Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X15: Liquid Crystals II |
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Sponsoring Units: DFD Chair: Peter Collings, Swarthmore College Room: 316 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X15.00001: The Taming of the Screw: Nonlinear Interactions in Smectic Liquid Crystals Elisabetta Matsumoto, Gareth Alexander, Randall Kamien From the twist grain boundary phase to the smectic phases of bent core liquid crystals, beautiful and intricate textures composed of screw dislocations appear time and again in a wide variety of smectic systems; yet, little is known about the interactions of screw dislocations. The linear smectic free energy is not sufficient to describe the energetics of single screw dislocations, and superposition cannot shed light on the interaction of many such defects. The full rotationally invariant nonlinear smectic free energy provides insight into systems of multiple screw dislocations. Such nonlinear interactions allow us to begin to understand the stability of the bulk phases observed in both smectic A liquid crystals and their chiral smectic C* counterparts. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X15.00002: Coalescence Dynamics Analysis Of Islands In Smectic A Freely Suspended Films Zoom Nguyen, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark We explore the coalescence dynamics of circular islands in smectic A freely suspended liquid crystal films. The process typically has two distinct stages. First, when the islands make contact initially, the thinner island wraps around the thicker one. These dynamics are fast and determined by the line tensions of the islands and by the film's viscosity. Then the region that used to be the thicker island expands and eventually covers the whole merged island. This process which is dependent on the permeation between layers in addition to the line tension and viscosity, is much slower. The shapes of the islands are extracted from high speed camera images and compared with model calculations. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X15.00003: Orientational fluctuation study in nematic liquid crystals by high speed micrograph Beom-Jin Yoon, Min Sang Park, Jung O. Park, Mohan Srinivasarao The orientational fluctuations in uniaxial and biaxial nematic liquid crystals were investigated with a polarized microscope and a high speed TV camera. Liquid crystals usually have fluctuations with respect to their director, even when the molecular axes tend to be aligned to each other. These fluctuations are sufficiently slow and large, have long wave length and increase with temperature. Herein, we describe our study on fluctuation dynamics by direct observations in real space, while it has been typically done by the photon scattering in reciprocal space. The twinkling of liquid crystals due to orientational fluctuations was observed with a high speed camera up to 500 frames/sec. The time correlation function of the intensity was computed via 2D spatial Fourier transform of each image and then the relaxation frequency was estimated from it. The elastic constant to the viscosity ratio was computed from the relaxation frequency. This approach provides facile route to analyze fluctuation dynamics in liquid crystals. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X15.00004: Modeling twisted distortions in nematic elastomers Vianney Gimenez, Badel Mbanga, Fangfu Ye, Jonathan Selinger, Robin Selinger Experimental studies have reported that nematic elastomers with a twisted nematic director--similar to the configuration in a twisted nematic cell--show a well-controlled deformation under change of temperature. Due to the difference in thermal expansion along and perpendicular to the nematic director, the sample twists and curls dramatically under heating and cooling [1]. We model this shape evolution using both analytical calculations and finite element elastodynamics simulations. In analytical calculations, we determine the optimal shape of an initially flat strip by minimizing a free energy functional that takes into account the coupling between orientational order and mechanical strain. We compare which of two final states--a helical or twisted ribbon shape--is lower in free energy, as a function of the sample's aspect ratio and material properties. We then use finite element simulations to model the dynamics of this spontaneous deformation and examine the resulting equilibrium shapes, which may be intermediate between helical and twisted. Results are compared to relevant experiments. We also use our simulation model to explore a wider variety of director configurations and sample geometries, beyond the ideal cases solvable via analytical methods. [1] G. Mol, K. D. Harris, C. W. M. Bastiaansen, and D. J. Broer, Adv. Funct Mater, 15, 1155 (2005). [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X15.00005: Behavior of Focal Conic Defects in Shear Flow Sourav Chatterjee, Shelley Anna The rheology of layered liquids is influenced to a large extent by defects present in the system, especially in small gaps. Toroidal focal conic defects are a common type of defect in small molecule layered liquids. We present a study of the influence of flow on focal conic defects in smectic liquid crystals, generated by antagonistic boundary condition at the surfaces. The defects are confined in gaps of the order of tens of microns and are subject to simple shear. The sizes of the focal conic defects vary with the gap size, and hence visual observations are made as to how the gap influences the dynamics of the focal conic defects in a shear flow. We also observe instabilities in initially defect free samples that lead to the creation of defects. The results offer insight into the complex relationship between defects and flow. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X15.00006: Polarization current as evidence of local anticlinic correlations in de Vries smectics Z. V. Smith, P. D. Beale, R.-F. Shao, L. Wang, D. M. Walba, N. A. Clark, M. A. Glaser Previous theoretical work on the electroclinic response of chiral de Vries SmA materials based on the electric field-induced reorientation of independent tilt domains [J. V. Selinger et al., Phys. Rev. E 64, 061705 (2001)] fails to account for the sigmoidal dependence of induced polarization (P) on field (E) seen in some materials. To account for this behavior, we model de Vries smectics as ensembles of small but finite anticlinic tilt domains. Within each domain, interlayer tilt coupling favors anticlinic interfaces, but the finite range of in-layer tilt correlations leads to thermally activated synclinic interfaces and a finite tilt correlation length along the layer normal. This model, equivalent to a generalized one-dimensional XY model in an external field with quadratic and quartic nearest-neighbor interactions, is studied by Monte Carlo simulation and transfer matrix methods. The model successfully reproduces the dependence of P on E for a specific material (W530), and yields physical parameters such as the in-layer correlation length and effective interlayer tilt coupling. The predicted anticlinic tilt correlations should be observable as diffuse superlattice reflections in polarized resonant x-ray scattering experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X15.00007: A one order parameter tensor description of biaxial nematic liquid crystals Xiaoyu Zheng, Peter Palffy-Muhoray We present a simple one order parameter tensor mean field theory of biaxial nematic liquid crystals. We construct the free energy from molecular interactions, identify the components of the order parameter tensors, and obtain self-consistent equations, which are then solved numerically. The phase behavior is described via a 3D phase diagram. We discuss the connection between molecular properties and the coefficients in the Landau expansion. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X15.00008: Elasticities and viscosities of a lyotropic chromonic nematic liquid crystal Krishna Neupane, Yuri Nastishin, Alan Baldwin, Oleg Lavrentovich, Samuel Sprunt We have performed dynamic light scattering studies of the elastic moduli and viscosity coefficients in a uniformly aligned sample of a lyotropic chromonic nematic formed by 14 wt. {\%} water solution of Disodium Cromoglycate [1]. These parameters show a significant anisotropy. In particular, the bend and splay moduli $K_{33}$ and $K_{11}$ are an order of magnitude higher than the twist modulus $K_{22}$, and the ratio $K_{33}/K_{11}$ shows an anomalous increase in temperature, which we attribute to the shortening of the aggregates. The bend viscosity is three orders of magnitude smaller than the splay and twist viscosities; all viscosity coefficients exhibit a strong temperature dependence. \\[4pt] [1] Nastishin \textit{et al.}, \textit{Phys. Rev. E.} \textbf{70}, 051706 (2004). [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X15.00009: Reflection and transmission coefficients of a cholesteric liquid crystal film with a negative dielectric coefficient Sabrina Relaix, Wenyi Cao, Peter Palffy-Muhoray A cholesteric liquid crystal (CLC) is a periodic dielectric structure where simple analytic solutions of Maxwell's equations exist: light propagating along the helical axis has been first described by Mauguin in 1911 [1], for wavelengths much smaller than the helical pitch, and was formulated more generally by de Vries in 1951 [2]. The analytical solutions are for bulk CLCs and do not describe the optical properties of a finite thickness CLC film. Recently, analytic expressions for the reflection and transmission coefficients of a CLC slab have been obtained by solving Maxwell's equations and satisfying boundary conditions [3,4], providing results for thick slabs which go beyond the limitation of numerical methods. We discuss how these results are modified when one of the dielectric coefficients is negative. We explore the connection with hyperbolic dispersion and negative index materials. [1] C. Mauguin, Bull. Soc. Fr. Miner. Cristallogr. 34, 6 (1911) [2] H. de Vries, Acta Crystallogr. 4, 219 (1951) [3] W. Cao, Ph.D. dissertation, Chemical Physics, Kent State University (2005) (http://e-LC.org) [4] S. Relaix, W. Cao and P. Palffy-Muhoray, to be published [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X15.00010: Maier-Saupe Theory of Nematics in 4D Peter Palffy-Muhoray, Xiaoyu Zheng We extend the Maier-Saupe theory of nematics to 4 dimensions. We consider the interaction of cylindrically symmetric particles, and derive an effective single particle potential. Using this, we obtain the free energy and the self-consistent equation for the order parameter -- a second rank traceless tensor. In 4D, the order parameter has three independent eigenvalues. We solve the self-consistent equation, and study the solutions as function of temperature. Our results give insight into the relation between orientational order parameters in different dimensions. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X15.00011: Statistical mechanics of the flexoelectric effect in nematic liquid crystals Subas Dhakal, Jonathan V. Selinger Flexoelectricity is the phenomenon in which polarization is induced by imposed deformations of the director field in nematic liquid crystals. Recent experiments [1,2] have found that the flexoelectric effect is three orders of magnitude greater for bent-core liquid crystals than for conventional rod-like liquid crystals. To understand this experimental result, we develop a lattice model for the statistical mechanics of the flexoelectric effect. We perform Monte Carlo simulations and mean-field calculations to find the behavior as a function of interaction parameters, temperature, and applied electric field. The resulting phase diagram has four phases: isotropic, uniaxial nematic, biaxial nematic, and polar. In the uniaxial and biaxial nematic phases, there is a large splay or bend flexoelectric effect, which diverges as the system approaches the nematic-polar transition. This model may explain the large bend flexoelectric coefficient observed in bent-core liquid crystals, which have a tendency toward polar order. [1] J. Harden, B. Mbanga, N. Eber, K. Fodor-Csorba, S. Sprunt, J. T. Gleeson, and A. Jakli, Phys. Rev. Lett. 97,157802 (2006). [2] J. Harden, R. Teeling, J. T. Gleeson, S. Sprunt, and A.Jakli, Phys. Rev. E 78, 031702 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X15.00012: Aggregates in Chromonic Liquid Crystal Phases of Aqueous Solutions of Sunset Yellow* Leela Joshi, Shin-Woong Kang, Dena Mae Agra-Kooijman, Satyendra Kumar Molecules of dye Sunset Yellow consist of flat poly-aromatic core and hydrophilic groups at the periphery. In aqueous environments, they self-organize into columnar aggregates mainly via $\pi -\pi $ interactions between aromatic cores. At high concentrations, dye aggregates develop orientational and positional orders to form the nematic (N) and columnar (C) mesophases. Synchrotron x-ray scattering and optical polarizing microscopy were used to better understand the growth of aggregates and mesophase formation. Average column height and their spatial organization strongly depend on concentration, temperature, and pH value of the solution. The aggregate size decreases with temperature exhibiting an Arrhenius behavior with mesophase dependent activation energy. A dramatic decrease in the aggregate size upon addition of \textit{HCl} highlights their sensitivity to electrostatic interactions. *Work supported by grant NSF/DMR-086991. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X15.00013: Aggregation Properties of the Chromonic Liquid Crystal Benzopurpurin 4B Christopher McKitterick, Peter Collings Benzopurpurin 4B (BPP) is a textile dye very similar to the common indicator Congo Red. As is true for all chromonics, the absorption spectrum is concentration dependent at low concentrations. If this dependence is used to estimate a free energy change for aggregation, it is higher than has been determined for other systems. Unlike other recently investigated chromonic liquid crystals, BPP forms a liquid crystal phase at extremely low concentrations, about 0.5 wt\%. Also unlike these other chromonic liquid crystals, the aggregation kinetics are exceedingly slow. X-ray diffraction and light scattering measurements indicate that the aggregates of BPP are much larger than for chromonic systems that form liquid crystals at higher concentrations. BPP aggregates can be imaged using confocal microscopy, revealing a length distribution centered at 3 $\mu$m for a solution forced through a 0.2 $\mu$m filter. Over days the aggregates lengthen to well over 10 $\mu$m. The diameter of the aggregate images is slightly greater than the diffraction limit of the microscope, placing an upper limit on the diameter of 0.14 $\mu$m. These dimensions are consistent with the light scattering results. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X15.00014: Generalized Nematohydrodynamic Boundary Conditions with Application to Bistable Twisted Nematic Liquid Crystal Displays Angbo Fang, Tiezheng Qian, Ping Sheng Parallel to the highly successful Ericksen-Leslie hydrodynamic theory for the bulk behavior of nematic liquid crystals (NLC), we derive a set of coupled hydrodynamic boundary conditions to describe the NLC dynamics near NLC-solid interfaces. In our boundary conditions, translational flux (flow slippage) and rotational flux (surface director relaxation) are coupled according to the Onsager variational principle of least energy dissipation. The application of our boundary conditions to the truly bistable $\pi$-twist NLC cell reveals that the thus far overlooked translation-rotation dissipative coupling at solid surfaces can accelerate surface director relaxation and enhance the flow rate. This can be utilized to improve the performance of electro-optical nematic devices by lowering the required switching voltages and reducing the switching times. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X15.00015: Soft Micro- to Nanolithography Using Highly Periodic Smectic Liquid Crystal Defects Hee-Tae Jung, Yun Ho Kim, Dong Ki Yoon, Hyeon Su Jeong Achieving perfect long-range order with soft building blocks at high speed and high resolution is one of the most exciting interdisciplinary research areas in current materials science and nano-biotechnology. Here, we have developed highly periodic patterns with sub-micrometer features over large-areas using toric focal conic domains (TFCDs) originated from smectic liquid crystal (LC). TFCDs are accomplished by precisely controlling the surface and interfacial properties of smectic LC. In order to apply the smectic liquid crystal defect arrays in lithography, the hexagonal arrays of domain patterns are used as molds for ultraviolet (UV) curable polymers, thereby providing LC defect stamps with high spatial resolution over large areas. Our method was further utilized to transfer patterns with sub-micrometer features from the polymer stamp surface to a secondary surface by microcontact printing ($\mu $-CP). The patterning method based on LC defects has significant advantages over existing lithographic approaches: 1) the masters and stamps are easy to fabricate, 2) the masters and stamps provide long-range surface ordering over large-areas, 3) the periodic arrays are formed quickly in several seconds, and 4) the stamps can generate feature sizes on the micrometer and submicrometer length scales, and 5) the methodology offers the possibility of controlling the array geometry by altering the geometry of the confining channels. [Preview Abstract] |
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