Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H30: Liquid Crystals: Experiments and Theory |
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Sponsoring Units: DFD Chair: Robin Selinger, Kent State University Room: Colorado Convention Center 304 |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H30.00001: Nonphotochemical Laser Induced Nucleation From a Supercooled Thermotropic Liquid Crystal Xiaoying Sun, Bruce Garetz, Michele Moreira, Peter Palffy-Muhoray A nonphotochemical laser induced phase transition was studied in a supercooled 4'-n-pentyl-4-cyanobiphenyl(5CB, also referred to as PCB and K15 ) liquid crystal system, using linearly polarized ps pulses from a Nd:YAG laser at a wavelength of 532nm. The result shows that light could induce nucleation from the metastable supercooled isotropic phase to the nematic phase in the case of a slow cooling rate and high laser intensity. The director of the induced nematic phase tends to align along the direction of polarization of the light. At the intensities used, there is no observable reorientation of the director once it is in the nematic phase. These experimental results are consistent with a mechanism based on optical Kerr alignment. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H30.00002: Stressed liquid crystals for fast phase retardation switching Anatoliy Glushchenko, Kevin Wood, Alex Rockwood, John West, Guoqiang Zhang, Ke Zhang Liquid crystalline materials are an attractive medium for many devices because they produce large electrically controllable shifts of the phase retardation. Relatively thick (10-100 micron) liquid crystal layers are needed for modern devices. However, the speed of these devices is inversely proportional to the square of the LC layer thickness. We must therefore decouple the speed of switching and thickness of the layer. Here we report on the development of stressed liquid crystals (SLCs) to achieve this goal. These new materials consist of a stressed polymer network that imposes unidirectional alignment of a liquid crystal matrix. We induced the stress by shearing the material. As expected from earlier research in PDLCs, shearing greatly reduces the switching time. Unlike the PDLCs the scattering of the films decreases drastically upon shearing. Indeed the SLC materials are optically clear throughout the visible and infrared. The SLC materials are also characterized by essentially no hysteresis and a completely linear voltage response. This greatly simplifies design of the electronics and driving schemes driving. The SLC's performance is determined by the component materials used, their relative concentration and the preparation conditions. We have produced SLC cells that produce almost 20 microns of phase retardation, that switch on and off in several milliseconds. In general the SLC cells require a driving voltage of about 1V/ $\mu $m. The SLCs produce the largest phase retardation at the fastest speeds. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 8:36AM |
H30.00003: Orientational multiplicity and transitions in liquid crystalline droplets Rajesh Goyal, Morton Denn Orientation distributions in droplets of liquid crystals with homeotropic anchoring are computed with a simulated annealing algorithm that minimizes the free energy of the Oseen-Frank continuum theory. The droplets exhibit multiple orientational steady states that are separated by finite energy barriers over the entire range of the dimensionless ratio of surface to elastic forces, with maximum transition energy densities of the order of 2,000 Pa for a typical liquid crystalline droplet with a spherical radius of 1 micron. The transition energy densities decrease with elongation to spheroidal droplets with aspect ratios of four or more, indicating that droplet elongation is favored to drive surface-induced transitions. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H30.00004: Optical Activity Produced by Layer Chirality in Bent-Core Liquid Crystals Loren Hough, Chenhui Zhu, Michi Nakata, Nattaporn Chattham, Gert Dantlgraber, Carsten Tschierske, Noel Clark Recent observations of large optical activity in chiral smectic liquid crystalline phases formed from achiral bent-core molecules have been attributed to both a helical superstructure and to layer optical chirality (LOC). The LOC model predicts that optical activity is produced by the local chiral layer structure formed by the simultaneous tilt and polar ordering of bent-core molecules (Hough and Clark PRL, 95, 107802 (2005)). The LOC model predicts that optical activity should be present in the well ordered B2 phase. However, in most materials, the optical activity is masked by birefringence. We studied the SmC$_{A}$P$_{A }$subphase of GDa104 (Dantlgraber, et al. Chem. Mater. 14, 1149 (2002).), which has a tilt angle of $\sim $45 degrees (orthoconic), and thus very low birefringence. In this system, we directly demonstrate that layer chirality produces optical activity consistent with the LOC model. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H30.00005: Observation of a possible tetrahedratic phase in bent-core David Wiant, Krishna Neupane, Sunil Sharma, Antal Jaklu, James Gleeson, Samael Sprunt Combined measurements of optical birefringence induced by high magnetic fields and dynamic light scattering have been performed on bent-core liquid crystals at temperatures above their clearing points. The results of these measurements provide compelling evidence of a phase transition between two optically isotropic phases that is consistent with the development of a novel ``tetrahedratic" form of orientational order. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H30.00006: Giant flexoelectricity of bent-core nematic liquid crystals John Harden, Badel Mbanga, Nandor Eber, Katalin Fodor-Csorba, Samuel Sprunt, James Gleeson, Antal Jakli Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLC's gigantic (more than three orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic (BCN) materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H30.00007: Bent-core fiber structure: Experimental and theoretical studies of fiber stability C. Bailey, E. Gartland, A. Jakli Recent studies have shown that bent core liquid crystals in the B7 and B2 phases can form stable freestanding fibers with a so called ``jelly-roll'' layer configuration, which means that the smectic layers would be arranged in concentric cylindrical shells. This configuration shows layer curvature is necessary for fiber stability. Classically this effect would destabilize the fiber configuration because of the energy cost of layer distortions and surface tension. We propose a model that can predict fiber stability in the experimentally observed range of a few micrometers, by assuming that layer curvature can be stabilized by including a term dealing with the linear divergence of the polarization direction if the polarization is allowed to have a component normal to the smectic layers. We show that this term can stabilize the fiber configuration if its strength is larger than the surface tension. We also propose an entropic model to explain the strength of this term by considering steric effects. Finally we will take results from this model and apply them to better understand experimental findings of bent-core fibers. Financial support by NSF FRG under contract DMS-0456221. Prof. Daniel Phillips, Particia Bauman and Jie Shen at Purdue Univ., Prof. Maria Carme Calderer at Univ. of Minnesota, and Prof. Jonathan Selinger at Kent State Univ. Liou Qiu and Dr. O.D. Lavrentovich, Characterization Facilities, Liquid Crystal Institute, Kent State Univ. Julie Kim and Dr. Quan Li, Chemical Synthesis Facilities, Liquid Crystal Institute, Kent State Univ. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H30.00008: Field-induced phase transitions in SmCP phases of bent-core liquid crystals S. Dhakal, Jonathan V. Selinger Liquid crystals composed of bent-core molecules have been studied extensively because they exhibit a wide variety of phases with potential applications. The smectic phases of these molecules have polar order in the layer plane due to the close packing. If the molecules are tilted with respect to the layer normal, the combination of tilt and polar order gives the layer chirality. The resulting liquid crystal can be either chiral or racemic (antichiral), even if the molecules are achiral. The reversibility of these two states with the application of electric field implies the possibility of making light shutters from antiferroelectric liquid crystals of bent-core molecules [1]. Extending an earlier model of chiral order [2], we develop a phenomenological theory involving three order parameters: chirality ($\chi )$, polarity (P) and tilt ($\theta )$ within each layer. By minimizing the free energy, we predict multiple phase transitions (SmC$_{A}$P$_{A}\to $SmC$_{S}$P$_{F}$, SmC$_{A}$P$_{A}\to $ SmC$_{A}$P$_{F})$ as a function of electric field, consistent with experiments. [1] A. Jakli \textit{et al}., \textit{Liq. Cryst.} \textbf{29}, 377 (2002). [2] J.V.Selinger, \textit{Phys. Rev. Lett.} \textbf{90,} 165501(2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H30.00009: Theory of ``scar'' defects in liquid-crystalline films Jonathan V. Selinger, Zhao Lu Recently, several researchers have studied crystalline order on the surface of a sphere, both theoretically [1] and experimentally [2]. In this system, one might expect to see twelve point disclinations, as required by topology. Instead, they find that the point disclinations extend into ``scar'' defects, which are finite grain boundaries. Our goal is to determine how general is the phenomenon of scar formation. Does it require crystalline order, and does it require curvature? For that reason, we investigate an xy model in a flat disk geometry, where boundary conditions require a total topological charge of +1, i.e. the vector order parameter must rotate through a total angle of 2$\pi $. In the classical xy model, the ground state would have a single vortex of charge +1. However, for certain slight variations on the xy model, the ground state has a scar defect, which looks like two vortices of charge +1/2 connected by an orientational domain wall. The formation of scars depends on details of the interaction energy in a lattice or continuum system. We discuss possible opportunities to observe these scar defects in experiments. [1] M. J. Bowick et al., \textit{Phys. Rev. B} \textbf{62}, 8738 (2000). [2] A. R. Bausch et al., \textit{Science} \textbf{299}, 1716 (2003). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H30.00010: Semi-soft Nematic Elastomers and Nematics in Crossed Electric and Magnetic Fields Fangfu Ye, Ranjan Mukhopadhyay, Olaf Stenull, Tom Lubensky Ideal nematic elastomers exhibit soft elasticity in which an elastic modulus of a uniaxial solid vanishes and in which it takes no stress to produce strains, up to a critical strain, in the direction perpendicular to the uniaxial direction. Elastomers crosslinked while stretched exhibit semi-soft elasticity with a nearly linear stress-strain curve at small strain followed by a nearly flat stress-strain relation up to a critical strain. Semi-soft elasticity above the so-called mechanical critical point, which terminates the line of coexistence of nematic phases, has not been established. We calculate global phase diagrams and stress-strain curves for a series of models, including a generalization of de Gennes-Maier-Saupe (dGMS) model and the neolclassical model for nematic elastomers, and demonstrate that semi-soft behavior can persist well above the mechanical critical point. The dGMS model also describes a normal nematic in perpendicular electric and magnetic fields and exhibits a Blume-Emery-Griffiths tricitical point topology. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H30.00011: Simulation studies of liquid crystal elastomers: response to light Robin Selinger, Jonathan V. Selinger, Badel L. Mbanga Azo-dye doped nematic elastomers bend and flex when subject to photoexcitation. We model this mechanical response at the continuum level using nonlinear finite element simulation. Our finite element algorithm uses explicit dynamics based on a Hamiltonian which couples mechanical strain and nematic order. Our explicit dynamics algorithm is structured essentially like molecular dynamics, and we discuss how this continuum level code can be easily integrated into a multiscale model. We use the model further to explore potential applications of nematic elastomers in microfluidics, fiber orientation control, and biologically-inspired robots. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H30.00012: Simulation studies of liquid crystal elastomers: soft elasticity Badel Mbanga, Jonathan V. Selinger, Robin Selinger Liquid crystal elastomers combine the elastic properties of rubbers with the order inherent in nematic liquid crystals. Stretching a monodomain strip of nematic elastomer in a direction transverse to the nematic director results in an energy-free rotation of the director, giving rise to a soft elastic response. In building a simulation model of this mechanism, we consider the limit in which the orientational order equilibrates rapidly compared to the strain, so that the local order tensor remains in continuously evolving quasi- static equilibrium as the strain relaxes. The order tensor in each volume element is determined by minimizing a free energy functional in the form of a Landau expansion that includes a term coupling the local orientational order with the local strain. The strain evolves via nonlinear finite element explicit dynamics. 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] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H30.00013: A dynamical approach for liquid crystal simulations Jones Wan A novel, efficient simulation scheme is proposed to determine the liquid crystal configurations under complex physical environments. In this simulation scheme, the liquid crystal elastic energy, the electrostatic interaction, and the surface anchoring effect are calculated by a dynamical approach, which is analogous to molecular dynamics simulation. As a result, various techniques established for molecular dynamics are readily adopted to the proposed scheme. We demonstrate the new method by calculating the director field of a liquid crystal under the effects of an external electric field and patchy pattens. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H30.00014: Smectic Defects with Riemann Reason Elisabetta Matsumoto, Randall Kamien, Christian Santangelo Minimal surfaces are natural starting points when considering smectics because they minimize the bending energy of the layers. Alternatively, ``sums'' of screw dislocations have been used to model twist-grain boundaries, and often lead to layers with the same topology as classic minimal surfaces. We use Riemann's minimal surface as a model for a smectic whose layers are joined by pores. We evaluate the energetics of this surface using an explicit phase field representation in terms of elliptic functions. We also build a surface with the same topological structure as Riemann's minimal surface using a configuration of oppositely-charged screw dislocations. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H30.00015: Angular momentum transport in soft anisotropic matter Mark Warner, Peter Palffy-Muhoray, Michael Shelley, Xiaoyu Zheng If an anisotropic crystal is placed in a magnetic field, it will tend to rotate into alignment as a rigid body. An anisotropic sample of soft matter, such as a liquid crystal or a liquid crystal elastomer, will behave differently; initially, only the anisotropic constituents rotate about their centers of mass, then interactions between them give rise to material currents and eventually to rigid body rotation. Since the external field exerts only a body torque -- but no force - on the sample, it is interesting to ask how the forces which drive translational motion arise. To gain insight into this problem, we consider the response of a cylindrical liquid crystal sample in a magnetic field in a geometry where both the director and the applied field are perpendicular to the cylinder axis. We solve the equations of motion for low molecular weight liquid crystals and for liquid crystal elastomers and trace the flow of angular momentum in order to understand the details of the dynamics. [Preview Abstract] |
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