Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A17: Liquid Crystals I - Nematics, Cholesterics. Skyrmions |
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Sponsoring Units: GSOFT Chair: Luz J Martinez-Miranda, University of Maryland Room: 276 |
Monday, March 13, 2017 8:00AM - 8:12AM |
A17.00001: Even Numbered Methylene-Linked LC Dimers in Magnetic Field Matthew Murachver, S. M. Saliil, S. N. Sprunt, J. T. Gleeson, A J\'akli Recently thermotropic liquid crystals composed of dimers with odd number methylene linkages showed an anomalously large (up to 13ºC in B=25T) increase of the isotropic – nematic phase transition. $^{[1]}$ This shift is orders of magnitude higher than Landau theory with realistic physical constants would suggest. Motivated by these results, here we studied dimers with even numbered methylene linkages which exhibit a linear shape \& nematic order. Even dimers contrast their odd-numbered counterparts in phase sequence \& range of magnetic enhancement. We propose, in contrast with conventional calamitic nematogens, dimeric molecules have a rich spectrum of confirmations with varying magnetic susceptibility. Under high magnetic fields both ordered states \& lower molecular bend angles are promoted. Research chemicals provided in collaboration with Georg Mehl (University of Hull) Gabi Tauba (Otto von Guericke University Magdeburg) Hao Wang (CPIP, KSU) Quan Li (CPIP, KSU) \\\\ $^{1.}$ Salili, S. M., Tamba, M.-G., Sprunt, S. N., Welch, C., Mehl, G. H., Jákli, A., and Gleeson, J. Physical Review Letters 116, (2016): 217801. [Preview Abstract] |
Monday, March 13, 2017 8:12AM - 8:24AM |
A17.00002: Dynamics and morphology of rigid and deformable shells in a nematic liquid crystal Arthur Evans, Michael Graham, Saverio Spagnolie When immersed in a nematic liquid crystal, colloids force topological defects to nucleate in the bulk director field. These defects, and the interactions between particles, are known to lead to assembly of complex structures. Much less is known regarding the dynamics of colloidal particles, and the effects of particle elasticity on assembly and interaction properties. In this talk I will present an immersed boundary method that models fully three-dimensional hydrodynamics of particles in a nematic liquid crystal, for both rigid body motion and deformable shells. For rigid body motion, viscous anisotropy, coupled with the dynamic interactions that occur in the case of strong anchoring or high Ericksen number, yield results for the linear and nonlinear microrheology of colloids. Additionally, soft membranes such as vesicles or polymerosomes may be deformed by the strong anchoring of liquid crystals; in this case, the defect structure affects long range interactions between soft particles, and also the final morphology of the membranes themselves. [Preview Abstract] |
Monday, March 13, 2017 8:24AM - 8:36AM |
A17.00003: Electron density distribution of symmetric liquid crystal dimer linked by flexible alkyl chain Dena Mae Agra-Kooijman, Michael Fisch, Gautam Singh, Muthukumaraswamy Vengatesan, Jangkun Song, Satyendra Kumar The results of x-ray scattering study combined with electron density distribution of a symmetric liquid crystal dimer linked by flexible chains, specifically 1",7"-bis(4-cyanobiphenyl-4'-yl) heptane (CB7CB) provide compelling new insights into the molecular organization in the nematic (N) and twist-bend nematic (N$_{TB}$) phases. The electron density ($\rho$) of the liquid crystal molecule has customarily been assumed to be uniform rather than a function of distance, $\rho$(z) along the molecular axis. We introduce a functional model of the electron density using Gaussian distribution for different parts of the molecule. The observed diffraction peak at $d \sim$ 0.41$L$ ($L$, molecular length) expected to appear at $d \sim L$, is found to primarily arise from the form factor, F(q) = $| f(q) \rvert| ^2$ where $f(q)$ is the Fourier transform of $\rho$(z). Modeling the functional form of $\rho$(z) was verified in the N phase of different rod-like molecules, with and without intercalation. The results further suggest that there is no intercalation of the dimer molecules in the N and N$_{TB}$ phases. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A17.00004: Triggered Release of Dispersed Microdroplets from Anisotropic Fluids Young-Ki Kim, Xiaoguang Wang, Emre Bukusoglu, Pranati Mondkar, Nicholas L. Abbott In contrast to isotropic fluids, molecules within nematic liquid crystals (NLCs) exhibit long-range orientational order (defined by a so-called director) that leads to elasticity. Micrometer-sized droplets dispersed in NLCs typically strain the director and generate topological defects. Consequently, microdroplets experience strong repulsive forces (elastic repulsion) near NLC interfaces, leading to their sequestration within the bulk of the phase. In this presentation, we will describe how the elasticity and phase behavior of NLCs can be used to trigger the escape of microdroplets. We will demonstrate that the release of sequestered microdroplets from NLC can be triggered by elastic repulsive forces that transport droplets ahead of an interface generated by nematic-isotropic phase transition. Alternatively, we will describe strategies to release microdroplets by tuning the elasticity of NLCs to permit buoyant or interfacial forces to override their sequestration. These mechanisms can be triggered both thermally and isothermally. Theoretical descriptions of these mechanisms establish their applicability to gas, liquid and solid microphases. These results hint at potential applications for drug delivery, sensors, and microfluidics. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A17.00005: Amphiphile-Induced Reorganization of Nematic Liquid Crystals at Aqueous Interfaces Amin Rahimi, Hadi Ramezani-Dakhel, Joel Pendery, Nicholas Abbott, Juan de Pablo Recent studies have shown that ordering transitions in 4-cyano-4'-pentylbiphenyl (5CB) molecules can be triggered by the self-assembly of specific amphiphiles near a flat aqueous-LC interface. In the absence of adsorbed amphiphiles, LC molecules adopt a parallel orientation at the aqueous interface. Self-assembly of amphiphile molecules at the LC-aqueous interface triggers a spontaneous reorientation of the LC at the aqueous interface. A number of observations indicate that the hydrophilic headgroup of the surfactant has marginal effect on the orientation of 5CB whereas the aliphatic tail structure, length, and conformation greatly affect the ordering of the LC. The structural reorganization of liquid crystals at aqueous interfaces has been primarily ascribed to a weakening of the surface anchoring strength induced by amphiphile molecules. Such explanations, however, have only been supported by \textit{a posteriori }microscopic observations. The underlying mechanism of such an ordering transition and the effect of amphiphile structure remain poorly understood. Here, we study the nature of molecular interactions between amphiphiles, 5CB, and water to understand the mechanism of ordering transitions using atomistic molecular dynamics simulations. [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A17.00006: A static elastic theory for usual, chiral, and twist-bend nematic liquid crystals orderings Luiz Roberto Evangelista, Michely P Rosseto, Roberta Rarumy Ribeiro de Almeida, Rafael Soares Zola, Giovanni Barbero, Ioannis lelidis To describe the elastic properties of the twist-bend nematic ($\rm N_{\rm TB}$) phase, a continuum description is proposed to tackle the orientational properties of the nematic, cholesteric, and twist-bend nematic phase. The elastic energy density is an extension of the usual Frank elastic energy density, by including an extra element of symmetry represented by the axis $ \textbf {t}$, allowing periodic distortions. This general energy density indicates the stability of at least three phases allowed by the elements of symmetry and can be faced as a framework to study the static distortion in the nematic ordering. The study of order transitions reveals a periodically modulated structure which appears as a ground state, exhibiting a twist-bend molecular organization. Similar arguments demonstrate that the nematic twist-bend $\rm N_{TB}$ phase is indeed a heliconical structure. It is possible to show analytically that the pitch of this structure is in the nanometric range, in agreement with experimental observations. [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A17.00007: Electrokinetics of Colloidal Particles in Nematic Liquid Crystals Christopher Conklin, Jorge Vinals Colloidal particles in a liquid crystalline matrix present a large variety of self-assembly behaviors through long range elastic interactions and topological constraints. When subjected to electric fields, electrokinetic effects provide an additional mechanism for colloidal particle interaction and manipulation. We present theoretical and numerical results of induced charge distributions, stresses, and fluid motion when colloidal particles are suspended in a nematic liquid crystal thin film that is subjected to an applied, uniform AC field. In our study, spatial charge separation and the resulting electrokinetic forces are due to anisotropic ionic mobilities and liquid crystal permittivity. We also include the effects of backflows on a time-dependent director orientation, which allows for the study of electrokinetic flows at non-negligible Ericksen number. The interplay between elastic and electrokinetic effects leads to new and complex interactions between colloidal particles. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A17.00008: Magnetically tunable selective reflection of light by heliconical cholesterics Seyyed Muhammad Salili, Jie Xiang, Hao Wang, Quan Li, Daniel Alexander Paterson, John Storey, Corrie Imrie, Oleg Lavrentovich, Samuel Sprunt, James Gleeson, Antal Jakli We present studies of chiral nematic liquid crystals composed of flexible dimer molecules subject to large dc magnetic fields between 0 and 31 T. We observe that these fields lead to selective reflection of light depending on temperature and magnetic field. The band of reflected wavelengths can be tuned from ultraviolet to beyond the IR-C band. A similar effect induced by electric fields has been presented previously, and was explained by a field-induced oblique-heliconical director deformation. The use of magnetic field here instead of electric field allows precise measurements of some material constants and holds promise for wireless tuning of selective reflection. References [1] S. M. Salili, J. Xiang, H. Wang, Q. Li, D. A. Paterson, J. M. D. Storey, C. T. Imrie, O. D. Lavrentovich, S. N. Sprunt, J. T. Gleeson, and A. Jakli, Phys. Rev. E 94, 042705 (2016). [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A17.00009: Contact Topology and Cholesteric Liquid Crystals Thomas Machon, Gareth Alexander Understanding the topological aspects of cholesteric liquid crystals has long stood as a challenge, with the standard homotopy theoretic techniques well-known to be both incomplete and conceptually flawed. We show how contact topology supplies a natural language in which one can describe and understand the many chiral structures observed in cholesterics, with familiar structures such layers, dislocations and double twist cylinders becoming essential topological objects in the theory. Using these techniques we establish the topological classification of cholesteric textures in a variety of domains, in particular we show how the mathematical concept of overtwistedness necessarily leads to the existence of topological solitons in cholesterics which have no analogue in non-chiral systems. Finally, we discuss the structure and topology of disclination loops in this context and sketch a connection between their entanglement and exotic topological invariants from contact homology theories. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A17.00010: Total Reflection in Cholesteric Liquid Crystal Cells Using Optical Transmission Grating Windows Ighodalo Idehenre, Vincent Tondiglia, Timothy Bunning, Dean Evans We present experimental and simulated results of utilizing optical transmission gratings to achieve total reflection in cholesteric liquid crystal systems. Total reflection occurs when the light incident upon a cholesteric liquid crystal cell at certain angle, is totally reflected regardless of the polarization state. Experimental as well as numerical studies using Bloch wave analysis have shown that a relatively steep angle of incidence with respect to the helical axis (\textgreater 55 degrees) is required to achieve strong polarization independent (near 100{\%}) total reflection. We propose the use of transmission grating windows as a means of introducing high angular incidence into the cholesteric liquid crystal region while using optical inputs at normal incidence to the windows. We analyze the total reflection behavior of these cells numerically using Fourier optical theory and 4 x 4 matrix method to simulate the angular and wavelength spectrum of the cell. The numerical simulations are then compared against experimental results. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A17.00011: A change in stripes for cholesteric shells via modulated anchoring Lisa Tran, Maxim Lavrentovich, Guillaume Durey, Alexandre Darmon, Martin Haase, Ningwei Li, Daeyeon Lee, Kathleen Stebe, Randall Kamien, Teresa Lopez-Leon Many of the patterns found in biological systems are also found to self-assemble into cholesteric liquid crystal (CLC) systems. In this work, we probe the effect of varying the perpendicular anchoring strength of a CLC that is confined to a spherical shell. The shell geometry gives the confinement and curvature conditions for the formation of a rich array of meta-stable states, revealing an unexplored region between degenerate parallel anchoring and strong perpendicular anchoring. We modulate the anchoring strength in experiments with two methods: by adjusting the surfactant concentration or, interestingly, by varying the temperature. We find two states not previously reported for CLC shells: a Bouligand arches state, where larger, lateral stripes on the shell can be filled with smaller, longitudinal substripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. We use a Landau-de Gennes model of the CLC to simulate the director configurations of these states. This work identifies the Bouligand arches state in CLC shells and builds upon the existing knowledge of cholesteric FCDs, structures that not only have potential for use as intricate, self-assembly blueprints but are pervasive in biological systems. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:24AM |
A17.00012: Properties of polymer stabilized cholesteric liquid crystals in the oblique helicoidal state Mariacristina Rumi, Timothy White, Timothy Bunning Dimeric liquid crystal molecules, constituted by two rigid units linked by a flexible spacer, exhibit different phase behavior and properties when the spacer has an odd number of methylene units, which imposes a bent shape onto the molecules, relative to monomeric analogues and dimers with even spacers. It has been shown that, when mixed with chiral dopants, these dimeric mesogens can assume an oblique helicoidal conformation of the director under the influence of an electric field directed along the helical axis. This conformation is similar to that of the twist-bend nematic phase, but with pitch controlled by the chiral dopant concentration. We are investigating how the introduction of a polymer network in dimer-containing cholesteric liquid crystals can be used to change and control the equilibrium states of the system, the range of existence of the oblique helicoid state, and the response to external stimuli. Comparison of the system properties with and without a polymer network can provide information on the relative role of boundary conditions, anchoring strength, and elastic energy in stabilizing an oblique helicoid arrangement of the director and in determining which textures it can assume. [Preview Abstract] |
Monday, March 13, 2017 10:24AM - 10:36AM |
A17.00013: Optical patterning and dynamics of torons and hopfions in a chiral nematic with photo-tunable equilibrium pitch Hayley Sohn, Paul Ackerman, Ivan Smalyukh Three-dimensional (3D) topological solitons arise in field theories ranging from particle physics to condensed matter and cosmology. They are the 3D counterparts of 2D skyrmions (often called ``baby skyrmions''), which attract a great deal of interest in studies of chiral ferromagnets and enable the emerging field of skyrmionics. In chiral nematic liquid crystals, the stability of such solitons is enhanced by the chiral medium's tendency to twist the director field describing the 3D spatial patterns of molecular alignment. However, their experimental realization, control and detailed studies remain limited. We combine experimental realization and numerical modeling of such light-responsive solitonic structures, including elementary torons and hopfions, in confined chiral nematic liquid crystals with photo-tunable cholesteric pitch. We show that the optical tunability of the pitch allows for using low-intensity light to control the soliton stability, dimensions, spatial patterning and dynamics. [Preview Abstract] |
Monday, March 13, 2017 10:36AM - 10:48AM |
A17.00014: Topology and energetics of skyrmions in chiral liquid crystals Ayhan Duzgun, Jonathan Selinger, Avadh Saxena Skyrmions are localized topological defects in the orientation of an order parameter field, without a singularity in the magnitude of the field. For many years, such defects have been studied in the context of chiral liquid crystals---for example, as bubbles in a confined cholesteric phase or as double-twist tubes in a blue phase. More recently, skyrmions have been investigated extensively in the context of chiral magnets. In this talk, we compare skyrmions in chiral liquid crystals with the analogous magnetic defects. Through simulations based on the nematic order tensor, we model both isolated skyrmions and periodic defect lattices. We determine how the topology and energetics of these defects are affected by easy-axis or easy-plane anisotropy, and by local biaxiality of the liquid-crystal order. [Preview Abstract] |
Monday, March 13, 2017 10:48AM - 11:00AM |
A17.00015: Generating arrays of defect arcs, loops, stripes and Skyrmions in liquid crystals Sajedeh Afghah, Yubing Guo, Miao Jiang, Oleg Lavrentovich, Qi-Huo Wei, Jonathan Selinger, Robin Selinger Using both simulation and experiment, we demonstrate that an ordered~array of defect structures---e.g. disclination arcs and loops---can be created in a nematic liquid crystal cell by patterning the confining~substrates~with~non-uniform~surface~anchoring. We study~liquid crystal cells with a lattice of~topological point $+$/- (1/2) or $+$/-1 defects in the anchoring pattern on one or both substrates. We find an ordered array of disclinations connecting pairs of point defects along one substrate or between substrates. Simulation studies are compared to experimental results.~We also perform analytical calculations to model defect patterns in cholesteric liquid crystals confined in thin cells with uniform homeotropic anchoring. We examine formation of a lattice of Skyrmions or a lattice of stripes, optimize the structure and spacing of each pattern, and determine which structure minimizes the free energy for a given cell thickness.~ We find that, as the inverse pitch of the liquid crystal increases, the resulting defect structure goes from uniformly homeotropic to a Skyrmion lattice and then to a stripe lattice.~ Results are compared to recent experiments and simulations [Y. Guo, S. Afghah, J. Xiang, O.D. Lavrentovich, R.L.B. Selinger, and Q.H. Wei, Soft Matter 12 (29), 6312 (2016).~] [Preview Abstract] |
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