Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L13: Liquid crystals: Mostly Nematics |
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Sponsoring Units: DFD Chair: Fangfu Ye, University of Illinois Room: B112 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L13.00001: Chirality and biaxiality in cholesteric liquid crystals Subas Dhakal , Jonathan Selinger Chiral liquid crystals commonly form a cholesteric phase, in which the molecular director is twisted into a helix. A longstanding problem in liquid-crystal science is how to determine the pitch of the cholesteric helix in terms of microscopic parameters, and how to explain why the pitch is so much larger than molecular length scales. One theory has argued that the pitch is large, i.e. the twist is small, because any twisting torque of one molecule on a neighbor requires at least short-range biaxial correlations between the molecules [1]. To investigate this concept, we develop a lattice model for chiral molecules interacting via anisotropic van der Waals forces. Through this model, we calculate the macroscopic pitch as a function of molecular chirality, molecular biaxiality, and temperature. These calculations show that the cholesteric twist decreases with increasing temperature, as seen in several experiments. Furthermore, they show that biaxial correlations enhance the twist, but are not required for a twist with this fluctuation-induced interaction. The simulation results are consistent with mean-field calculations for this model. [1] A.B. Harris, R.D. Kamien, and T.C. Lubensky, Phys. Rev. Lett. 78, 1476 (1997). [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L13.00002: Simulation studies of defect textures and dynamics in 3-d cholesteric droplets Vianney Gimenez-Pinto , Shin-Ying Lu , Jonathan Selinger , Robin Selinger We model defect texture evolution in droplets of cholesteric liquid crystals by solving for the dynamics of the nematic director field. In order to accommodate defects in the simulated texture, we use a finite difference formulation that is explicitly independent of sign reversal of the director at any position in the sample. Textures are visualized using either the Berreman 4x4 matrix method or by mapping free energy density. We study both planar and focal conic cholesteric textures in 3-d spherical and cylindrical droplets, with the goal to optimize device geometries for bistable display applications. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L13.00003: Photopolymerization-Induced Mesophase Transition in Relation to Phase Diagram of Reactive Nematic Mesogen and Hexagonal Columnar Liquid Crystal Mixture Tsang-Min Huang , Thein Kyu The phase diagram of columnar liquid crystal, 2,3,6,7,10,11-hexakispentyloxy triphenylene (HPTP), and nematic monomer, 4-(3-Acryloyloxypropyloxy)-benzoic acid 2-methyl-1,4-phenylene ester (RM257) mixtures has been investigated experimentally. Phase transition temperatures are determined by using polarized optical microscopy (POM), and differential scanning calorimetry (DSC). The phase diagram shows a eutectic phase behavior and consists of isotropic, nematic, order hexagonal, crystalline, and an induced mesophase which is not existed in the neat components. Wide-angle x-ray diffraction (WAXD) result shows this induced mesophase is the disordered hexagonal phase. By virtue of photocurable capability of RM257, the evolution of phase morphology of the LC mixtures subjected to photopolymerization has been also studied under UV illumination. Depending on reaction temperature and composition, the morphology of the cured mixtures can be fixed in the isotropic or anisotropic states. The existence of columnar phase of HPTP after reaction can be identified in some compositions by WAXD that undergoes reaction-driven phase transformation. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L13.00004: Glassy correlations and thermal fluctuations in nematic elastomers Bing Lu , Xiangjun Xing , Fangfu Ye , Paul Goldbart By means of the vulcanization theory framework we address the properties of nematic elastomers prepared in the isotropic liquid state and subsequently randomly cross-linked beyond the gelation point. We base our analysis on a model replica Landau free energy, in which the vulcanization order parameter is coupled to the order parameter describing the local degree of nematic ordering, retaining fluctuation terms to the Gaussian level. We explore how the cross-linking renormalizes the thermal correlations of the local nematic order, and also results in frozen-in, glassy nematic correlations. We examine these thermal and glassy correlations for two different preparation histories of the system: in the first, the cross-linking is done at temperatures close to the isotropic-nematic transition; in the other, the cross-linking is done at higher temperatures, but the system is subsequently cooled to near this transition temperature. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L13.00005: Minimal model for polydomain nematic elastomers Fangfu Ye , Bing Lu , Xiangjun Xing , Paul Goldbart Nematic elastomers are materials that possess both the elastic properties of cross-linked rubber and the orientational properties of nematic liquid crystals. When cross-linked in the isotropic phase, and subsequently cooled into the nematic regime, these materials usually form polydomain structures. To understand the formation of these polydomain structures, we develop a microscopic model based on an elastic network medium formed by the random end-linking, via springs, of rigid dimers. We assume that the dimers interact with one another through a Maier-Saupe term, and that the springs impose soft constraints, not only on the separations between the dimers that they link but also on the relative orientations of these dimers. We then use vulcanization theory to investigate: (i) how these orientational constraints lead to the emergence of polydomain structures, as the temperature is decreased; and (ii) the dependence of the characteristic size of the domains on temperature and link density. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L13.00006: Real space analysis by direct observation in liquid crystal dynamics Beom-Jin Yoon , Min Sang Park , Jung Ok Park , Mohan Srinivasarao We describe a facile route for studying dynamics of nematic liquid crystals. We analyzed real space images of the system that can't be resolved by optical microscopes, for which the typical investigations have been done in reciprocal space. We have imaged the director fluctuations, which are slow and large enough to be detected by an optical microscope. A series of the digital images of a homogeneously aligned nematic liquid crystals were obtained under the cross-polarized microscope with a non-coherent light, white light source, and a high speed camera. We made Fourier analysis of the time and spatial series of the images, and this procedure provided a wide range of the wave vector dependent information, which is equivalent to multi-angle light scattering of the nematic liquid crystals. The characteristic times of the correlation function were measured. Dynamic properties of the liquid crystals and temperature dependence of them were evaluated through combination of real space intensity information, Fourier optics, and well known theories developed in scattering method. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L13.00007: Prediction of flow-aligning and tumbling in a bent-core nematic liquid crystal using measurements of orienation order parameters Min Sang Park , Beom-Jin Yoon , Jung Ok Park , Mohan Srinivasarao The flow behavior of bent-core nematic liquid crystal (A131), which has been known to exhibit a biaxial nematic phase, is predicted by measurements of 2$^{nd}$ and 4$^{th}$ rank orientation order parameters. Using experimentally determined uniaxial, $<$P$_{200}>$ and $<$P$_{400}>$ , and biaxial orientation order parameters, $<$P$_{220}>$, $<$P$_{420}>$ and $<$P$_{440}>$ from polarized micro-Raman spectroscopy, we compute the tumbling parameter, $\lambda $. The relationships between the order parameters and tumbling parameter derived by 2 different groups are used and the results are computed: a molecular theory by Archer and Larson (1995), that by Kroger and Seller (1995) for uniaxial system, and Leslie's theory for 2-director continuum. Temperature evolution of tumbling parameter shows the transition from a flow alignment regime to a tumbling instability. The results of the temperature evolution of tumbling parameter of bent-core nematic LC are compared to those of pure nematic LC (5CB) and LC mixture (E7). [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L13.00008: Elastic constants and viscosities of a bent-core nematic liquid crystal studied by dynamic light scattering and magnetic Frederick transition Madhabi Majumdar , Peter Salamon , James Gleeson , Antal Jakli , Samuel Sprunt We present a study of determining absolute magnitudes of orientational Frank elastic constants and corresponding viscosities together with the viscoelastic ratios for director fluctuations in one of the bent-core nematic compounds \textit{CIPbis10BB} by both the dynamic light scattering method and the magnetic and electric field induced director reorientation (Frederick transition). The values of the splay, twist and bend Frank elastic constants and viscosities are as follows K11=3.4$\times $10$^{-7}$ dynes, K22=3.4$\times $10$^{-8}$ dynes, K33=1.8$\times $10$^{-7}$ dynes, $\eta _{splay}$=12P, $\eta _{twist}$=4.1P and, $\eta _{bend}$=23P. Our result shows that the orientational elastic constants of the BCN studied are 3 to 5 times lower than those of typical calamitics (5CB) and K11$>$K33$>>$K22. A dramatic enhancement of orientational viscosities (4 to 100 times larger than calamitics) was confirmed in BCNs. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L13.00009: Bulk Structure and Interface Ordered Focal Conic Domains of the Dark Conglomerate Phase of a 4,4'-diphenylmethane Based Bent-Core Mesogen J. Maclennan , D. Chen , Y. Shen , C. Zhu , L. Hough , M. Glaser , N. Clark , N. Gimeno , M. Ros The saddle-splay topology of the dark conglomerate (DC) phase has been established recently [L. E. Hough, \textit{et al. Science} \textbf{325}, 452 (2009)]. On cooling, usually amorphous or disordered focal conic domains are obtained in the bulk. However, in the dark conglomerate phase of a 4,4'-diphenylmethane based bent-core mesogen at the liquid crystal/air interface, the air imposes strong homeotropic alignment at the free surface of the liquid crystal, forcing the smectic layers to form parallel to the surface. The bulk preference for saddle-splay curvature in the DC phase is then manifested at the surface as toric focal conic domains (TFCDs). The internal fluidity of the phase allows the TFCDs to anneal into a quasi ordered array, essentially forming a hexagonal structure with a periodicity of about 400 nm, correspond to the minimum elastic free energy. This directly confirms the proposed plumber's nightmare structure for the DC phase and indicates that ordered TFCDs may be produced in this phase under suitable conditions. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L13.00010: Blue Phase Mixtures of Bent-Core Liquid Crystals and Chiral Dopants Stefanie Taushanoff , Khoa Van Le , Robert Twieg , Hideo Takezoe , Antal Jakli Stable blue phase materials are made using nematogenic bent-core liquid crystals doped with a high twisting power chiral material. Studies show the existence of stable BPIII (blue fog) phase in a relatively wide 10-20\r{ }C temperature range. Polarizing optical microscopy, optical rotation and electro-optical studies were used to characterize the material. [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L13.00011: Cones and Anticones: Spontaneous Mechanical Response of Disclinations in Nematic Glasses Carl Modes , Kaushik Bhattacharya , Mark Warner Nematic elastomers and glasses respond strongly to changes in ambient heat or light, and the response along the director differs significantly from that in the normal directions. This phenomena is well characterized for simple nematic director fields, less so for more complicated textures. We analytically examine the elastic ground states of a nematic glass in the membrane approximation as a function of temperature for some of these more complicated director fields. In particular, we are interested in textures arising from disclination defects with an eye towards fabricating three-dimensional shapes from flat sheets of material, at the nano-scale all the way to macroscopic objects. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L13.00012: A Line Defect Structure in Soft-Mode Turbulence Rinto A Nur Qomaru Zaman , Tatsuhiro Ueki , Yoshiki Hidaka , Michael I. Tribelsky , Shoichi Kai Defects have been much investigated in various physical systems. The property and symmetry in a system can be reflected by the existence of defects. For example in spin models, symmetries in the 2D XY and 2D Ising models generate point and line defects, respectively. In the soft-mode turbulence (SMT) in electroconvection of homeotropic nematic systems which is a kind of spatiotemporal chaos induced by nonlinear interaction between the Nambu-Goldstone modes and the convective modes, a curious line structure called $blackline$ has been discovered. We measured the density of the blackline as a function of control parameters, ac voltage and frequency. By detailed observations and analysis, it is clarified that the blackline is a structure of the nematic director in the x-y plane and includes a sequence of point defects. We discussed similarity with the density of the blackline and that of the point defect in the conventional 2D XY model. The occurrence of this type of defects is only due to the symmetry in the SMT and independent of the properties of fluctuations. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L13.00013: Macroscopic Behavior of Nematics with D$_{2d}$ Symmetry Harald Pleiner , Helmut R. Brand We discuss the symmetry properties and the macroscopic behavior of a nematic liquid crystal phase with D$_{2d}$ symmetry. Such a phase is a prime candidate for nematic phases made from banana-shaped molecules where the usual quadrupolar order coexists with octupolar (tetrahedratic) order. The resulting nematic phase is non-polar. While this phase could resemble the classic D$_{\infty h}$ nematic in the polarizing microscope, it has many static as well as reversible and irreversible properties unknown to non-polar nematics without octupolar order. In particular, there is a linear gradient term in the free energy that selects parity leading to ambidextrously helical ground states when the molecules are achiral. In addition, there are static and irreversible coupling terms of a type only met otherwise in macroscopically chiral liquid crystals, e.g. the ambidextrous analogues of Lehmann-type effects known from cholesteric liquid crystals. Finally, we discuss certain nonlinear aspects of the dynamics related to the non-commutativity of three-dimensional finite rotations as well as other structural nonlinear hydrodynamic effects. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L13.00014: A novel soft-core spherocylinder model for liquid crystals Jing Zong , Xinghua Zhang , Qiang WANG Interaction models with soft-core repulsions that allow particle overlapping give orders of magnitude faster/better sampling of configurational space than those with hard-core repulsions (e.g., hard spherocylinders or Gay-Berne potential). While several soft-core spherocylinder models were recently proposed$^{1}$, their repulsive interaction depends only on the shortest distance between two spherocylinders. Here we present a novel, computationally efficient soft-core spherocylinder model, which gives exact treatment of the excluded-volume interactions and anisotropic shape of two particles (thus the orientational interaction between them favoring their parallel alignment). It further takes into account the degree of overlap between the two particles, thus superior to other soft-core models. This model has great potential applications in the study of liquid crystals, block copolymers containing rod blocks, and liquid crystalline polymers. [1] \textit{Z. E. Hughes et al.}, \textbf{Comput. Phys. Commun., 178}, 724 (2008); \textit{J. S. Lintuvuori and M. R. Wilson}, \textbf{J. Chem. Phys., 128}, 044906 (2008). [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L13.00015: Nematic order on curved spherical spaces Alberto Fernandez-Nieves , Teresa Lopez-Leon , Sharan Devaiah , Ekapop Pairam When nematic liquid crystals are confined to spherical shells, complex defect structures emerge. These structures are characterized by a varying number of point defects and disclination lines, depending on the elastic energy of the liquid crystal, the thickness of the shell, and the boundary conditions for the director at the confining spheres. Topology establishes restrictions that must be fulfilled, but it is the energy landscape that ultimately determines the final state of the system. By using double emulsion droplets, we can experimentally address this fascinating interplay between topology and energy. We find a wealth of defect structures in our shells and propose that the shell thickness inhomogeneity is the key parameter enabling the broad range of configurations we observe; these include long-time predicted configurations, as well as new structures and transitions between them that where never considered before. In addition, we hope to extend our studies to non-spherical surfaces, such as the torus and higher-genus surfaces. For this purpose, we have recently generated toroidal droplets and have studied their hydrodynamic stability. On these closed surfaces, the nature of the defect structure is expected to be qualitatively different from that of the spherical case. [Preview Abstract] |
Tuesday, March 16, 2010 5:30PM - 5:42PM |
L13.00016: Diffusion in Rodlike Polymer Liquid Crystals Paul Russo , Garrett Doucet , Jianhong Qiu The optical tracer self diffusion of the rodlike polymer PBLG has been studied in solutions spanning the isotropic-liquid crystalline phase boundary for two different molecular lengths ($L$ = 92.1 nm, axial ratio $x $= 58 and $L$ = 16.8 nm, $x$ = 10.5). The results are compared to previously published data at $L $= 159 nm, $x $= 99 (\textit{J. Chem. Phys.}, 1999, 111(4), 1746-1752). Under the conditions of the measurements, the two longer rodlike polymers align in the liquid crystalline phase with their cholesteric screw axis in the vertical direction. The diffusion is measured in a particular direction normal to this over distances long compared to the polymer lengths. Except for the shortest polymer, whose cholesteric screw axis does not assume the vertical alignment, the diffusion suddenly increases as the liquid crystalline phase is entered, then declines, signaling the disappearance of topological constraints in the isotropic phase having a vertical component. The parallel component of diffusion is little reduced in the least concentrated liquid crystalline regime, compared to dilute solutions. The solutions also contain a small, fluorescent component whose mobility was not at all affected by the isotropic-liquid crystalline phase transition. Additional measurements have been made for magnetically aligned rods, in which case the differences between parallel and perpendicular diffusion components can be measured directly. [Preview Abstract] |
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