Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J18: Liquid Crystals III: Mostly Nematics and Cholesterics |
Hide Abstracts |
Sponsoring Units: DCMP GSNP Chair: Matthew Glaser, University of Colorado, Boulder Room: 403 |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J18.00001: Twist-bend nematic liquid crystals in high magnetic fields Antal Jakli, Pavan Challa, Volodimyr Borshch, Owain Parri, Samuel Sprunt, Oleg D. Lavrentovich, James T. Gleeson We present magneto-optic measurements on two odd-numbered dimer molecules that form the recently discovered twist-bend nematic (N$_{TB}$) phase, which represents a new type of 3-dimensional anisotropic fluid with about 10 nm periodicity and accompanied optical stripes. We show that B $=$ 25T shifts downward the N-N$_{TB}$ phase transitions by almost 1$^{\circ}$C, and explain it quantitatively. We also show that the optical stripes can be unwound by a temperature and material dependent magnetic induction in the range of B $=$ 5-25T. Finally, we propose a Helfrich-Hurault type mechanism for the optical stripe formation. Based on this model we calculate the unwinding magnetic field, and find agreement with our experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J18.00002: Second harmonic light scattering from a bent-core nematic liquid crystal Shokir Pardaev, Antal Jakli, Robert Twieg, Jarrod Williams, James Gleeson, Brett Ellman, Samuel Sprunt We study the angular distribution of second harmonic (SH) light scattered from the aligned nematic phase of a bent-core liquid crystal. Throughout the nematic range and for certain combinations of polarizations of the fundamental and second harmonic fields, we detect peaks in the SH signal at non-zero scattering vectors ($\pm$q) along the nematic director, while the signal for q = 0 (forward direction) remains at the background level. The value of q, which corresponds to a length scale in the micron range, decreases with temperature when heating toward the nematic to isotropic transition. We will present a model to explain the major aspects of our results and indicate their significance in terms of the structure of bent-core nematics. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J18.00003: Direct Observation of Heliconical Pitch in the Twist-bend Nematic Liquid Crystal Phase of Bent Molecular Dimers Min Shuai, Michael Tuchband, Dong Chen, Arthur Klittnick, Joseph Maclennan, Matthew Glaser, Noel Clark, Eva Korblova, David Walba Nanometer-scale modulation of the director field is directly observed using freeze-fracture transmission electron microscopy (FFTEM) in the heliconical twist-bend nematic (N$_{\mathrm{TB}})$ phase, a periodic mesophase with no detectable modulation of the electron density [Chen, D., \textit{et al}., PNAS, 2013, 110(40):15931--15936]. A homologous series of achiral odd-methylene-linked~dimers CB$m$CB ($m =$ 5, 7, 9, and 11) and binary mixtures with simple cyanobiphenyl $n$CBs ($n =$ 5, 6, 7, and 8) in the N$_{\mathrm{TB}}$ phase has been studied. The helix pitch is found to vary between 6 and 11 nm. Increase the m or n value increases the helix pitch. Meanwhile, surprisingly, the helix pitch becomes shorter as the monomer concentration in the mixtures increases. FFTEM images show homogenous phases and preliminary measurements of the transition temperature versus concentration indicate that the binary mixtures are close to ideal. Polarized optical microscopy and calorimetry are carried out to study the nature of the N-N$_{\mathrm{TB}}$ transition in detail. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J18.00004: Distortion of cholesteric helical structures by in-plane fields and effect on the efficiency of second-order Bragg reflections Mariacristina Rumi, Timothy Bunning When an electric field is applied perpendicular to the helical axis of a chiral nematic liquid crystal, as at the center of gap regions in cells with interdigitated electrodes, materials with positive dielectric anisotropy and in a planar conformation not only experience an elongation of the pitch, but also undergo a deformation of their helical structure so that the refractive index deviates from exhibiting a sinusoidal variation in any direction perpendicular to the axis. Under these conditions, Bragg reflections of all orders are in principle active even for light at normal incidence. We will show that for various chiral nematic mixtures with the main (first-order) reflection in the near-infrared range, second and third-order reflections can easily be observed in the visible range when an electric field is applied and the material optical properties are probed selectively in the center of gap regions. The efficiency of the second-order reflections increases with the magnitude of the applied field and can become comparable to that of the first order reflection at high fields. This process can be exploited in the design of switchable reflective filters whose reflection band can be electrically tuned. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J18.00005: Colloidal Transport and Periodic Stick-Slip Motion in Cholesteric Finger Textures Kui Chen, Linnea Metcalf, Daniel H. Reich, Robert L. Leheny We have investigated the transport of colloidal particles within cholesteric finger textures formed by mixtures of the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) and the chiral dopant4-(2-methylbutyl)-4-cyanobiphenyl (CB15) with cholesteric pitches between 24 and 55 micrometers. Spherical silica colloids (radius 5-10 micrometers) moving under the force of gravity through the texture translated strictly perpendicular to the cholesteric axis and had no measurable mobility parallel to the axis. Thus, when the applied force was oriented at an oblique angle to the axis, the spheres moved at an angle to the force. Nickel disks, 20 micrometers in radius and 300 nanometers thick, driven by gravity similarly showed no mobility parallel to the cholesteric axis for small pitch. For larger pitch, the disks displayed a periodic stick-slip motion caused by elastic retardation followed by yielding of the finger texture. Effective drag viscosities obtained from the sphere and disk motion were anomalously large compared with those of pure 5CB. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J18.00006: Deformations in chiral liquid crystals Petr Shibaev, Kathryn Reddy, Daniel Bateman, Andrey Iljin Deformations and their relaxation in chiral liquid crystals are studied experimentally and theoretically in planar geometry for liquid crystalline mixtures of varying viscosities. It is shown by both methods that shear deformation in liquid crystals results in the inclination and extension of cholesteric helix in samples with high viscosity [1,2]. Stretching deformation results in shrinking cholesteric helix. This leads to a possibility of detecting deformations on a nanometer scale by observing changes in selective reflection spectra. Theoretical model takes into account elastic strain of physical network formed by the entanglements between components of liquid crystalline mixture, viscosity of the matrix and elasticity of the liquid crystalline subsystem. This allows to model mechanical response of the matrix with different viscosities to stretching and shear of various amplitudes. It is shown that relaxation of the cholesteric helix takes much shorter time than mechanical relaxation of the mixtures. The model perfectly agrees with experimental data. The model is compared with theoretical model describing behavior of elastomers. \\[4pt] [1] P.V. Shibaev, L. Newman, \textit{Liquid Crystals}, 40, 428 (2013);\\[0pt] [2] P.V. Shibaev,C. Schlesier, \textit{Applied Physics Letters, 101, 193503 (2012)} [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J18.00007: Electrooptic response of chiral nematic with conical helicoidal director deformation Oleg Lavrentovich, Jie Xiang, Sergij Shiyanovskii, Corrie Imrie Electrically induced reorientation of liquid crystal (LC) director caused by dielectric anisotropy is a fundamental phenomenon widely used in modern technologies. We report on the experimental observation of an electrooptic effect with a distinct conical helicoid deformation of the director. The effect is observed in a chiral nematic in which the ground state of the director represents a right-angle helicoid. Application of the electric field along the helicoid axis transforms it into an oblique-angle helicoid. Further increase of the field causes complete unwinding into a uniaxial nematic state, in agreement with the theory proposed by R.B. Meyer in 1968. The effect is observed in a dimer nematic material in which the bend elastic constant is smaller than its twist counterpart. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J18.00008: A Simple Procedure for The Determination of Refractive Indices of Liquid Crystals from High-Resolution Birefringence Measurements E. Kutlu, H. Ozbek, S. Ustunel, M. Cetinkaya We present a simple procedure to determine the temperature dependence of the extraordinary and ordinary refractive indices of liquid crystals based on the high precision birefringence measurements. We show that the procedure needs only a single value for refractive index, namely n$_{\mathrm{I}}$ being the value of the refractive index in the isotropic phase just above the nematic-isotropic (NI) transition temperature apart from the birefringence. In most studies the calculation of the order parameter is based on the Haller approximation known to be inconsistent with the weakly first-order character of the N-I transition and to lead systematically lower values for the critical exponent. Here, we revisit the methodology for the determination of the orientational order parameter in the N phase We have calculated the order parameter by applying Vuks and Neugebauer models and the procedure by Kuczynski \textit{et al.} We conclude that the approximation for the average refractive index \textless n\textgreater $\approx $ n$_{\mathrm{I}}^{2}$ is plausible to determine the temperature dependence of the refractive indices together with the birefringence data. This procedure allows one to obtain the normalised polarizabilities of the extraordinary and ordinary rays without addressing density measurements. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J18.00009: Simulated Textures of Toroidal Nematic Liquid Crystal Droplets Perry Ellis, Alberto Fernandez-Nieves Nematic liquid crystals under confinement by curved surfaces can produce complex hierarchical structures whose design principles and properties have yet to be unraveled. Here we focus on toroidal geometries and perform computer simulations of the nematic textures seen between crossed-polarizers. We find agreement with experiments using director fields that exhibit pronounced twist deformations with contributions from bend and splay. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J18.00010: Liquid crystals confined inside toroids Karthik Nayani, Jayalakshmi Vallamkondu, Eric Dancu, Jung Ok Park, Mohan Srinivasarao, Alberto Fernandez-Nieves We generate stable toroidal droplets of liquid crystals stabilized inside a yield stress material. In this talk we discuss aspects of both nematic and cholesteric tori: from double twist to low pitch organization. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J18.00011: Magnetic field-induced suppression of the amorphous Blue Phase Pavan Challa, Samuel Sprunt, Antal Jakli, James Gleeson We present magneto-optical measurements on two liquid crystals that exhibit a wide temperature-range amorphous blue phase (BPIII). Magnetic fields up to 25T are found to suppress the onset of BPIII in both materials by almost 1 $^{\circ}$C. This effect appears to increase non-linearly with the field strength. The effect of high fields on established BPIII's is also reported, in which we find significant hysteresis and very slow dynamics. Possible explanations of these results are discussed. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J18.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J18.00013: Observation of NanoDNA Liquid Crystal Phases from Four Base Pair Duplexes at Subambient Temperatures Gregory Smith, Tommaso Fraccia, Tommaso Bellini, David Walba, Noel Clark Based upon conventional Onsager model considerations, liquid crystal (LC) formation in DNA-water mixtures was originally thought to be impossible for DNA polymers of very short length (\textless 100 bases). We originally reported the discovery of chiral nematic (N*), columnar C$_{\mathrm{U}}$ and C$_{2}$ LC phases in NanoDNA oligomers as short as 6 bases in length and have since described additional LC phases involving DNA with random sequences and various blunt or sticky-end duplex architecture, all in the regime of \textless 20 bases. These results suggested a self-assembly motif where hydrophobic forces or hydrogen bond mediated base-pairing enable unusually short polymers to stack into functionally longer units that permit them to exhibit LC phase behavior. We report now the existence of LC phases of ultra short duplexed NanoDNA, 4 bases in length, in blunt-end, sticky-end and random sequence configurations, all observed at temperatures of $\sim$ 5 $^{\circ}$C and not stable \textgreater 13-15 $^{\circ}$C. These oligomers demonstrate an unusual wealth of phase behavior, including the typical N*, C$_{\mathrm{U}}$ and C$_{2}$ phases as well as higher order dark and bright phases, including what we believe to be a Blue Phase. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J18.00014: Direct observation of lens-shaped nematic tactoids and micro-phase separation in aqueous suspensions of $\alpha $-zirconium phosphate nanosheets Yue Shi, Min Shuai, Yongqiang Shen, Dong Chen, Joseph Maclennan, Zhengdong Cheng, Noel Clark We study the ordering of monolayer $\alpha $-zirconium phosphate nanosheets in aqueous suspensions. As the concentration increases, we confirm that the inter-plate spacing decreases, the X-ray correlation increases, and there is nematic ordering even at the highest concentrations. The inter-plate spacing shows linear swelling behavior, but not as expected for a uniform swelled system. The micro-phase separation is proposed and further demonstration by centrifugation, optical microscope, confocal fluorescent microscope and freeze fracture TEM. Self-assembling of stacks of the platelets are proposed to form liquid crystal phases. Lens-shaped tacoids with radial director field are observed, and the quantitative analysis of the tactoid properties gives estimates of ratios of the bulk elastic constant, anchoring strength to the bare surface tension. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J18.00015: Modulated liquid-crystal phases induced by polarity: Twist-bend, splay-bend, and blue phases Jonathan Selinger, Shaikh Shamid, David Allender Nematic liquid crystals exhibit flexoelectric couplings between polar order and gradients in the director field. When the couplings become strong enough, the uniform nematic phase can become unstable to the formation of a modulated polar phase. The question is then: What is the structure of the modulated polar phase? Classic work by Meyer and further studies by Dozov predicted two possible structures, known as twist-bend and splay-bend. One of these predictions, the twist-bend phase, has recently been identified in experiments on bent-core liquid crystals. Here, we investigate modulated polar phases through a combination of Landau theory and lattice simulations. We find a range of possibilities, including the twist-bend and splay-bend phases as well as polar blue phases, with 2D or 3D modulations of the director field and the polar order. We compare these polar blue phases with chiral blue phases, and discuss opportunities for observing them experimentally. [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