Twist Distortion Research Articles

Flexural capacity of hollow flange beams

The hollow flange beam (HFB) is a unique cold-formed steel section developed in Australia for use as a flexural member. Research has identified that the HFB section's flexural capacity for intermediate span members is limited by lateral distortional buckling, which is characterized by simultaneous lateral deflection, twist, and web distortion. This buckling behaviour is mainly due to the unique geometry of the section, comprising two torsionally stiff triangular flanges connected by a slender web. This paper presents a finite element analytical model suitable for non-linear analysis of HFB flexural members. The model includes all significant effects that may influence the ultimate capacity of such members, including material inelasticity, local buckling, member instability, web distortion, residual stresses, and geometric imperfections. It was found to accurately predict both the elastic lateral distortional buckling moments and the ultimate capacities of HFB flexural members, and was therefore used in the development of design curves and suitable design procedures.

Using Liquid Crystals To Image Reactants and Products of Acid−Base Reactions on Surfaces with Micrometer Resolution

We report that nematic liquid crystals (LCs) of 4-cyano-4‘-pentylbiphenyl (5CB) and p-methoxybenzylidene-p-n-butylaniline (MBBA) can be used to image reactants and products of reversible, acid−base reactions on surfaces with micrometer resolution. When supported on obliquely deposited films of gold, azimuthal orientations of LCs measured on self-assembled monolayers (SAMs) formed from HOOC(CH2)10SH are found to be orthogonal to those measured on SAMs of the corresponding sodium carboxylate salt. Conversion of the carboxylic acid to its sodium salt is amplified into a change in orientation of 104−105 mesogens per surface-confined acid group. The change in orientation of the LC is easily transduced into an optical output by forming twist distortions within the LC upon conversion of the carboxylic acid to its sodium carboxylate salt. The threshold conversion of acid to salt that drives the formation of the twist distortion within the supported LC can be systematically shifted between 8% and 100% by using mix...

The structures of S0 spheroidene in the light-harvesting (LH2) complex and S0 and T1 spheroidene in the reaction center of Rhodobacter sphaeroides 2.4.1 as revealed by Raman spectroscopy

The Raman spectrum of S0 spheroidene in the light-harvesting (LH2) complex (LHC) and those of S0 and T1 spheroidene in the reaction center (RC) of Rhodobacter sphaeroides 2.4.1 were recorded. Comparison of the S0 Raman spectrum of the all-trans isomer bound to the LHC of R. sphaeroides 2.4.1 with that free in n-hexane solution suggests that the LHC-bound carotenoid takes a flat, all-trans configuration with possible distortion in the plane of the conjugated chain. On the other hand comparison of the S0 Raman spectrum of the 15-cis isomer bound to the RC with that free in n-hexane solution suggests that the RC-bound carotenoid takes a 15-cis configuration which is twisted around the C15(double bond)C15′ bond and distorted in the (single bond)C15H(double bond)C15′H(single bond) plane. The T1 Raman spectra of the RC-bound spheroidene indicated substantial twisting and in-plane distortion of the conjugated backbone. Based on the above results, a mechanism of triplet-energy dissipation by the RC-bound carotenoid, which involves an internal rotation around the C15(double bond)C15′ bond, is proposed. © 1996 John Wiley & Sons, Inc.

Surface-induced nematic ordering and the localization of a twisted distortion in a nematic cell

A Landau--de Gennes phenomenological model is used to analyze the coupling between the surface-induced scalar order parameter variation and the orientational ordering in a nematic slab, considering also nonplanar distortions where the twist angle $\ensuremath{\omega}$ can vary. The results show that a weak variation $\ensuremath{\delta}\ensuremath{\omega}$ additional to that predicted for a pure twist deformation by the Frank elastic theory can appear. This variation can be either globally spread over the whole sample or localized to a subsurface region of thickness comparable to the nematic correlation length. In the former case $\ensuremath{\delta}\ensuremath{\omega}$ results from a global tilt angle variation, while in the latter it is caused by a variation of the scalar order parameter S close to the surface. In contrast to the recently analyzed S variation-induced subsurface deformation of the tilt angle [see, e.g., G. Ska\ifmmode \check{c}\else \v{c}\fi{}ej et al., Phys. Rev. E 57, 1780 (1998)], an intrinsic (chiral nematic) or confinement-induced global twist is needed to yield the localized $\ensuremath{\delta}\ensuremath{\omega}$ effect.

Atomic resolution STM imaging of a twisted single-wall carbon nanotube

We present atomically-resolved STM images of single-wall carbon nanotubes (SWNTs) embedded in a crystalline nanotube rope. Although they may be interpreted as of a chiral nanotube, the images are more consistently explained a an achiral armchair tube with a quenched twist distortion. The existence of quenched twists in SWNTs in ropes might explain the fact that both as-grown bulk nanotube material and individual ropes have insulator-like conductivity at low temperature.

Ab initio MO study on the periodic trends in structures and energies of hypervalent compounds: five-, six-, and seven-coordinated XF 5, XH −6, XF −6, XH 2−7 and XF 2−7 species containing a group 15 central atom (where X is P, As, Sb, Bi)

A systematic ab initio study employing effective core potentials (ECP) on the central atoms has been carried out to predict structures, stabilities and the resultant periodic trends for the five hypervalent series XF 5, XH − 6, XF − 6, XH 2− 7 and XF 2− 7, where X is P, As, Sb and Bi. The D 3h and C 4v structures of XF 5 correspond to minima and pseudorotation transition states, respectively. The pseudorotation barriers are found to decrease with increasing size of X, being 4.3 (X=P), 3.0 (X=As), 1.8 (X=Sb) and 1.3 kcal mol −1 (X=Bi). This trend parallels the trend in the differences in axial and equatorial XF distances in XF 5 ( D 3h). XH − 6 ( O h) and XF − 6 ( O h) are found to be minima for all X, whereas their D 3h counterparts correspond to transition states for trigonal twist distortion. In contrast to the fluxional five-coordinated neutrals XH 5 and XF 5, the six-coordinated anions XH − 6 and XF − 6 are predicted to be rigid with the barriers to the O h− D 3h distortion ranging from 25 to 65 kcal mol −1. These barriers also decrease monotonically down the column following the trend in the differences [X-L( D 3h)]−[X-L( O h)], where [X-L()] is the distance between X and L=H or F in structure D 3h or O h. All XH − 6 and XF − 6 are indicated to be thermodynamically stable with respect to loss of H − and F −, respectively. On the other hand, XH − 6 appear to be unstable toward the loss of H 2 molecule, a feature also shown by the four-coordinated XH − 4 and five-coordinated XH 5 hypervalent hydrides studied by us earlier. The periodic trends revealed in the thermodynamic stabilities of XH − 6 and XF − 6 are irregular. No stable structure is found for the XH 2− 7 heptahydrides. By contrast, all XF 2− 7 heptafluorides are predicted to be structurally stable at the RHF level (zero imaginary frequencies), exhibiting pentagonal bipyramidal D 5h geometries for X=P, As, Sb, but a distorted pentagonal bipyramid of C 1 symmetry for X=Bi. XF 2− 7 species are unknown and their synthesis is expected to be very difficult to achieve, mostly due to the significant thermodynamic instabilities of these anions toward the loss of F −.

Chiral symmetry breaking in three-dimensional smectic-Cliquid-crystal domains

We report an observation of a unique type of spontaneous chiral symmetry breaking in three-dimensional domains of a smectic-C material consisting of achiral molecules. The observed helical structure clearly demonstrates the effect of an elastic coupling between the bend and the twist distortions in the c field. The sign and the magnitude of the coupling coefficient are determined experimentally. We also demonstrate that an external chiral bias field favors domains of one handedness.

A numerical technique for predicting microstructure in liquid crystalline polymers

A numerical technique has been developed to model texture in nematic liquid crystals. The technique differentiates between splay, twist and bend distortions and includes splay-splay compensation. The technique is tested by the simulation of the Freedericksz transition and by the determination of minimum energy director fields for specific boundary conditions. To model the bulk, periodic boundary conditions are imposed. The effect of elastic anisotropy on disclination character has been investigated by terminating simulations before all the defects have been annihilated. With a low twist constant, twist disclinations are observed; with a high twist constant, wedge disclinations are observed. With a low twist constant and high splay constant, realistic for polymeric liquid crystals, features observed experimentally are simulated.

Monolithic Crystal Bender for Dynamical Sagittal Focusing with Compensation for Anticlastic Curvature and Twist Distortions

Two types of diamond shaped monolithic crystal benders have been developed for dynamical sagittal focusing on the general purpose EXAFS beamline BM29 at the ESRF. The commissioning tests proved to be fairly encouraging and confirmed that the most convenient option was to provide an active correction for the residual twist deformations combined with a static control of the antielastic curvature.

On the Frank Elastic Constants of Lyotropic Polymer Liquid Crystals

The Frank elastic constants of lyotropic polymer liquid crystals are formulated using an equivalent freely-jointed chain model, which can be uniquely defined by the parameters of the corresponding wormlike chain and the polymer concentration. The contribution of the higher virial terms to the elastic constants was taken into account on the basis of the scaled particle theory used previously for undistorted nematic polymer solutions. The fomulated elastic constants are applicable to systems with any polymer conformation from the rod limit to the coil limit and at arbitrary concentration and degree of orientation. The theoretical elastic constants for twist and bend distortions agree semiquantitatively with most experimental results for the elastic constants of lyotropic liquid crystals of poly(γ-benzyl glutamate) with no adjustable parameters.

Intermittency at the DSM1-DSM2 Transition in Nematic Liquid Crystal Films

Abstract In this letter we report on the turbulence to turbulence transition, which occurs in a homogeneously oriented nematic liquid crystal (NLC) film under the action of an external alternate voltage. Our observation show that the transition is characterized by a NLC-substrate anchoring breaking, which results in the on set of the twist distortion of the molecular director field. We give the characterization of the different dynamic regimes in terms of the probability distribution functions (PDF's) of the director field curvature energy. The PDF's, which are undoubtedly non-gaussian, indicate that, in both turbulent regimes, the curvature energy is highly intermittent.

Active monolithic crystal bender for dynamical sagittal focusing

We describe a monolithic crystal bender with compensated anticlastic curvature or twist distortions. Interferometric measurements confirm that with sagittal radii of the order of 5 m, the meridional curvature can be kept larger than 800 m. An improved version is yet in progress.

Bend-stripes in hybrid aligned nematic layers

Abstract The occurrence of bend-stripes also involving twist distortion is investigated in hybrid aligned nematic layers, with stronger tilt-anchoring at the homeotropic substrate than at the planar one. The modulated structure is found to exist above the thickness threshold d a for the aperiodic bend-splay deformation. The periodicities of the director azimuth and polar angle are incommensurate along the normal to the cell plates. In principle, two independent periodic modes are possible, the first one stable, with threshold d p = d a, the second one metastable, with threshold d p > d a, according to the values of the torsional extrapolation lengths. Both transitions are continuous with respect to the in-plane wavenumber. The second mode can appear only if the difference between the torsional and the tilt extrapolation lengths is positive at the planar substrate. Moreover, both thresholds are independent of the saddle-splay elastic constant.

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer–solvent mixtures

Abstract The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K 22 and viscosity coefficient γ1. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K 22 coupled with large increases in γ1. The decrements in K 22 are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.

Freeze-fracture TEM of the helical smectic A* phase

A new liquid crystal (called the smectic-A* phase) that combines cholesteric twist and smectic layering was a surprise as smectic phases preclude twist distortions. However, the twist grain boundary (TGB) model of Renn and Lubensky predicted a defect-mediated smectic phase that incorporates cholesteric twist by a lattice of screw dislocations. The TGB model for the liquid crystal analog of the Abrikosov phase of superconductors consists of regularly spaced grain boundaries of screw dislocations, parallel to each other within the grain boundary, but rotated by a fixed angle with respect to adjacent grain boundaries. The dislocations divide the layers into blocks which rotate by a discrete amount, Δθ, given by the ratio of the layer spacing, d, to the distance between grain boundaries, lb; Δθ ≈ d/lb (Fig. 1).

Dynamic light scattering from nematic monodomains containing mesogenic polymers of differing architectures

Nematic monodomains which contain liquid crystal polymers [CH 2 C(Me)(COO(CH 2 ) 6 OPhC(Me)=C(Me)PhOMe)] n , [CH 2 CH(O[CH 2 ] m OPhPhCN)] n and [OPhPhCH 2 CH(Et)PhO(CH 2 ) m ] n dissolved in a low molar mass nematogen, 4'-n-pentyl-4-cyanobiphenyl (5CB), are described. The relaxation rates of the splay, twist, and bend director distortions have been measured and compared with those of pure 5CB at comparable temperature decrements T N-I from the nematic to isotropic transition. Addition of both side-chain and main-chain polymers results in substantial decreases in the relaxation rates of all three modes

Magnetic Field Induced Instability on an Aligned Nematic Solution of a Rodlike Polymer

Abstract The reorientation of an initially well aligned nematic rodlike solution of the rodlike poly(1,4-phenylene-2,6-benzobisthiazole), PBT, in an external magnetic field in the sample plane is studied. For angle β between the magnetic field H and the initial director n less than 45 degrees, the reorientation occurs by a homogeneous twist distortion of the director field, permitting assessment of the Frank elastic constant KT and the Leslie viscosity ηT. For larger β, the reorientation is nonhomogeneous, with a component of the distortion out of the sample plane, resulting in a phase-grating, made manifest by its focusing effects for transmitted light.

Dynamic Light Scattering from a Side-Chain Liquid Crystal in a Nematic Solvent

Abstract Nematic monodomains consisting of a side-chain liquid crystal polymer dissolved in a low-molar mass nematogen, pentylcyanobiphenyl, were prepared in planar and homeotropic alignment at polymer concentrations up to 15% w/w. The elastic and viscosity constants associated with the splay, bend and twist distortions of the nematic matrix were determined by analysis of the dynamic light scattering data on these monodomains over a wide range of scattering angles. The addition of the side-chain liquid crystal polymer results in a substantial decrease in the relaxation frequencies for dynamic light scattering. These changes are due to the combined contributions of a decrease in all three elastic constants and larger increases of the three viscosities. The effect is largest for the bend distortion.

INFLUENCE OF TWIST DISTORTION ON THE TRANSITION POINT BETWEEN TWO NEMATIC SUBPHASES N1 AND N2 OF HOBA

The liquid crystal HOBA exhibits a nematic phase between the achiral smectic C region and the isotropic one. In the nematic phase two subphases N1 and N2 can be distinguished. A shift of the transition temperature N1 − N2 has been observed, if an elastic deformation is involved, with respect to the unidirectional homogeneous case. In the present paper the experimental data concerning a twist distortion are discussed, and a theoretical model is given.

Ferronematics in Magnetic and Electric Fields

Abstract We consider the effect of a magnetic field on a ferronematic. It is shown that first order Freedericksz transitions are possible in such systems as a consequence of elastic and diamagnetic anisotropies. Interestingly the possibility exists for a first order transition from a splay-bend distortion to a pure twist distortion or vice versa at a second threshold. A super-imposed electric field can also result in a first order Freedericksz transition.