Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y53: Liquid Crystals |
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Sponsoring Units: GSOFT DBIO DPOLY Chair: Doug Cleaver, Sheffield Hallam University Room: LACC 513 |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y53.00001: Polymer-Stabilized Blue Phase Liquid Crystal Droplets Alexander Cohen, Monirosadat Sadati, Xiao Li, Hadi Ramezani-Dakhel, Ye Zhou, Juan Pablo Hernandez-Ortiz, Juan De Pablo Cholesteric liquid crystals with a self-organized helical structure exhibit a photonic band gap along the helical axis. At high chirality, the so-called cholesteric “blue phases” emerge. The blue phases possess highly ordered lattice structures that are stabilized by line defects. These structures exhibit selective reflection of incident light; however, they only appear in a narrow temperature window. Blue phase temperature range can be expanded by incorporating photo-polymerized mesogens into the defect networks. Past studies of polymer stabilized blue phases have mainly focused on bulk properties. In this work, we show that the confinement of the polymer-stabilized blue phase liquid crystal into the spherical droplets greatly influences their structure, color, and stability. We have found that the defect structure can be manipulated by controlling the droplet size and polymerization time, thereby offering tremendous opportunities for designing optical devices based on the highly chiral liquid crystals. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y53.00002: Ostwald ripening of two-dimensional islands in thin, spherical bubbles of smectic liquid crystal in microgravity Pavel Dolganov, Nikita Shuravin, Vladimir Dolganov, Cheol Park, Eric Minor, Joseph MacLennan, Matthew Glaser, Noel Clark, Christoph Klopp, Torsten Trittel, Ralf Stannarius We have recently made experimental observations of 2D coarsening in which thickness islands grow via coalescence and Ostwald ripening in molecularly thin, tethered smectic liquid crystal bubbles. We report here on the dynamics of island growth and collapse without coalescence in island emulsions at locations far from the meniscus around the bubble inflation needle. This Ostwald ripening of essentially 2D smectic A and C islands occurs as a result of the system’s tendency to reduce its energy through a decrease in the total length of the boundaries of the islands, a process that is generally dependent on the surface and line tension, the disjoining pressure of the island, and the excess air pressure inside the bubble. Smaller islands generally have a higher disjoining pressure than their larger neighbors, which results in permeative flow from the smaller to the larger islands. We will describe our observations of Ostwald ripening behavior in smectic bubbles and compare these with the dynamics of island coarsening dynamics in flat smectic films. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y53.00003: Melting of Nematic Liquid Crystals with Predesigned Topological Defect Arrays Miao Jiang, Yubing Guo, Xiayu Feng, O Lavrentovich, Qihuo Wei Dynamics of transitions between phases of different translational and/or orientational ordering is a fascinating area of intense studies. Topological defects are well known to play an important role in the phase transitions as an essential means of symmetry breaking. In this talk, we will present experimental studies on the melting transition of thin nematic liquid crystals with predesigned molecular orientation patterns of topological defect arrays. We found that the heating rates and topological charges have critical impacts on whether defect-mediated heterogeneous or homogeneous nucleation and growth dominate the process. We also found that in all situations, the growth of isotropic domain sizes follows a simple power law of the temperature and that the exponent can be either 0.5 or 1, depending on the heating rate and topological charges. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y53.00004: Simulating the dynamics of liquid-crystal skyrmions Ayhan Duzgun, Jonathan Selinger, Avadh Saxena In magnetic systems, extensive research has investigated skyrmion motion, which is achieved by small electric currents. Similarly, in liquid crystals, recent experiments demonstrate that skyrmions can be moved by fluid flow, light, and electric fields [1]. Here, we use analytic and numerical methods, including Monte Carlo and relaxational dynamics simulations, to model the particle-like motion and interaction of liquid crystal skyrmions. First, we determine the effect of electric field changes on skyrmion shape, and demonstrate that field gradients can induce skyrmion motion. Next, we consider the exposure of certain liquid crystals to light which changes the helical pitch. To model the effect of such pitch changes, we apply gradients in the natural twist and find that these gradients also induce skyrmion motion. These results regarding electric fields and light are in qualitative agreement with experiments. Finally, we show that nonuniform surface anchoring can also be used to manipulate skyrmion motion, and we investigate the elasticity of skyrmions subjected to force. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y53.00005: Dielectric Controlled Self-assembly in Binary Colloidal Mixtures Ziwei Wang, Erik Luijten Electrostatic interactions play a key role in many self-assembly processes, including colloidal aggregation. Prior simulation work has reported a rich variety of aggregates formed in simple binary suspensions of oppositely charged spherical colloids. However, dielectric effects, which are complicated to compute due to their many-body nature, are generally ignored in these studies. By employing a novel efficient method to handle the polarization charges via a combination of the method of images and the method of moments, we are able to examine the role of polarization in colloidal aggregation and crystallization in large-scale simulations. We systematically vary size ratio and relative permittivity, and determine the phase diagram, showing that dielectric effects qualitatively alter self-assembled structures. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y53.00006: Self-assembly of Poly(p-phenylene terephthalamide) Nanofibrils: Emergence of Structural Defects Santhosh Mogurampally, Giacomo Fiorin, Michael Klein We provide molecular level insights on the self-assembly mechanisms of poly(p-phenylene terephthalamide) (poly(PPTA)) chains in a strong solvent by using large scale molecular dynamics simulations based on a rigorous development of a coarse-grained potential model. Ordered liquid crystalline phases of poly(PPTA)n chains facilitated by the solvent are reported for various polymer lengths, n and initial configurations. We find that the degree of hydrogen bond (H-bond) plane ordering depends sensitively on the polymer length and initial polymer alignments. Our largescale simulations suggest an increased propensity of grain boundary formation parallel to the H-bond direction with guided initial configuration. Our results reveal that the trapping of solvent nanoclusters within the ordered domains of poly(PPTA) fibrils give rise to structural defects. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y53.00007: Charge Transport and Phase Behavior of Ionic Liquid Crystals from Fully Atomistic Simulations Michael Quevillon, Jonathan Whitmer Ionic liquid crystals occupy an intriguing middle ground between room-temperature ionic liquids and mesostructured liquid crystals. These systems are composed of bulky ionic groups attached to mesogenic "tails" in a way which resembles common surfactants; the charge and tail structure are tunable, and useful for applications in green solvents or charge transport media. Both of these properties are tunable for applications such as specific conductive materials or as greener solvents. As the liquid crystal behavior gives rise to local ordering, the ionic moieties form mesostructures that are conducive to ion transport by separating charged regions and aliphatic apolar regions. Layers form in these materials that effect an anisotropy in molecular mobility and hence ion transport. Here, we examine a fully atomistic model of ionic liquid crystals using molecular dynamics in the constant pressure–constant temperature ensemble. We discuss the implications that phase behavior and layering of these materials may have for electrolyte applications. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y53.00008: Geometry-induced Orientational Ordering in Chiral Liquid Crystals Monirosadat Sadati, Jose Martinez-Gonzalez, Ye Zhou, Khia Kurtenbach, Viviana Palacio Betancur, Juan Pablo Hernandez-Ortiz, Juan De Pablo In this work we explore the effect of geometrical confinements on the orientational ordering and transition of topological defects in chiral liquid crystal (LC) droplets. The geometrical confinements are induced by uniaxial stretching as well as compressing of a polymer-dispersed liquid crystal films. Our experimental and computational results show that compressing a chiral liquid crystal droplet with radial spherical structure (RSS) embedded in a polymer matrix with planar anchoring flattens the spherical droplet to an oblate. The chiral liquid crystal in the outer part of the oblate exhibits the structure similar to that of spherical droplets with RSS rings while the inner part adopts a disc-shape topology with helical axis aligned normal to the surface. Upon uniaxial stretching, the RSS configuration becomes unstable and the reflection band of the frustrated chiral liquid crystal shifts toward shorter wavelengths. This study allows discovery of new phases of chiral liquid crystals for applications in mechano-optical metamaterials. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y53.00009: ‘Nemator’ Model of Orientational Distortions in Liquid Crystals with Defects Natalie Aryasova, Sergij Shiyanovskii The Oseen-Frank (OF) and Landau-de Gennes (LdG) models of liquid crystals (LCs) are classical tools for analysis and simulations of static and dynamic orientational distortions. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y53.00010: Twist fluctuations and rotational diffusion of the director near hedgehog defects in nematic droplets Alexis de la Cotte, Madeline Van Winkle, Peter Collings, Tom Lubensky, Arjun Yodh When strong homeotropic boundary conditions are set at the interface of emulsion droplets containing nematic liquid crystal (NLC), the spherical confinement combined with the system bulk elasticity is known to produce a point-like defect called a radial hedgehog with topological charge +1. However, this configuration is not necessarily the ground state. For favorable ratios of the elastic constants, a twist perturbation has been observed near the defect. Here we report experimental observation and quantification of twist fluctuations in emulsion droplets of the NLC 5CB. These fluctuations can be understood as a new kind of rotational diffusion corresponding to a collective motion of the whole system. To explain this phenomenon, we build a model for the nematic director that accounts quantitatively for the origin of our experimental observations and also provides insight about the physical properties of the defect. The model offers a more complete understanding of the transition between pure radial and twisted configurations in spherically confined NLC, and it could useful for future investigation of trapped colloidal particles in nematic hosts. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y53.00011: Defect dynamics of circularly confined 2D active nematicl iquid crystals Achini Opathalage, Michael Norton, Michael Juniper, Blake Langesley, Seth Fraden, Zvonimir Dogic We study the role of boundary conditions on a simplied experimental model of biological active matter system composed of extensile filamentous bundles of microtubules driven by clusters of kinesin motors, to elucidate the structure and dynamics of active nematic liquid crystals. These bundles form a dense quasi-2D active nematic liquid crystals when sediment onto a surfactant-stabilized oil-water interface. We further confine this system ontocircular boundary conditions, imposing total topological charge. We observe unique dynamical behavior under high confinement in the order of hundred micrometers. The system produces two circulating +/- 1/2 defects, driving the material toward the edge of the circle. The circulating behavior is eventually destroyed by buckling of the nematic at the container wall which nucleates a +/- defect pair. This behavior is remarkably periodic until the energy supply of the system; ATP is drained. We study the defect-defect and defect-boundary |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y53.00012: Density functional theory for two-dimensional hard rods René Wittmann, Christoph Sitta, Frank Smallenburg, Matthieu Marechal, Klaus Mecke, Hartmut Loewen Despite their simplicity, (non-spherical) hard particles became a standard model for colloidal systems. Fundamental measure theory (FMT) and its recent generalizations allow to predict the phase behavior of such a fluid solely from the shape of the individual particles. Such density functionals, which are exact in the low-density limit have been successfully applied to hard spherocylinders in three dimensions. However, the implementation of the most general framework, fundamental mixed measure theory (FMMT), usually requires systematic approximations of the comprised two-body term. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y53.00013: Macroscopic orientation control of graphene flakes by magnetic field and broad device applications Feng Lin, Guang Yang, Chao Niu, Yanan Wang, Zhuan Zhu, Jonathan Hu, Xufeng Zhou, Zhaoping Liu, Zhiming Wang, Jiming Bao We demonstrate the orientation control of graphene flakes by a weak static magnetic field and subsequently achieve high order parameter alignment by rotating magnetic field. The orientation control is made by weak static magnetic field for large diamagnetic susceptibility of exfoliated graphene. Viewed as a non-magnetic material, a liquid suspension of graphene flakes is firstly used for magnetic field sensing with high sensitivity and spatial resolution. The graphene suspension is then packaged as a writing and/or display board that can be controlled by magnets or magnetic field. Graphene liquid suspension was firstly aligned with high order parameter of 0.8 by rotating magnetic field. Macroscopic optical properties of aligned graphene flakes such as birefringence and diffraction were investigated. After mixing with UV cured resin, graphene flakes suspension was aligned as high performance polarizer. By combination lithography process with rotating magnetic field alignment, graphene flakes were assembled into patterns of different orientations with lithography mask. The macroscopic control and alignment of graphene can not only transfer unique properties of graphene from microscopic to macroscopic scale, but also be used to align other nanomaterials. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y53.00014: Ion trapping by the graphene electrode in a graphene-ITO hybrid liquid crystal cell Rajratan Basu, Andrew Lee A monolayer graphene coated glass slide and an indium tin oxide (ITO) coated glass slide with a planar-aligning polyimide layer were placed together to make a planar hybrid liquid crystal (LC) cell. The free-ion concentration in the LC was found to be significantly reduced in the graphene-ITO hybrid cell compared to that in a conventional ITO-ITO cell. The free-ion concentration was suppressed in the hybrid cell due to the graphene-electrode’s ion trapping process. The dielectric anisotropy of the LC was found to increase in the hybrid cell, indicating an increase in the nematic order parameter of the LC due to the reduction of ionic impurities. |
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