Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B37: Liquid Crystals II |
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Sponsoring Units: DFD Chair: Luz J. Martinez-Miranda, U of Maryland Room: LACC 512 |
Monday, March 21, 2005 11:15AM - 11:27AM |
B37.00001: Structure within bent-core liquid crystals films at the air/water interface Lu Zou, Ji Wang, Elizabeth K. Mann, Antal J\'akli, Wolfgang Weissflog Films of three different classes of bent-core liquid crystals in Langmuir thin films at the air-water interface are considered. X-ray diffraction studies complement characterization of the films by surface pressure, Brewster angle microscopy, x-ray, and surface potential measurements. Both the end chains and the dipole moment of the core are varied, to allow an exploration of different possible layer structures at the interface. Depending on the end chains, we see either liquid phases of molecules flat on the surface or ordered phases with correlation length $\sim $10nm; both cases give homogeneous, optically isotropic films. This material is based upon work supported by the National Science Foundation under Grant No.9984304. The surface potential work was supported by the Petroleum Research Fund, under grant ACS PRF{\#} 35293-G 7. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No W-31-109-Eng-38. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B37.00002: Investigation of the physical properties of banana liquid crystal fibers Christopher Bailey, Antal Jakli It has been known that smectic liquid crystals of rod-shape molecules can produce stable free-standing thin films which are created by stacks of smectic layers. An analogous system occurs for liquid crystals of bent-core molecules (`banana' liquid crystals) in which stable fibers are formed by multiple concentric cylindrical layers. These fibers can be created with very large length to diameter ratios. We have shown that these fibers can be formed in the liquid crystal phase and then stabilized in a crystalline state at room temperature. We investigated the conductivity of these fibers and the dynamics of fiber breaking in the liquid crystal and crystalline phases. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B37.00003: Adiabatic Scanning Calorimetric Investigation of the Smectic-A to Hexatic-B Phase Transition in the Liquid Crystal 65OBC. Jan Thoen, Bert Van Roie, Katleen Denolf, Guido Pitsi Adiabatic scanning calorimetry (ASC) was used to investigate the smectic-A to hexatic-B (SmA -- HexB) phase transition in the liquid crystal n-hexyl-4'-n-pentyloxybiphenyl-4-carboxilate (65OBC). The high- resolution ASC technique allows one to measure simultaneously the enthalpy and specific heat capacity as a function of temperature and separate the true latent heat at (weakly) first-order phase transitions from pretransitional enthalpy changes. We were able to prove in a direct way that the SmA -- HexB transition in 65OBC is very weakly first-order with a latent heat of 0.04 +/- 0.02 J/g. Our experiments also confirm the high value of 0.64 +/- 0.05 for the critical specific heat capacity exponent earlier reported in literature, suggesting closeness of the transition to a tricritical point. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B37.00004: Optical Rotation of Layered Liquid Crystalline Phases in the Absence of Helical Winding of Molecular Orientation L.E. Hough, Noel Clark We show that periodic structures consisting of chiral layers can have significant optical activity. This optical activity is due entirely to the chiral structure of the individual layers, not to any chiral superlayer structure. In particular, we consider structures in which any large scale manifestations of chirality such as the helical winding of molecular orientation are either suppressed or absent. This optical activity has recently been observed in phases which lack any macroscopic birefringence - for instance orthoconic anticlinic smectics. Though very strong, the optical activity of such phases is still a small effect relative to the typical birefringence of liquid crystalline phases, and so is only apparent in such special cases. In this model we derive an expression for the optical rotation using a linear expansion of the induced polarization. We apply this simple model to the B2 phases of bent core liquid crystals and find that the magnitude of the calculated optical rotation is 0.1 to 1 deg/um. We find that the chiral SmCsPf and SmCaPa phases show strong optical rotation. In addition, the SmCaPf gives optical rotation but only for light propagation oblique to the layer normal and tilt plane. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B37.00005: Subdiffusive Dynamics of Photo-Addressable Self-Assembled Monolayers Guanjiu Fang, Noel Clark, Joseph Maclennan, Matthew Farrow, David Walba, Youngwoo Yi, Thomas Furtak Self-Assembled Monolayers (SAMs) synthesized on glass and incorporating azobenzene are illuminated with green light at normal incidence to study the effect of photo-isomerization on in-plane molecular orientation. Measurements of the monolayer birefringence at the glass-gas interface show that the SAM orientation and order parameter dynamics are subdiffusive, characterized by a stretched exponential relaxation with a distribution of relaxation times. Order decays with power law relaxation and exponents that decrease with increased initial writing intensity and/or duration, indicative of orientational trapping wells with a distribution of depths. Results on dynamics of reorientation at the glass-solvent interface will also be presented. This work was supported by NSF Grant CHE-0079122 and NSF MRSEC Grant DMR-0213918. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B37.00006: Optical Tweezer Studies of Liquid Crystals Using Multiple Optical Traps Apichart Pattanaporkratana We have constructed an acousto-optically scanned CW YAG laser system to generate dynamically controllable multiple optical traps. This multiple optical tweezer is being employed to probe the static and dynamic interactions of defects and textures in two and three dimensional liquid crystal (LC) systems. Results will be presented on 2D systems, where interactions between islands, thicker circular regions on few-layer thick freely suspended liquid crystal (LC) films, have been studied in the smectic C phase, in which the islands interact via the c-director orientation field. In the Smectic A phase, it has been found that the elastic interactions between islands are much smaller than in the Smectic C, and it is easy to induce coalescence using the optical tweezers. Studies of motion of suspended particles in 3D nematics and smectics will also be presented. *This research is supported by NASA Grant NAG3-2457, and NSF MRSEC Grant DMR 0213918 [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B37.00007: Measurement of strain birefringence in isotropic and liquid crystal elastomers and ferrogels Chris Bailey , Antonio M. Figueiredo Neto, Maria H. Godinho, Peter Palffy-Muhoray, Tibor Toth-Katona We have constructed an apparatus for the sensitive measurement of strain induced birefringence in elastomer samples. Rather than rotating the sample, we rotate the polarization of the probe beam while applying stress to the sample, and analyze the transmitted intensity. We have studied the strain induced birefringence in isotropic elastomers, nematic liquid crystal elastomers, and elastomers with dissolved ferrofluid particles. We present the results of our measurements, and interpret them in terms of the stress-optic theorem, and the coupling between orientational order parameters and strain. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B37.00008: Modeling the Dynamics of Shape Change in Liquid Crystal Elastomers Peter Palffy-Muhoray , Roland Ennis, Luis Malacarne, Michael Shelley Because of the strong coupling between orientational order and mechanical strain, liquid crystal elastomers can exhibit large and fast responses to external stimuli. The system dynamics can be characterized in terms of the motion of the conserved cross-links and the evolution of the non-conserved orientational order parameter. We have developed a fully non-local free energy for nematic elastomers, and a corresponding dissipation function. Using a Lagrangian formalism, we have obtained from these the equations of motion. We discuss these dynamical equations, the associated boundary conditions, and approaches to providing a numerical solution. We present the current status of our numerical results, and compare these with the results of experiments. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B37.00009: Dynamics of the Growth of Twist Fingers in Cholesteric Liquid Crystals Clinton Braganza, Deng-Ke yang Cholesteric liquid crystals can be switched to the homeotropic state, where the helical structure is unwound, by externally applied fields higher than a threshold. When the field is reduced below the threshold, the helical structure is restored by nucleation of twist fingers from the homeotropic state. We studied the structure of the twist fingers and their evolution. We observed that the fingers grew with a constant speed which is determined by the field and the cell thickness. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B37.00010: Dynamics of the acousto-optic effect in a nematic liquid crystal Jonathan V. Selinger, Viktoria A. Greanya, Anthony P. Malanoski, Brian T. Weslowski, Mark S. Spector In a nematic liquid-crystal cell, the application of an ultrasonic wave induces a rotation of the director, leading to a change in the optical transmission through the cell. In earlier work, we have experimentally investigated the key physical parameters that control the steady-state acousto-optic effect, and we have developed a theoretical model for this effect. In this study, we investigate the dynamic response of the optical intensity after the ultrasonic wave is switched on or off. Our experiments show that the optical intensity follows a double-exponential function of time, indicating that the system has two relaxation modes with widely different time scales. The fast mode has an amplitude and time scale consistent with the dynamics of the Freedericksz transition, but the slow mode shows novel behavior associated with the acousto-optic effect. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B37.00011: Molecular simulation of nematic liquid crystals under acoustic fields Anthony P. Malanoski, Jonathan V. Selinger The behavior of a nematic liquid crystal under an acoustic field is investigated in a model system. The anisotropic properties of liquid crystals allow them to be aligned by ultrasonic waves, leading to a change in optical transmission known as the acousto-optic effect. This effect is exploited to image the pattern of acoustic intensity reflected off a target. To gain a better understanding of the fundamental basis of the acousto-optic effect, we have carried out molecular dynamics simulations of a model system of rigid molecules under an acoustic wave. These simulations show that the molecules align with the major axis perpendicular to the direction of propagation of the acoustic wave. The acoustic response increases with increasing molecular chain length. The dependence of the acoustic response on variations in the density and the molecular rigidity has also been studied. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B37.00012: Dynamics of pulse laser induced holographic grating in azo-dye doped nematic liquid crystal host Chie-Tong Kuo, Shuan-Yu Huang Dynamics of pulse laser induced holographic grating in azo-dye doped nematic liquid crystals has been investigated. The dependence of diffraction efficiency on the temperature and the dye concentration is characterized using a single exponential function. The diffusion coefficients of dye in the directions parallel and perpendicular to the LC director are determined. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B37.00013: The escape-configuration lattice near the nematic-isotropic transition: tilt analogue of blue phases Yashodhan Hatwalne, Buddhapriya Chakrabarti, N.V. Madhusudhana We predict the possible existence of a new phase of liquid crystals near the nematic-isotropic ($ NI $) transition. This phase is an achiral,tilt analogue of the blue phase and is composed of a lattice of {\em double-tilt}, escape- configuration cylinders. We discuss the structure and the stability of this phase and provide an estimate of the lattice parameter. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B37.00014: Bogomol'nyi, Prasad, Sommerfield Configurations in Smectics Christian Santangelo, Randall Kamien It is typical in smectic liquid crystals to describe elastic deformations with a linear theory when the elastic strain is small. In smectics, certain essential nonlinearities arise from the requirement of rotational invariance. By employing the Bogomol'nyi, Prasad and Sommerfeld (BPS) decomposition and relying on boundary conditions and geometric invariants, we have found a large class of exact solutions. [Preview Abstract] |
Monday, March 21, 2005 2:03PM - 2:15PM |
B37.00015: Density of States in Photonic Band Gap Liquid Crystals Wenyi Cao, Peter Palffy-Muhoray, Bahman Taheri Liquid crystals with modulated ground states are periodic dielectric structures, hence they are self-assembled photonic band gap materials. Fluorescent emission is suppressed in the stop band, but is enhanced at the band edges, and, above a pump threshold, distributed feedback lasing occurs. We interpret experimental observations of fluorescence alteration and lasing in terms of the density of states, and give a detailed description for helical cholesteric liquid crystals. Lasing occurs due to the singularity in the density of states at the band edges. [Preview Abstract] |
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B37.00016: Freeze Fracture and AFM Studies of the Liquid Crystal Dark Conglomerate and B4 Phases L.E. Hough, M. Spannuth, H.J. Jung, J. Zasadzinski, D. Kruerke, G. Heppke, David Walba, Noel Clark Using freeze fracture transmission electron microscopy (FFTEM) and AFM, we show that the Dark Conglomerate and B4 phases are layered structures dominated by saddle splay. The B4 layer structure consists of a TGB like phase made up of parallel arrays multiple burgers vector screw dislocations (grain boundaries) giving pi/2 rotations across the grain boundaries. Models of the layer structure based on periodic arrays of grain boundaries, each described by Scherk's first surface, yield key features of the observed structures. The dark conglomerate phase is an optically isotropic thermotropic analog of the lyotropic sponge phase and appears to be a disordered form of the saddle splay stabilized structure proposed for the smectic blue phase by DiDonna and Kamien [PRL 89, 215504]. We propose that the mechanism for saddle splay is intralayer frustration produced by mismatch in the in-layer plane area of different molecular subfragments. This work is supported by a NSF Graduate Research Fellowship and by NSF MRSEC Grant DMR0213918. [Preview Abstract] |
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