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Full-Text Articles in Physics

Statistical Mechanics Of Bend Flexoelectricity And The Twist-Bend Phase In Bent-Core Liquid Crystals, Shaikh M. Shamid, Subas Dhakal, Jonathan V. Selinger May 2013

Statistical Mechanics Of Bend Flexoelectricity And The Twist-Bend Phase In Bent-Core Liquid Crystals, Shaikh M. Shamid, Subas Dhakal, Jonathan V. Selinger

Chemical Physics Publications

We develop a Landau theory for bend flexoelectricity in liquid crystals of bent-core molecules. In the nematic phase of the model, the bend flexoelectric coefficient increases as we reduce the temperature toward the nematic to polar phase transition. At this critical point, there is a second-order transition from high-temperature uniform nematic phase to low-temperature nonuniform polar phase composed of twist-bend or splay-bend deformations. To test the predictions of Landau theory, we perform Monte Carlo simulations to find the director and polarization configurations as functions of temperature, applied electric field, and interaction parameters.


Simulation Study Of Seemingly Fickian But Heterogeneous Dynamics Of Two Dimensional Colloids, Jeongmin Kim, Chanjoong Kim, Bong June Sung Jan 2013

Simulation Study Of Seemingly Fickian But Heterogeneous Dynamics Of Two Dimensional Colloids, Jeongmin Kim, Chanjoong Kim, Bong June Sung

Chemical Physics Publications

A two-dimensional (2D) solid lacks long-range positional order and is diffusive by means of the cooperative motion of particles. We find from molecular dynamics simulations of hard discs that 2D colloids in solid and hexatic phases show seemingly Fickian but strongly heterogeneous dynamics. Beyond translational relaxation time, the mean-square displacement is linear with time, t, implying that discs would undergo Brownian diffusion and the self-part of the van Hove correlation function [G(s)(r, t)] might be Gaussian. But dynamics is still heterogeneous and G(s)(r, t) is exponential at large r and oscillatory with multiple peaks at intermediate ...


Comment On "Raman Scattering Study Of Phase Biaxiality In A Thermotropic Bent-Core Nematic Liquid Crystal'' Reply, Bharat R. Acharya, Hyunchul Choi, Mohan Srinivasarao, Satyendra Kumar Feb 2011

Comment On "Raman Scattering Study Of Phase Biaxiality In A Thermotropic Bent-Core Nematic Liquid Crystal'' Reply, Bharat R. Acharya, Hyunchul Choi, Mohan Srinivasarao, Satyendra Kumar

Physics Publications

No abstract provided.


Homogeneous And Homeotropic Alignment Of Bent-Core Uniaxial And Biaxial Nematic Liquid Crystals, Hyungguen Yoon, Shin-Woong Kang, Matthias Lehmann, Jung Ok Park, Mohan Srinivasarao, Satyendra Kumar Jan 2011

Homogeneous And Homeotropic Alignment Of Bent-Core Uniaxial And Biaxial Nematic Liquid Crystals, Hyungguen Yoon, Shin-Woong Kang, Matthias Lehmann, Jung Ok Park, Mohan Srinivasarao, Satyendra Kumar

Physics Publications

The effect of well-established homeotropically and homogeneously aligning polyimides, developed for calamitic liquid crystals, on the alignment of the nematic phases of bent-core mesogens was investigated using polarizing optical microscopy and conoscopy. Two polyimides, the homogeneously aligning SE7792 and the homeotropically aligning S659, were used to align three bent-core mesogens. Two of the mesogens, with an azo group in one of their arms, are found to align in a manner similar to the calamitic mesogens, while a fluorenone core based mesogen counter-intuitively aligns homogeneously on S659 coated surface and homeotropically on SE7792. Attractive interactions between unidirectionally oriented polyimide side-chains and ...


Lyotropic Chromonic Liquid Crystal Semiconductors For Water-Solution Processable Organic Electronics, V. G. Nazarenko, O. P. Boiko, M. I. Anisimov, A. K. Kadashchuk, Yuriy A. Nastishin, A. B. Golovin, Oleg Lavrentovich Dec 2010

Lyotropic Chromonic Liquid Crystal Semiconductors For Water-Solution Processable Organic Electronics, V. G. Nazarenko, O. P. Boiko, M. I. Anisimov, A. K. Kadashchuk, Yuriy A. Nastishin, A. B. Golovin, Oleg Lavrentovich

Chemical Physics Publications

We propose lyotropic chromonic liquid crystals (LCLCs) as a distinct class of materials for organic electronics. In water, the chromonic molecules stack on top of each other into elongated aggregates that form orientationally ordered phases. The aligned aggregated structure is preserved when the material is deposited onto a substrate and dried. The dried LCLC films show a strongly anisotropic electric conductivity of semiconductor type. The field-effect carrier mobility measured along the molecular aggregates in unoptimized films of LCLC V20 is 0.03 cm(2) V-1 s(-1). Easy processibility, low cost, and high mobility demonstrate the potential of LCLCs for ...


Surface Alignment, Anchoring Transitions, Optical Properties, And Topological Defects In The Nematic Phase Of Thermotropic Bent-Core Liquid Crystal A131, B. Senyuk, H. Wonderly, M. Mathews, Q. Li, Sergij V. Shiyanovskii, Oleg Lavrentovich Oct 2010

Surface Alignment, Anchoring Transitions, Optical Properties, And Topological Defects In The Nematic Phase Of Thermotropic Bent-Core Liquid Crystal A131, B. Senyuk, H. Wonderly, M. Mathews, Q. Li, Sergij V. Shiyanovskii, Oleg Lavrentovich

Chemical Physics Publications

We study optical, structural, and surface anchoring properties of thermotropic nematic bent-core material A131. The focus is on the features associated with orientational order as the material has been reported to exhibit not only the usual uniaxial nematic but also the biaxial nematic phase. We demonstrate that A131 experiences a surface anchoring transition from a perpendicular to tilted alignment when the temperature decreases. The features of the tilted state are consistent with surface-induced birefringence associated with smectic layering near the surface and a molecular tilt that changes along the normal to the substrates. The surface-induced birefringence is reduced to zero ...


Nematic Biaxiality In A Bent-Core Material, Hyungguen Yoon, Shin-Woong Kang, Ronald Y. Dong, Alberto Marini, Kattera A. Suresh, Mohan Srinivasarao, Satyendra Kumar May 2010

Nematic Biaxiality In A Bent-Core Material, Hyungguen Yoon, Shin-Woong Kang, Ronald Y. Dong, Alberto Marini, Kattera A. Suresh, Mohan Srinivasarao, Satyendra Kumar

Physics Publications

The results of a recent investigation of the nematic biaxiality in a bent-core mesogen (A131) are in apparent disagreement with earlier claims. Samples of mesogen A131 used in the two studies were investigated with polarized optical microscopy, conoscopy, carbon-13 NMR, and crossover frequency measurements. The results demonstrate that textural changes associated with the growth of biaxial nematic order appear at similar to 149 degrees C. The Maltese cross observed in the conoscopic figure gradually splits into two isogyres at lower temperatures indicating phase biaxiality. Presence of the uniaxial to biaxial nematic phase transition is further confirmed by temperature trends of ...


Distance Of Closest Approach Of Two Arbitrary Hard Ellipses In Two Dimensions, Xiaoyu Zheng, Peter Palffy-Muhoray Jun 2007

Distance Of Closest Approach Of Two Arbitrary Hard Ellipses In Two Dimensions, Xiaoyu Zheng, Peter Palffy-Muhoray

Chemical Physics Publications

The distance of closest approach of hard particles is a key parameter of their interaction and plays an important role in the resulting phase behavior. For nonspherical particles, the distance of closest approach depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed for use in computer simulations [ Vieillard-Baron J. Chem. Phys. 56 4729 (1972); Perram and Wertheim J. Comput. Phys. 58 409 (1985)], no analytic solutions have been obtained for the distance of closest approach of ellipsoids in three dimensions, or, until now, for ellipses in two dimensions. We have derived an analytic expression ...


Distance Of Closest Approach Of Two Arbitrary Hard Ellipsoids, Xiaoyu Zheng, Wilder Iglesias, Peter Palffy-Muhoray Jun 2007

Distance Of Closest Approach Of Two Arbitrary Hard Ellipsoids, Xiaoyu Zheng, Wilder Iglesias, Peter Palffy-Muhoray

Chemical Physics Publications

The distance of closest approach of hard particles is a key parameter of their interaction and plays an important role in the resulting phase behavior. For nonspherical particles, the distance of closest approach depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed for use in computer simulations [ Vieillard-Baron J. Chem. Phys. 56 4729 (1972); Perram and Wertheim J. Comput. Phys. 58 409 (1985)], no analytic solutions have been obtained for the distance of closest approach of ellipsoids in three dimensions, or, until now, for ellipses in two dimensions. We have derived an analytic expression ...


Nonlocal Model For Nematic Liquid-Crystal Elastomers, R. Ennis, L. C. Malacarne, Peter Palffy-Muhoray, M. Shelley Jan 2006

Nonlocal Model For Nematic Liquid-Crystal Elastomers, R. Ennis, L. C. Malacarne, Peter Palffy-Muhoray, M. Shelley

Chemical Physics Publications

We have developed a fully nonlocal model to describe the dynamic behavior of nematic liquid-crystal elastomers. The free energy, incorporating both elastic and nematic contributions, is a function of the material displacement vector and the orientational order parameter tensor. The free energy cost of spatial variations of these order parameters is taken into account through nonlocal interactions rather than through the use of gradient expansions. We also give an expression for the Rayleigh dissipation function. The equations of motion for displacement and orientational order are obtained from the free energy and the dissipation function by the use of a Lagrangian ...


Biaxial Nematic Phase In Bent-Core Thermotropic Mesogens, Bharat R. Acharya, Andrew Primak, Satyendra Kumar Apr 2004

Biaxial Nematic Phase In Bent-Core Thermotropic Mesogens, Bharat R. Acharya, Andrew Primak, Satyendra Kumar

Physics Publications

A biaxial nematic phase had been predicted with D2h symmetry, wherein the mesogen’s long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, n and m, respectively. The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality. The results of x-ray diffraction can be readily reproduced by ab initio calculations that explicitly include the bent-core shape in the form factor and assume short-range positional correlations.


Deuteron Nmr Investigation Of A Photomechanical Effect In A Smectic-A Liquid Crystal, Boštjan Zalar, Oleg Lavrentovich, Huairen Zeng, Daniele Finotello Aug 2000

Deuteron Nmr Investigation Of A Photomechanical Effect In A Smectic-A Liquid Crystal, Boštjan Zalar, Oleg Lavrentovich, Huairen Zeng, Daniele Finotello

Chemical Physics Publications

Deuteron quadrupole-perturbed NMR is used to study the perturbation of orientational order in a smectic-A liquid crystal (octylcyanobiphenyl or 8CB) caused by photoinduced trans-to-cis isomerization of a photosensitive dopant (diheptylazobenzene or 7AB). The time and temperature dependences of the orientational order were independently studied for 8CB, 7AB, and their mixtures. Upon UV irradiation that causes trans-to-cis isomerization of 7AB, the orientational order parameter of the smectic-A phase is reduced. Relaxation in the dark exponentially restores the equilibrium value of the order parameter. The characteristic time for this process closely matches the lifetimes of the 7AB excited state. While ...


Critical Behavior At The Nematic To Lamellar Phase Transitions: A Synchrotron X-Ray Scattering Study, S. T. Shin, J. D. Brock, Mark Sutton, J. D. Lister, Satyendra Kumar Apr 1998

Critical Behavior At The Nematic To Lamellar Phase Transitions: A Synchrotron X-Ray Scattering Study, S. T. Shin, J. D. Brock, Mark Sutton, J. D. Lister, Satyendra Kumar

Physics Publications

The nematic (N) to lamellar (Lα) phase transition in binary mixtures of cesium-perfluoro-octanoate (CsPFO) and water has been studied by high-resolution synchrotron x-ray scattering at 46.6 weight % CsPFO. The longitudinal correlation length ξ and the susceptibility σ associated with the lamellar phase fluctuations in the N phase, measured over three decades of reduced temperature, diverge with critical exponents ν=0.86±0.04 and γ=1.37±0.11, respectively. These results show that the N to Lα phase transition is quantitatively similar to the N to smectic-A phase transition of thermotropic liquid crystals with a ...


Phase-Transitions In Finite-Thickness Nematic Liquid-Crystal Films - Planar Anchoring, Natasha Kothekar, David W. Allender, R. M. Hornreich Oct 1995

Phase-Transitions In Finite-Thickness Nematic Liquid-Crystal Films - Planar Anchoring, Natasha Kothekar, David W. Allender, R. M. Hornreich

Physics Publications

Phase transitions in finite-thickness nematogenic materials bounded by two planar surfaces and characterized by identical surface interactions linearly proportional to the order parameter have been studied theoretically by solving the coupled nonlinear Euler-Lagrange equations. The surface interaction was assumed to favor molecular orientation in the surface plane with no rubbed or preferred direction. The related problem of a semi-infinite film having a single surface has been studied previously at temperatures above the bulk nematic-isotropic phase transition point T-NI. For that geometry and physically relevant elastic constants, it was shown that, in addition to the bulk transition, there is a second ...


Ac-Calorimetry Studies At The Hexatic-B Smectic-A And Crystal-B Hexatic-B Phase-Transitions In 2 Compounds With Hydrogen-Bonding, Germano Iannacchione, Ewa Gorecka, Satyendra Kumar, Daniele Finotello Apr 1995

Ac-Calorimetry Studies At The Hexatic-B Smectic-A And Crystal-B Hexatic-B Phase-Transitions In 2 Compounds With Hydrogen-Bonding, Germano Iannacchione, Ewa Gorecka, Satyendra Kumar, Daniele Finotello

Physics Publications

Two compounds of the 1-(4’-alkoxyphenylamino-3-[5’’-(2’’-menthylpirydil)]-propen-1-one-3 (PIRn) series that exhibit the hexatic-B (Hex-B) to smectic-A (Sm-A) and the crystal-B (Cry-B) to Hex-B phase transitions were studied by high resolution ac calorimetry. The Hex-B–Sm-A phase transition was found to be of first order and continuous for PIR7 and PIR9, respectively. The tricritical point was located at the concentration for which TCry-B–Hex-B/THex-B–Sm-A =0.980±0.004, i.e., a hexatic range of 7 K. At both sides ...


X-Ray Studies Of The Hexatic Phase In Liquid-Crystals With A Crystal-B-Hexatic-B-Smectic-A Phase Sequence, Ewa Gorecka, Li Chen, Wieslaw Pyzuk, Adam Krowczynski, Satyendra Kumar Oct 1994

X-Ray Studies Of The Hexatic Phase In Liquid-Crystals With A Crystal-B-Hexatic-B-Smectic-A Phase Sequence, Ewa Gorecka, Li Chen, Wieslaw Pyzuk, Adam Krowczynski, Satyendra Kumar

Physics Publications

Free-standing monodomain thick films of three liquid crystalline compounds were examined by x-ray scattering. These compounds exhibit a unique enantiotropic crystal-B-hexatic-B-smectic-A phase sequence with no evidence of herringbone order. The hexatic order parameter Cg changes in the hexatic phase with critical exponent beta=0.15+/-0.03. Temperature dependence of the in-plane positional order correlation length suggests a strongly first-order hexatic-B to crystal-B phase transition with indiscernible enthalpy change.