Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W8: Liquid Crystals II: Dynamics and Topology |
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Sponsoring Units: DFD Chair: Robin Selinger, Kent State University Room: Morial Convention Center RO6 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W8.00001: Finite element studies of the soft elastic response in liquid crystal elastomers Badel Mbanga, Jonathan Selinger, Robin Selinger When a liquid crystal elastomer film is stretched in a direction transverse to the nematic director, the resulting stress-strain curve typically displays a plateau region, showing a characteristic soft elastic response. Using 3-d nonlinear finite element simulation, we model the formation of orientational domains controlling this mechanical behavior. We investigate the force-displacement response as a function of strain rate, and explore geometric frustration arising from boundary conditions imposed by clamps. We also model mechanical response of polydomain films aligned under external strain. Our 3-d finite element algorithm is based on a Hamiltonian with terms representing elastic potential energy, kinetic energy, and coupling between elastic strain and nematic order. We assume that orientational order of the material relaxes quickly and remains in local quasistatic equilibrium with the instantaneous local strain. Internal dissipation is also included. We intend through this model to further our understanding of the basic physics governing the dynamic mechanical response of nematic elastomers and also provide a useful computational tool for design and testing of potential engineering device applications. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W8.00002: Fluctuating hydrodynamics of nematics for models of liquid-crystal based biosensors via lattice Boltzmann simulations Orlando Guzman, Jose Antonio Velez, David Casta\~neda Experimental biosensors based on liquid crystals (LC) use nematics to detect the presence of specific analytes, via the optical textures exhibited by the LC at long times. Efforts to model the time evolution of these textures have relied on relaxational models, ignoring transport phenomena. In this work we include hydrodynamics into a model for these LC biosensors, using lattice Boltzmann (LB) methods and assess the effect on the lifetime of multidomain structures, characteristic of high concentrations of analyte. We apply Yeoman's et al. LB algorithm, which reproduces the hydrodynamic equations developed by Beris and Edwards for LCs. We also take into account thermal fluctuations, by adding random perturbations to the hydrodynamic modes. Following Adhikari et al., their amplitude is determined by the Fluctuation-Dissipation theorem and we excite both hydrodynamic and the sub-hydrodynamic modes (also called ghost modes). As a result, we analyze the influence of the fluctuations and hydrodynamics on the movement of topological defects. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W8.00003: Viscous properties of bent core nematic liquid crystals measured using a nanoliter viscometer Christopher Bailey, John Harden, Antal Jakli Since the development of bent core liquid crystal mesogens showing the nematic phase, much work has been done to study the physical properties of these materials in the form of dielectric spectroscopy, dynamic light scattering, and magnetic field induced phase transitions. Some results of these studies showed interesting behaviors such as slow biaxial fluctuations and the possibility of long range tetrahedratic ordering above the nematic phase. Here we report rheological and optical studies on several bent core mesogens in their isotropic and nematic phases. For the rheological studies, we built a viscometer capable of measuring viscoelastic properties with two centipoise resolution with only using 10nL of fluid. Results show abnormal viscosity and optical behavior near the isotropic to nematic phase transition. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W8.00004: Electrically induced twist in smectic liquid-crystalline elastomers Jonathan Selinger, Robin Selinger, Christopher Spillmann, Jawad Naciri, B. R. Ratna Liquid-crystalline elastomers are cross-linked polymer networks covalently bonded to liquid-crystalline units, so that any change in the orientational order influences the shape of the polymer network. As a new approach to developing electrically controllable actuators, we prepare elastomers of chiral smectic-A liquid crystals, which have an electroclinic effect, i.e. a molecular tilt induced by an applied electric field. For thin films in a bookshelf orientation, one would expect the electroclinic effect to cause an in-plane shear of the elastomer, leading to a distortion from a rectangle to a parallelogram. Surprisingly, our experiments find instead that an electric field causes a twisting of the film out of the plane, leading to a helically curved shape. The twist is rapid and reversible, with a helical sense that depends on the sign of the applied field. To explain this electrically induced twist, we develop a continuum elastic theory based on the assumption that the film has an asymmetry between front and back, which can be attributed to the preparation conditions. We further present finite-element simulations of the twist, which show the dynamic shape change. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W8.00005: Topography induced homeotropic alignment of nematic liquid crystals Youngwoo Yi, Noel Clark, Joseph Maclennan, Vaibhav Khire, Christopher Bowman We probe the orientation of a nematic liquid crystal on a planar aligning polymer film, where the film is topographically modified with sub micron scale checkerboard patterns. Alignment is studied in hybrid LC cells with the patterned polymer film on one plate and a self-assembled monolayer (homeotropic) on the other plate. A transition to homeotropic alignment on the pattern is observed as the pattern scale is reduced. An analysis shows that as the size of a square well becomes smaller the elastic energy of LCs decreases more slowly than the surface anchoring energy of the well, so that when the size of the well becomes small enough the elastic energy in planar configuration becomes comparable or larger than the polar anchoring energy of the surface. [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W8.00006: Flexoelectric effect in a bent-core liquid crystal measured by Dynamic Light Scattering Madhabi Majumdar, K. Neupane, James. T. Gleeson, Antal Jakli, Samuel Sprunt Flexoelectricity is a linear coupling between electric polarization and elastic flexure in liquid crystals [1]. Although typically quite weak in calamitic LCs, the flexoelectric effect has recently been shown, by direct (electromechanical) measurement of the flexure-induced polarization, to be enhanced by several orders of magnitude in certain bent-core nematic (BCN) liquid crystals [2]. We report here an application of dynamic light scattering to measure the flexoelectric coefficient ($e1 + e3$) of BCNs through coupling of polarization to elastic fluctuation modes of the optic axis. Our results agree in order of magnitude with the values obtained by the electromechanical method.\\~[1]~R.B. Meyer, \emph{Phys. Rev. Lett}. 22, 918 (1969).\\~[2]~J. Harden, B. Mbanga, N. Eber, K. Fodor-Csorba, S. Sprunt, J. T. Gleeson, A. Jakli, \emph{Phys. Rev. Lett}. 97, 157802 (2006). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W8.00007: Measurement of the Converse Flexoelectric Effect of a Bent-Core Nematic Liquid Crystal John Harden, Richelle Teeling, Samuel Sprunt, James Gleeson, Antal Jakli Flexoelectricity is a linear coupling between bend or splay distortions and electric polarization$^{1}$. It is a unique property of orientationally ordered materials of which liquid crystals are the best known example. It has been shown that the bend flexoelectric coefficient in ``banana'' bent-core liquid crystals is three orders of magnitude higher than the effect found in calamitic liquid crystals$^{2}$. Using a Mirau interferometer attached to the objective port of a microscope, we were able to measure the converse effect. This polarity dependent flexing of a thin cell yielded displacements of 100nm when 100V DC was applied to a 1cm x 2cm x 25$\mu $m cell filled with the bent-core nematic liquid crystal 4-chloro-1,3-phenylene bis 4-[4'-(9-decenyloxy) benzoyloxy] benzoate (ClPbis10BB). The substrates were 100$\mu $m thick Mylar with ITO as a conducting layer. These preliminary experiments show the promise of new types of soft actuators or beam steering devices. References: $^{1}$Meyer R.B. (1969). \underline {Physical Review Letters} \textbf{22}(18): 918-921. $^{2}$Harden, J., B. Mbanga, et al. (2006). \underline {Physical Review Letters} \textbf{97}(15). Acknowledgement: NSF DMR-0606160 and NSF REU-0649017 [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W8.00008: Envelope Dynamics of an Experimental Electroconvection Pattern G. Acharya, G. Dangelmayr, I. Oprea, J.T. Gleeson A video displaying electroconvection of the nematic I52 is analyzed. Spatial Fourier transforms of the frames reveal that the dynamics is driven by four groups of oblique (relative to the director) modes corresponding to counterpropagating pairs of traveling waves. This is consistent with a stability analysis of the electrohydrodynamic equations, which predict for I52 a Hopf bifurcation with four oblique critical wave numbers. Beyond the linear stability analysis, a weakly nonlinear analysis tells that the evolution of the pattern is governed by four slowly varying (in space and time) envelopes. These four envelopes are extracted from the pattern using Fourier analysis, and analyzed using several diagnostic tools such as statistical analysis, Karhunen Loeve decomposition, and the computation of correlation lengths and times as well as Lyapunov exponents. The results of this analysis indicate that the pattern shows extensive spatiotemporal chaos. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W8.00009: Orientational order and topological defects on curved surfaces Robin Selinger, Jonathan Selinger, Alex Travesset Recent studies show a close relationship between geometry of surfaces and orientational order within the surfaces. Positive topological defects are attracted to regions of positive Gaussian curvature, and vice versa. To investigate this relationship, we develop a new computational approach to simulate orientational order on surfaces of arbitrary shape. We place xy spins on the surface in a disordered mesh constructed via random sequential absorption. We apply this approach to a sphere, a torus, and an ``egg-crate" surface, $z = A sin(kx) sin (ky)$. For the sphere, we find a total topological charge of +2 as required by the Gauss-Bonnet theorem. For the torus, defects form in pairs, +1 defects on the outer edge (with positive Gaussian curvature) and -1 defects on the inner edge (with negative Gaussian curvature). For the egg-crate surface, with a coarse mesh, a +1 defect forms at each max/min and a -1 defect at every saddle point; while for a finer mesh, defects anneal away. We analyze these simulation results in terms of a continuum elastic model. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W8.00010: Local photo-reorientation of a liquid crystal using a laser focused on an azo dye-based monolayer Yue Shi, Noel Clark The orientation adopted by molecules in an azobenzene-based self-assembled monolayer (azo-SAM) is perpendicular to the polarization of incident green light due to isomerization, aligning liquid crystal (LC) correspondingly. To study the local photo reorientation of the LC, the exciting laser is focused into a small spot on the azo-SAM, of a hybrid cell made with nematic LC sandwiched between the azo-SAM and a homeotropic surface. Under irradiation with changing polarization, a variety of interesting phenomena are observed, including winding of rings of reorientation and orientational slipping. Results vs. exciting light intensities and different rotating frequencies will be reported. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W8.00011: Topology and Geometry of 1D Translational Order on Curved Surface Xiangjun Xing It is shown that one dimensional translational order on two dimensional curved substrate is naturally described by differential forms. A new type of global dislocation defects is identified and its relation with the topological properties of the embedding (compact) manifold is explored using algebraic topological methods. The associated topological charge classifies all ground states with no local defects. The energetics of smectic order on curved substrate is also discussed. Coupling between nematic director field and extrinsic curvature is shown to be important. As a simple application, the phase diagram of smectic order on a torus is analyzed. Two phases are identified: a small/thin phase where the nematic director is locked by curvature and a large/fat phase where the director varies continuously with system parameters. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W8.00012: Coarsening of two-dimensional islands in freely suspended smectic A films Duong Nguyen, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark We have observed coalescence driven coarsening of islands (edge dilocation loops) in freely suspended Smectic A liquid crystal films. This is a good realization of a two-dimensional system, with films as thin as one molecular length (3-4 nm). The film is drawn by spreading material across a circular hole of about 4 mm in diameter, after which the film is suspended by the meniscus in contact with the edge of the hole. Islands are generated by blowing air parallel to the film surface, which produces a shearing force that breaks a thick region of the film into circular islands. Depending on the Smectic A material, we observe either strong or weak short-range repulsion between dislocation loops, leading to slow or fast coalescence, repectively. Over time, the average size of islands increases as the number of islands drops. The observed coasening dynamics is compared with theoretical predictions. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W8.00013: The Ground States of Nematic Order on a Sphere and Topological Defects Homin Shin, Mark Bowick, Xiangjun Xing We study the ground states of a spherical nematic order and the resulting configuration of topological defects. To emulate the ground state, we use hard rods confined on the surface of a sphere and very gradually compress the system upto the maximum packing density with Monte Carlo simulations. The nematic phases with four +1/2 disclination defects are clearly observed. Although the tetrahedral structure of four +1/2 defects is expected, we find all the defects most likely sitting on a great circle. The theoretical reasoning is provided with the calculation of defect energies in terms of the elastic anisotropy. Finally, we present that the allowance of some softness to the rods gives arise to qualitative changes in the director field surrounding the defect core. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W8.00014: Thermal-Cycle Memory Functions and Ising Dynamics Brad Johnson, David Patrick The Ising model provides a rich system for the study of a variety of correlated systems. In this talk, we present the results of numerical studies of 2- and 3-dimensional Ising spin systems subjected to thermal cycling from an ordered state to states with a fixed order parameter ($<$1), but with differing overall morphologies, and back to a quenched state. We find that for systems with initial states generated by thermal disordering above T$_{c}$, the initial state of a given order parameter has larger `islands' of like-spin (than the case for random disorder with the same overall order parameter) and consequent quenches of the state to T$<$T$_{c}$ result in a strong correlation to a particular final average order parameter. The function we find is given by $\left\langle S \right\rangle \approx \tanh (B\cdot S_{init} )$, where S$_{init}$ is the order parameter of the initial state, $<$S$>$ is the average quenched order parameter, and B is a constant that depends upon the morphology of the initial state. The reason for the strong correlation stems from the energies associated with spins at the borders of large clusters. This `memory effect' does not occur in 3D (due to the larger number of near-neighbors). Finally, we discuss the `memory function' in the context of interfacial states of liquid crystals. [Preview Abstract] |
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