by hybrid micro‐macro‐modeling techniques, which combine a purely macroscopic modeling with selected microstructural bridging techniques. Of particular importance are crystal reorientation mechanisms, which cause texture‐based anisotropies at the macroscale. The coupling is realized by homogenization‐based definitions of macroscopic structural tensors, which account for the evolving orientation texture. A crucial aspect is the combination of this hybrid texture modeling with further anisotropy effects, caused by dislocation‐structures. They are mainly responsible for hardening effects related to changes of the strain path. Furthermore, a particular account is taken on size effects and length scales by embedding the hybrid modeling technique into an extended theory of gradient‐plasticity. This results in a computationally efficient hybrid two‐scale model for polycrystalline plasticity, whose capability is demonstrated by means of representative benchmarks. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanical and functional properties of amorphous-crystalline ribbons of Ti40.7Hf9.5Ni44.8Cu5 alloy

The mechanical behavior and functional properties in amorphous-crystalline thin ribbons of Ti40.7Hf9.5Ni44.8Cu5 alloy with a different volume fraction (from 0 to 100%) of crystalline phase were studied. The results obtained showed that a fully amorphous sample was deformed elastically up to 6%. The existence of the crystalline phase in the sample resulting in the deformation was realized by reorientation of martensite at the early stage. The strain accumulated at this stage was fully recoverable during subsequent heating through the temperature range of reverse martensitic transformation. It was found that an increase in the volume fraction of crystalline phase led to a rise in the value of the shape memory effect. This was due to the increase in the volume fraction of the sample deforming by the mechanism of martensite reorientation. It was observed that the amorphous-crystalline Ti40.7Hf9.5Ni44.8Cu5 alloy was deformed by three mechanisms of unelastic deformation: reorientation of martensite crystals, plastic deformation of the amorphous phase, and dislocation slip. It was shown that the change in the deformation mechanism was determined by the volume fraction of crystalline phase and the value of preliminary strain. It was found that the Ti40.7Hf9.5Ni44.8Cu5 alloy exhibited a two-way shape memory effect only if the amorphous and the crystalline phase coexisted in the alloy.

Aggregation of colloidal particles in a non-equilibrium backflow induced by electrically-driven reorientation of the nematic liquid crystal

We study the dynamics of anisotropic aggregation of spherical colloidal particles with dipolar interactions, dispersed in a nematic liquid crystal. The director is perpendicular to the surface of spheres so that each particle creates a dipolar distortion when placed in a uniformly aligned planar nematic slab. A backflow of the nematic caused by periodic voltage pulses, propels the spheres in a bidirectional fashion, so that the dipoles of one polarity move in the direction antiparallel to the dipoles of opposite polarity. The dipoles experience inelastic collisions, head to tail and head to head. The head-to-tail collisions of similarly oriented dipoles result in linear aggregation parallel to the backflow, while the head-to-head collisions lead to aggregation in a transversal direction. The two flows are separated by an impact parameter that controls the frequency of head-to-head collisions and thus the resulting shape of aggregates, their anisotropy and fractal dimension. We discuss in detail the numerous forces of elastic, electrophoretic and dielectrophoretic nature, as well as gravity forces, acting on colloidal spheres suspended in an electrically driven slab of a nematic liquid crystal. The proposed “nematic collider” offers opportunities in the quantitative studies of non-equilibrium processes such as anisotropic aggregation and jamming, as well as in the development of systems capable of self-assembly through anisotropic interactions of the building units.

Mobile phone as a platform for numerical simulation

In this work numerical simulations performed on mobile devices equipped with ARM microprocessors are shown. Calculations include: light propagation in linear and nonlinear media based on one-dimensional Schrödinger equation and molecules reorientation in nematic liquid crystals. The purpose of this publication is to show advantages and disadvantages of using mobile devices as a platform for education and research. Discussion about software development is provided.

The Orientational Fluctuations in the Re-Orientation Process of a Nematic Liquid Crystal

The field-induced molecular reorientation of liquid crystals is usually described by the Frank-Oseen equation with several simplifying assumptions. Among them the “classical” theory ignores thermal fluctuations in the liquid crystal structure. As a consequence, the static Frank-Oseen equation may give inconsistent results in some details of the reorientation process. This work is aimed to include formally the orientational fluctuations in the field-induced reorientation process in nematics. The simplified approach presented here is a “zero-order” approximation of the problem, nevertheless even in this form it allows for correct physical interpretation of the process for fields up to the threshold.

Optical Kerr Constant of Azo-Dye-Doped Nematic and Polymer-Dispersed Liquid Crystals Determined by Biphotonic Z-Scan Technique

We have recently investigated the optical Kerr constant (n2) of azo-dye-doped nematic liquid crystal (ADDLC) and polymer-dispersed liquid crystal (ADDPDLC) films using the Z-scan technique. The measurements of ADDLC samples are firstly performed with the simultaneous application of two pump beams, which are referred as the biphotonic Z-scan technique. The results show that n2 for ADDLC films is contributed primarily from the reorientation of liquid crystals induced by the photoisomerization of azo dyes and thermal effect, while that of ADDPDLC films with nano-sized LC droplets results from the thermal effect, and changes markedly with temperature.

Optical Reorientation and Trapping of Nematic Liquid Crystals Leading to the Formation of Micrometer-Sized Domain

We report on observation of optical reorientation of homogeneous nematic liquid crystals (LCs) thin slab of 4′-penthyl-4-cyanobiphenyl induced by a tightly focused near-infrared laser beam. We foun...

Efficient modeling of microstructure evolution in magnesium by energy minimization

The description of the complex interplay between deformation-induced twinning and dislocation slip, typical for metals showing an hcp structure such as magnesium, is of utmost importance for understanding their deformation behavior. In the present paper, an incremental energy principle is presented for that purpose. Within this principle, dislocation slip is modeled by crystal plasticity theory, while the phase decomposition associated with twinning is considered by a mixture theory. This mixture theory naturally avoids the decomposition of the twinning process into so-called pseudo- dislocations followed by a reorientation of the total crystal. By way of contrast, the proposed model captures the transformation of the crystal lattice due to twinning in a continuous fashion by simultaneously taking dislocation slip within both, possibly co-existent, phases into account. The shear strain induced by twinning as well as the deformation history are consistently included within the twinned domain by an enhanced multiplicative decomposition of the deformation gradient. Kinematic compatibility between the different phases is enforced by a Hadamard-type compatibility condition, while compatibility with respect to the boundary conditions requires the introduction of a boundary layer. The evolution of all state variables such as the twinning volume and the plastic strains associated with dislocation slip follow jointly and conveniently from minimizing the stress power of the total crystal. This canonical variational principle is closely related to the postulate of maximum dissipation and guarantees thermodynamical consistency of the resulting model. Particularly, the second law of thermodynamics is fulfilled. In contrast to previous models suitable for the analysis of the deformation systems in magnesium, the Helmholtz energy of the twinning interfaces and that of the aforementioned boundary layer are considered. Analogously, the energy due to twinning nucleation and that related to twinning growth are accounted for by suitable dissipation functionals. By doing so, the number of twinning laminates becomes an additional unknown within the minimization principle and thus, the thickness of the lamellas can be computed. Interestingly, by interpreting this thickness as the mean free path of dislocations, a size effect of Hall–Petch-type can naturally be included within the novel model. The predictive capabilities of the resulting approach are finally demonstrated by analyzing the channel die test. For that purpose, a certain rank-two laminate structure is considered. However, it bears emphasis that the proposed framework is very general and consequently, it can also be applied to other materials.

Alignment and Reorientation in Nematic Liquid Crystals Doped with Red Dye and Carbon Nanotube

Four different dyes (Methyl red, Disperse red 1, 13 and 60) were separately doped to each of four different nematic liquid crystals (E7, E8, E63 and ZLI-1132). Their solubilities, textures, phase transition temperatures and order parameters were determined. At the second stage, single-walled nanotubes in a small amount were separately added to each of these solutions, and the experiments were repeated as similar to previous ones. The solubility of dyes was sufficient for using in experiments. When adding dyes and nanotubes, an appreciable change in textures of liquid crystals was not observed. The phase transition temperatures stayed within the limitations required. The changes observed in order parameters depending on the voltage were very small. The highest value of order parameter (0.77) was attained when co-using red 60 dye and nanotube as dopants in the liquid crystal E63.

Tunable light diffractor associated with liquid crystal molecular reorientation from homogeneous alignment to periodic twist deformation in micrometre scale

We have demonstrated the formation of a transmissive tunable light diffractor with a fine grating period using a fringe-field switching (FFS) device. The presented device operates as a superposed amplitude and phase grating. The phase grating is formed by a thin slab of a nematic liquid crystal (LC) film and the amplitude grating is constructed by the electrode pattern of the FFS device. The electrode-position-dependent periodic deformation of the LC director as a function of the applied electric field is analysed and the respective occurrence of phase difference in monochromatic, coherent, linearly polarized He–Ne (632.8 nm) laser light is also calculated using the Jones matrix method at operating voltage. The same calculation results show that the transmitted beam of the present FFS diffractor is right-handed elliptically polarized. The operation characteristics of the thin diffracting device are investigated using a He–Ne laser source and a photodetector which shows a significantly large angular deviation of the diffracted beam owing to the small periodicity of LC deformation.

Retention of the Twinning Σ3 Misorientation in the Process of Lattice Transformation during Cold Rolling of a Fe3 Pct Si Single Crystal

The article investigates the twinning evolution of two active systems during cold rolling of a (110)[001]-oriented Fe3 pct Si single crystal. An intriguing fact is revealed that for both twinning systems the Σ3 misorientation is retained, regardless of the fundamental difference in their transformation in the course of the matrix crystal lattice reorientation. One of the twinning systems retains the regular habit plane position, while the other acquires an irregular one. A model is proposed that describes the mechanism of such transformations.

Mechanical recrystallization of ultra-strength tungsten nanoneedles

We present field-ion microscopy observation and crystallographic analysis which shows that plastic deformation mode in the tungsten nanotip in the field of high mechanical stresses occur by combining formations of strain localization bands and crystal lattice reorientations. Nanomechanical tensile tests in situ at about 15 GPa have revealed the large-angle reorientations of grains corresponding to the coincident site lattice with Σ = 33, which can be regarded as a result of reactions between lattice dislocations.

透過型Four Detectors Polarimeterの試作と液晶配向解析への応用

透過型Four Detectors Polarimeterの試作と液晶配向解析への応用

Translation calibration of inverse-kappa goniometers in macromolecular crystallography.

Precise and convenient crystal reorientation is of experimental importance in macromolecular crystallography (MX). The development of multi-axis goniometers, such as the ESRF/EMBL mini-κ, necessitates the corresponding development of calibration procedures that can be used for the setup, maintenance and troubleshooting of such devices. While traditional multi-axis goniometers require all rotation axes to intersect the unique point of the sample position, recently developed miniaturized instruments for sample reorientation in MX are not as restricted. However, the samples must always be re-centred following a change in orientation. To overcome this inconvenience and allow the use of multi-axis goniometers without the fundamental restriction of having all axes intersecting in the same point, an automatic translation correction protocol has been developed for such instruments. It requires precise information about the direction and location of the rotation axes. To measure and supply this information, a general, easy-to-perform translation calibration (TC) procedure has also been developed. The TC procedure is routinely performed on most MX beamlines at the ESRF and some results are presented for reference.

Molecular weight dependence of growth and morphology of it-poly(butene-1) spherulites

Molecular weight dependence of growth and morphology of spherulites of isotactic poly(butene-1), iPB-1, and those of the mixtures with atactic poly(butene-1), aPB-1, were examined by atomic force microscopy (AFM) and polarizing optical microscopy (POM) in order to examine the mechanism of the structural evolution by the branching and re-orientation of lamellar crystals at the growth front. The width of lamellar crystals and the characteristic size of the inner structure of spherulites decreased with increasing molecular weight. The result suggests that the mobility of the melt determines the sizes in spherulites and supports the growth front instability induced by a gradient triggering the branching. The sizes in the mixtures also decreased with increasing weight-averaged molecular weight, M w. The size dependence in low M w region, however, was too strong and that in high M w was too weak in comparison with the predicted dependence for the prepared M w. It has been concluded that the peculiar behaviors should be discussed with effective M w influenced by the occurrence of separation and exclusion of non-crystallizing aPB-1 at the growth front.

Electro-optic Kerr effect in polymer-stabilized isotropic liquid crystals

The electro-optic Kerr effect with a high Kerr constant around 10−9 m/V2 was observed in the polymer-stabilized isotropic liquid crystals over a wide temperature range. Optical isotropy at the field-off state was attained with randomly oriented domains of (chiral) nematic liquid crystals formed by a polymer matrix cured in the isotropic phase. Optical birefringence in the field-on state was induced by the reorientation of liquid crystals in the domains. Rapid rising and falling response times of the cells around 0.26 ms (0.52 ms total response time) were achieved. A theoretical model was proposed to explain the experimental results and agreed well with the results.

Numerical Simulations of Electrically Induced Birefringence in Photonic Liquid Crystal Fibers

It has been recently experimentally demonstrated that propagation and polarization properties of the photonic liquid crystal fibers can be effectively tuned with the electric field. In particular, effective electric tuning of the phase birefringence has been obtained in the photonic liquid crystal fibers based on the high index glasses. Accurate numerical simulations of the impact of electric field on the guiding properties of the photonic liquid crystal fibers require complex methods, in which all important physical properties of the liquid crystal are taken into account (optical anisotropy, molecular orientation and relatively high losses). In this paper we present two different numerical approaches based on the finite element method. First one utilizes the simplified assumption of the collective tilt of molecules, and gives rough estimation of the phase birefringence tuning range. The second approach is much more rigorous, since electrically induced reorientation of the liquid crystal is calculated with a Q-tensor method giving the values of the inhomogeneous permittivity tensor. The value of the electrically induced birefringence calculated with the second method is in a good agreement with the experimental results.

A Hybrid Micro­Macro­Model for the Description of Evolving Anisotropy in Finite Polycrystal Plasticity

AbstractThe paper outlines recent developments of an efficient computational micro‐macro modeling of evolving anisotropies in metallic polycrystals. Main focus is put onto large strain deformation processes where the anisotropy is caused by developments of crystallographic texture. We construct a hybrid micro‐macro framework that mixes ingredients of a purely macroscopic modeling with scale bridging operations of selected micromechanisms. On the micromechanical side, we develop a new algorithmic procedure to capture the crystal reorientation for evolving fcc and bcc textures based on a parametrization of rotations in the Rodigues space. The computational model provides a fast and robust method for the estimation of evolving textures. This computational tool for texture estimation is incorporated in a modular format into a micro‐macro‐model, where it governs the evolution of macrostructural tensors due to texture development. The general framework for the hybrid embedding is a purely phenomenological setting of anisotropic finite plasticity in the logarithmic strain space. The model provides an efficient and computationally handable two‐scale approach for the prediction of effects caused by complex microstructural changes in polycrystals. The capability of the proposed method is demonstrated by means of representative numerical examples. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)