Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V8: Liquid Crystals I: Structure and Defects |
Hide Abstracts |
Sponsoring Units: DFD Chair: Oleg Lavrentovich, Kent State University Room: Morial Convention Center RO6 |
Thursday, March 13, 2008 11:15AM - 11:27AM |
V8.00001: Effect of Concentration Variations on the Interaction of a Sm-A Liquid Crystal and a Nanoparticle Luz J. Martinez-Miranda, Lynn K. Kurihara, Rahina S. Rabiu We have observed the evolution of the interaction between a Sm-A Liquid Crystal (8CB) and a nanoparticle as the concentration of the nanoparticle is reduced from 30{\%} wt to 0.1{\%}wt. We have observed that a linear structure is observed as the concentration of the particle falls below 15{\%} wt. There is a difference between the functionalization compounds as observed in the study of the 30{\%} wt mixtures. In addition, we have observed that the influence of the nanoparticle in the ordering or disordering of the liquid crystal can be quantified through the integrated intensity of the x-ray signal. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V8.00002: Simulating defect structures in nematic liquid crystal shells Lena Lopatina, Andrew Konya, Jonathan Selinger, Robin Selinger, Alex Travesset Recent theoretical and experimental studies have investigated nematic liquid crystals confined to a shell between two spheres. When the shell is very thin, the structure provides an experimental realization of nematic order and defects in a 2D curved geometry. As the shell becomes thicker, the behavior crosses over to a 3D liquid crystal, with different types of defects. To study this dimensional crossover, we perform simulations of nematic order in a shell. For these simulations, we use a disordered lattice, or mesh, constructed through random sequential adsorption on the inner surface, the outer surface, and within the bulk of the shell, with a nematic director on each site of the mesh. By minimizing the energy, we determine the nematic texture as a function of the radii and thickness of the shell, and as a function of the off-center displacement of the inner sphere. The results show a crossover between half-charged vortex line defects for thin shells and boojum pairs for thicker shells, and demonstrate a new equilibrium state with two vortex lines and one boojum pair. They also show a complex evolution of the structures and energy as the inner sphere moves off-center. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V8.00003: String defects in smectic-C monolayers and hybrid nematic films Fangfu Ye, Zhao Lu, Lena Lopatina, Jonathan Selinger, Alex Travesset Defects are important in determining the structure and statistical mechanics of liquid crystals. In this project, we study the structure of topological defects in smectic-C monolayers and hybrid nematic films. When subjected to boundary conditions requiring a total topological charge of +1, the classical xy model in a flat disk geometry has a ground state of a single vortex of charge +1. By comparison, perfect nematics with directors in the same 2D geometry have two +1/2 vortices repelling each other. We show that slightly tilting directors out of the plane to form a smectic-C monolayer yields two +1/2 vortices bound together by a domain wall, which we call a string defect. We also develop a model easily testable in experiments, in which a thick nematic film is constrained in a cell with weak homeotropic anchoring on one surface and strong planar anchoring on the other surface. For this model, we derive the phase diagram and investigate how the length of string defects changes with the thickness of the cell. We further present numerical simulations of the nematic director in the hybrid film. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V8.00004: Coherent anti-Stokes Raman scattering polarized microscopy of 3D director structures in liquid crystals Ivan Smalyukh, Alexander Kachynski, Andrey Kuzmin, Paras Prasad We demonstrate labeling-free three-dimensional imaging of director structures in liquid crystals using coherent anti-Stokes Raman scattering (CARS) polarized microscopy [1]. Spatial mapping of the structures is based on the strong sensitivity of a polarized CARS signal to the orientation of selected chemical bonds of anisotropic molecules in liquid crystals. As an example, we study director structures in cholesteric, nematic, and smectic materials. We demonstrate that the CARS images of molecular orientation patterns are consistent with the structure models and with the respective computer-simulated CARS textures. \newline [1]. A.V. Kachynski, A.N. Kuzmin, P.N. Prasad, and I.I. Smalyukh, Appl. Phys. Lett. 91, 151905 (2007). [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V8.00005: Light Scattering Study of Biaxiality in Nematic Liquid Crystal Tetrapodes Krishna Neupane, Shinwoong Kang, Sunil Sharma, D. Carney, T. Meyer, George H. Mehl, David W. Allender, Satyendra Kumar, Samuel Sprunt We have performed dynamic light scattering studies on thermotropic liquid crystalline tetrapodes [1], which reportedly exhibit a uniaxial to biaxial nematic phase transition. Our results [2] support the existence of the biaxial nematic phase in tetrapodes. The uni - to biaxial transition is found to be weakly first-order in a 4-ring tetrapode and second-order in a 3-ring tetrapode, while the isotropic to uniaxial nematic transition is weakly first order in both materials. The temperature dependence of the relaxation rates of the biaxial order parameter modes, and of the intensity associated with biaxial director fluctuations, is explained by a Landau-deGennes model of the free energy. \newline [1] R. Elsasser, J. W. Goodby, G. H. Mehl, D. Rodriguez-Martin, R. M. Richardson, D. J. Photinos, and M. Veith, Mol. Cryst. Liq. Cryst. \textbf{402}, 237 (2003) \newline [2] K. Neupane, S. W. Kang, S. Sharma, D. Carney, T. Meyer, G. H. Mehl, D. W. Allender, S. Kumar, and S. Sprunt, \textit{Phys. Rev. Lett.} \textbf{97}, 207802 (2006) [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V8.00006: Effects of dielectric relaxation on the director dynamics of uniaxial nematic liquid crystals Mingxia Gu, Ye Yin, Sergij V. Shiyanovskii, Oleg D. Lavrentovich We derive the reorienting dielectric torque acting on the director, considering the frequency dependence of the dielectric tensor. The model takes account into the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor and predicts the ``dielectric memory effect'' (DME), i.e., dependence of the dielectric torque on both the ``present'' and ``past'' values of the electric field and the director. In a sharply rising electric field, the DME slows down director reorientation for the materials whose dielectric anisotropy is positive at low frequencies, but speeds up the response for the dielectrically negative materials. We also demonstrate, both theoretically and experimentally that an induced ``memory'' polarization leads to a dielectric torque in the switch-off phase which has an opposite sign to that of the LC's dielectric anisotropy, when a specific switching-off profile is used; this reverse torque accelerates the director relaxation back to the equilibrium state. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V8.00007: Condensation of lyotropic chromonic liquid crystals by additives Luana Tortora, H.-S. Park, S.-W. Kang, S. Kumar, K.V. Kaznatcheev, O.D. Lavrentovich Lyotropic chromonic liquid crystals (LCLCs) are formed by molecules with rigid polyaromatic cores and ionic groups at the periphery that aggregate in water. Condensation of LCLCs can be driven by polyamines, organic salt and neutral polymers. At a suitable concentration of additives, a nematic LCLC demixes into a coexisting isotropic phase and a condensed phase with birefringence higher than that in the original N. By employing synchrotron X-ray scattering we demonstrate the formation of a columnar hexagonal (C) phase. Scanning transmission X-ray microscopy, LC PolScope and fluorescent confocal microscopy allow us to map the relative concentration of components in the condensed and isotropic regions. Both electrostatic and entropy effects contribute to the condensation. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V8.00008: Interactions in the NOBOW and 8CB Mixtures Dong Chen, Chenhui Zhu, Noel Clark Mixtures of a bent-core mesogen (NOBOW) and a calamitic mesogen (8CB) are studied using X-ray diffraction (XRD), polarized light microscopy and freeze fracture electron microscopy (FFEM). XRD shows that as the 8CB concentration increases, the transition temperature of Iso-B4 decreases and the correlation length of NOBOW B4 decreases while the correlation length of 8CB SmA increases. Polarized light microscopy reveals that the mixtures have larger chiral domains than pure NOBOW and when the phase of 8CB changes to SmA, they show the same boundary as the chiral domains. FFEM images show more details on the structure of the mixtures. Along with the experiments, we will present theoretical studies on the interactions in the NOBOW and 8CB mixtures. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V8.00009: Effect of Solvent Concentration on the Liquid Crystal Phase Transitions of Octylcyanobiphenyl-Hexane Mixtures Krishna Sigdel, Germano Iannacchione The effect of a non-mesogenic, low-molecular weight, solvent on the phases of a liquid crystal can be profound. High-resolution ac-calorimetry has been carried out on the isotropic to nematic (\emph{I}-\emph{N}) and the nematic to smectic-\emph{A} (\emph{N}-Sm\emph{A}) phase transitions of the liquid crystal octylcyanobiphenyl (8CB) as a function of hexane concentration. Temperature scans were performed above and below these transition temperatures for all samples. Six 8CB+hexane samples were studied having molar concentrations of 0.017 (0.5\% by mass), 0.033 (1\%), 0.063 (2\%), 0.078 (2.5\%), 0.092 (3\%) and 0.119 (4\%) of hexane. Upon increasing dilution of 8CB by the linear form of hexane, the transition temperatures shift lower while the order of both transitions evolves. These effects may be the consequence of the weakening of the liquid crystal molecular interactions due to the presence of the solvent. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V8.00010: Landau potential of polymer stabilized ferroelectric liquid crystals Paul Archer, Ingo Dierking Polymer stabilized liquid crystals (PSLC) [1] consist of a relatively low concentration of a photo-polymerized monomer (typically less than 10{\%}wt) which is phase separated from the continuous liquid crystal medium. For the case of a polymer stabilized ferroelectric liquid crystal, photo-polymerized in the SmA* phase, the network forms parallel to the smectic layer normal. This results in an elastic coupling between the polymer network and the liquid crystal which alters the characteristics of the SmA* to SmC* phase transition. The generalized model of ferroelectric liquid crystals has been modified to encompass this additional interaction through a polymer coupling coefficient. Analysis of experimental tilt angle and polarization data allows the determination of the polymer coupling coefficient and hence the full Landau potential. Results will be shown and discussed for varying polymer concentration. \newline [1] I. Dierking, Adv. Mater. \textbf{12}, 167 (2000) [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V8.00011: Varying the optical properties of cholesteric liquid crystals Sabrina Relaix, Michele Moreira, Peter Palffy-Muhoray, Michel Mitov Cholesteric Liquid Crystals (CLCs) are of particular interest as they form self-assembled photonic band gap (PBG) structures - a macroscopic helical structure, leading to a selective reflection of light - which can be easily tuned by external fields. As PBG materials, CLCs have been used as mirrorless lasers with low lasing thresholds since the density of photon states is suppressed in the reflection band and is enhanced at its edges [1]. The modification of the cholesteric organization -- either by the introduction of a pitch gradient across the cell or by the incorporation of nanoparticles in the medium -- has direct consequences on the PBG and hence the reflected intensity [2,3]. In this presentation, I will describe the variations in the optical properties of CLC caused by these modifications and will discuss possible applications, such as tuning the CLC laser wavelength or adjusting the laser threshold. \newline [1] P. Palffy-Muhoray et al., Phil Trans R Soc A 364, 2747 (2006) \newline [2] S. Relaix et al., Appl. Phys. Lett. 89, 251907 (2006) \newline [3] S. Relaix et al., Liq. Cryst. 34, 1009 (2007) [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V8.00012: Three-dimensional imaging of chemical bond orientation in liquid crystals by coherent anti-Stokes Raman scattering microscopy. Oleg D. Lavrentovich, Heung-Shik Park, Brian G. Saar, X. Sunney Xie Coherent anti-Stokes Raman scattering (CARS) microscopy is used to provide three-dimensional chemical maps of liquid crystalline (LC) samples without the use of external labels. CARS is a polarization-sensitive optical imaging technique that derives contrast from Raman-active molecular vibrations in the sample. Compared to other three-dimensional imaging techniques, CARS offers the most rapid chemical characterization available without the use of external dyes or contrast agents. Examples that illustrate the applicability of CARS microscopy to LCs include textures and defects in nematic and smectic LC, electric Frederiks transition. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V8.00013: Low electric field induced phase transition of the B1 bent-core liquid crystal phase to a switching phase J. Kirchhoff, L.S. Hirst Liquid crystal materials that have ferroelectric and antiferroelectric phases are useful in applications due to their switching properties. The B1 bent-core liquid crystal phase is a columnar phase that does not exhibit switching. A transition from the B1 liquid crystal phase to a switching phase has been seen at an electric field of 10 V/$\mu $m, which is much lower than previously seen fields of greater than 25 V/$\mu $m [1]. This transition is irreversible upon reduction of the applied field and switching continues almost threshold-less down to an applied field of 40 mV/$\mu $m, which has not been previously reported. Any amount of a chiral rod-like dopant increases the field required to transition from the B1 to the switching phase, and the transition becomes reversible with the mixture relaxing back to the B1 phase after a decrease in the electric field. A small concentration of the rod-like dopant also induces a change from the B1 phase to a new liquid crystal phase. These effects were studied using polarized optical microscopy, calorimetry (DSC), and x-ray measurements. \newline [1] J. Ortega et. al., Phys. Rev. E, \textbf{69}, 011703 (2004) [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V8.00014: Evidence of Broken Reciprocity in Chiral Liquid Crystals Michele Moreira, Nithya Venkataraman, Bahman Taheri, Peter Palffy-Muhoray Reciprocity in light scattering is predicated on bounded scattering media with symmetric and linear permittivity, conductivity and permeability. Due to their anisotropy and chirality, cholesteric liquid crystal form periodic dielectric structures. If the periodicity is comparable to the wavelength of light, these phases are self-assembled photonic band gap structures. There appear in the permittivity odd powers of the wave vector resulting from nonlocality and broken inversion symmetry. Evidence of non-reciprocity has been found in optically active crystals by Bennett [1] and in stacks of cholesteric and nematic liquid crystal cells by Takezoe [2]. We present experimental data showing broken reciprocity in transmittance and reflectance in cholesteric cells with different pitches having overlapping but distinct reflection bands. We explain our results in terms of simple analytic descriptions of material properties and propagating modes. \newline [1] P.J. Bennett, S. Dhanjal, Yu. P. Svirko and N. I. Zheludev, \textit{Opt. Lett}. \textbf{21}, 1955 (1996) \newline [2] J. Hwang; M.H. Song; B. Park; S. Nishimura; T. Toyooka; J.W. Wu; Y. Takanishi; K. Ishikawa; H. Takezoe, \textit{Nat. Mat.} \textbf{4}, 383 (2005). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700