Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U21: Liquid Crystals I |
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Sponsoring Units: DFD Chair: P. Palffy-Muhoray, Kent State University Room: Baltimore Convention Center 318 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U21.00001: Aggregation in Two Dye Systems That Form Chromonic Liquid Crystals Peter J. Collings, Viva R. Horowitz, Michelle R. Tomasik X-ray scattering and various optical techniques are utilized to study the aggregation process and aggregate structure for two water-soluble dyes that form chromonic liquid crystal phases. The x-ray measurements indicate that the molecules stack in columns with a cross-section approximately equal to the area of a single molecule. The optical measurements point to an aggregation process that occurs at all concentrations, with the distribution of aggregate size shifting to larger and larger aggregates as the concentration is increased. A simple theory based on the law of mass action and an isodesmic aggregation process is in excellent agreement with the experimental data, yielding a value for the ``bond energy'' between the molecules in an aggregate. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U21.00002: Orientational order of an ideal rodlike nematic: Rewriting the theory of nematic liquid crystals? Edward Samulski, Louis Madsen, Theo Dingemans Order in rodlike nematic liquid crystals (LCs) represents a rich field described by myriad theories and studied using various analytical methods. We have made deuterium (D) NMR observations on the labeled para-quinquephenyl LC, which closely approximates a rigid rod. To investigate this high-melting nematic (range 390 - 427 deg. C), we have fabricated a high-efficiency oven on an NMR probehead using fumed silica tiles and utilizing only ambient air cooling. Observations on p-quinquephenyl clearly and drastically deviate from the Maier-Saupe theory and all other molecular theories of nematics, thus indicating the necessity for more a complete theory (e.g., including microscopic director fluctuations) to describe nematic order. We measure the complete order tensor for this LC using combinations of quadrupole and dipolar NMR coupling constants and different D label positions. We will discuss progress on refinements to nematic order theory, relations to NMR measurements, and fits using the phenomenological Landau-deGennes theory. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U21.00003: Interlayer interactions in ferroelectric liquid crystals Mehdi Hamaneh, Philip Taylor We have recently drawn attention to a physical mechanism that can lead to an aligning interaction between distant layers in a ferroelectric smectic-C$^*$ liquid crystal. This effect arises because the amplitude of thermal fluctuations in layer shape is sensitive to correlations in $c$-director orientation in layers that are not nearest neighbors. This makes the entropy of the system dependent on the relative alignment of the $c$-director in all the smectic layers. In earlier treatments of this problem, a mean-field approximation was made in order to obtain an order-of-magnitude estimate of the strength of the interlayer interaction. While this was sufficient to demonstrate the significance of the mechanism, it led to an overestimate of the overall strength of the interaction because it included a self-energy term related to the anisotropy of a single layer. We have now remedied this shortcoming by evaluating in more detail the interlayer interaction due to layer shape fluctuations. We find that the rate at which the interaction decays as a function of interlayer distance does not follow any simple power-law form, but depends on a number of material parameters. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U21.00004: The B4 phase: layer curvature driven by frustrated intralayer packing L.E. Hough, D. Kruerke, C.D. Jones, H.T. Jung, J. Zasadzinski, G. Heppke, J. Rabe, W. Stocker, E. Korblova, D. Walba, N.A. Clark We combine freeze fracture transmission electron microscopy, atomic force microscopy, and x-ray diffraction to show that the B4 phase is a smectic phase with highly curved layers (mean radius $\sim $ 4 layer spacings). The layer structure of the phase is a TGB-like phase made up of parallel arrays of multiple burgers vector screw dislocations (grain boundaries) giving 45 degree rotations across the grain boundaries. Models of the layer structure are based on periodic arrays of grain boundaries, each described by Scherk's first surface, and yield key features of the observed structures. This layered structure is dominated by saddle splay and we propose that the energy cost of defects required to make such a structure is offset by an energy gain of the layer curvature. We show that analysis of the wide angle x-ray diffraction of this phase indicates that layer curvature relieves the intralayer frustration produced by the packing of bent-core molecules. This work is supported by a NSF GRF and by NSF MRSEC Grant DMR0213918. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U21.00005: Polarization-Enhanced Interaction between Islands on Freely-Suspended Smectic C* Liquid Crystal Films Apichart Pattanaporkratana Smectic liquid crystals can be made to form freely suspended films, two-dimensional systems locally quantized in thickness by an integral number of smectic layers, on which islands, circular regions of greater thickness than the surrounding film area, can be generated. In smectic C films, each such island is accompanied by a topological defect pair, an s = +1 topological defect inside and an s = -1 defect nearby on the background film. The distortions of the in-plane orientational order of the smectic C director field result in elastic interactions between the islands, with a short-range repulsion and a long-range dipolar attraction governing their stability and leading to their organization in chain-like structures with an equilibrium island separation. We have directly measured the repulsive and attractive forces between smectic C* islands using multiple optical traps and have compared the results quantitatively with theory. We find that the interactions between islands are much smaller in the racemic smectic C case than in the chiral smectic C*, an effect we attribute to long- range coulombic forces arising from polarization charges. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U21.00006: Riverbottom texture: Patterns of compressional stress in an SSFLC cell Christopher D. Jones, David A. Coleman, Noel A. Clark, Michael D. Wand We have been studying the texture of remnant compressional stress in a bookshelf aligned SmA phase of the Displaytech mixture MDW8068. MDW8068 exhibits isotropic - nematic - SmA - SmC phases, and throughout the range of the SmA phase the layers show significant expansion on cooling. This layer expansion causes layer compression, which is relieved by dislocation formation and surface depinning events throughout the cell. The resulting SmA has essentially perfect alignment, but with a pattern of remnant stress that can be visualized near the SmA - SmC transition because of the divergent tilt susceptibility and resulting compression-induced tilt near the SmA - SmC transition. Low dislocation density areas are the areas of greatest layer compression, implying that the edge dislocations relieve the compressive stress. Temperature cycling shows the texture is set near the N - SmA transition, though x-ray diffraction data shows that the layer expansion occurs through the entire range of the SmA. X-ray diffraction from oriented samples has been done which shows that the texture is a result of competition between smectic ordering and surface pinning. Work supported by ED GAAN Fellowships P200A030179 and P200A000839, and NSF MRSEC Grant DMR-0213918. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U21.00007: Random Lasing in Multidomain Cholesteric Liquid Crystals Michele Moreira, Mingxia Gu, Oleg Lavrentovich, Bahman Taheri, Peter Palffy-Muhoray A conventional laser consists of a pumped amplifying medium and an optical cavity to provide feedback for light amplification. In disordered materials, light can be trapped by multiple scattering processes and, if a gain medium is added, random lasing can occur. This random laser source does not require a regular cavity, but instead depends on multiple scattering in a random medium. Random lasers have attracted considerable attention recently because of their low cost and ease of construction. We present recent results of our random lasing experiments in dye-doped multidomain cholesteric liquid crystals, with submicron pitch, where the highly reflective cholesteric domains are the scattering elements. We discuss the underlying physics, compare the performance of these systems with others, consider the effects of temperature on the emission spectrum, and suggest possible applications. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U21.00008: Wavelength Hopping in Cholesteric Liquid Crystal Lasers Michele Moreira, Christopher Bailey, Wenyi Cao, Bahman Taheri, Peter Palffy-Muhoray Due to their birefringence and periodic structure, cholesteric liquid crystals (CLCs) in the helical cholesteric phase are one-dimensional photonic band gap materials. Gain enhancement and distributed feedback effects give rise to low threshold mirrorless lasing at the band edges. Since the wavelength at the band edge is proportional to the cholesteric pitch, which is a smooth function of temperature, one would expect the lasing wavelength to vary smoothly with temperature. Observations show, however, that the lasing wavelength does not depend smoothly on temperature, but instead exhibits periodic jumps between regions of smooth monotonic behavior. We have determined the reflection band dynamics, observed multiple lasing peaks at the hopping wavelength, and see evidence of hysteresis on measuring the reflection band and lasing peaks at different heating rates. We compare our observations with theoretical models, and propose an explanation for the observed dynamics. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U21.00009: Poisson-bracket formulation of the dynamics of polar liquid crystals William Kung, M. Cristina Marchetti, Karl Saunders We develop the dynamical theory of polar liquid crystals with local $C_{\infty v}$-symmetry using the general Poisson-bracket formalism. We obtain dynamical equations for the slow macroscopic fields that govern the dynamics in both the polarized and the isotropic phases. Starting from a microscopic definition of an alignment vector proportional to the polarization, we obtain Poisson bracket relations for the director field. The hydrodynamic equations differ from those of nematic liquid crystals ($D_{\infty h}$) in that they contain terms violating the ${\bf{n}} \rightarrow -{\bf{n}}$ symmetry. We find that the $\mathcal{Z} _2$-odd terms induce a general splay instability of a uniform polarized state in a range of parameters. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U21.00010: Finite Element Elastodynamics Studies of Shape Evolution in Liquid Crystal Elastomers Robin Selinger, Badel Mbanga, Jonathan Selinger Liquid crystal elastomers change shape under heating/cooling, applied fields, or optical illumination, with induced strains up to 400{\%}. We present a novel finite element elastodynamics technique to model dynamics of shape change in these materials, with explicit coupling between nematic order and elastic strain. Without added dissipation, the elastodynamics algorithm conserves the sum of kinetic and potential energy to one part in 10$^{6}$, even for large strains and rotations. In initial studies, we model shape evolution during a transition from the isotropic phase to nematic and back again, and model the induced curvature of an elastomer strip under local optical illumination. This method allows modeling of complex geometries and dynamic perturbations, and can serve as a bridge between fundamental soft condensed matter theory and engineering design. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U21.00011: Temperature Dependence of Acousto-Optic Effect in a Nematic Liquid Crystal Cell Steven Sundbeck, Anthony Malanoski, Brian Weslowski, Devanand Shenoy, Jonathan Selinger The acousto-optic effect occurs in a nematic liquid crystal cell when an incident ultrasonic wave causes a rotation of the director. This effect is observable as a change in the optical transmission through a cell, and has been exploited as a means of nondestructive imaging. The sensitivity and speed of this rearrangement are dependent on the viscosity of the liquid crystal material. Because of this, the effect is sensitive to the temperature. In this work we investigate quantitatively how the acousto-optic response is affected by the temperature of the liquid crystal cell. We present the results of studies of changes to the acoustic sensitivity of the cells and changes of their dynamic responses to the introduction of the ultrasonic wave. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U21.00012: Elliptic Phases: A Study of the Nonlinear Elasticity of Twist- Christian Santangelo, Randall Kamien The twist-grain boundary phase in smectic-A liquid crystals, constructed from rotating walls of parallel screw dislocations, is a prime example of a stable, ordered configuration of defects. In smectics, nonlinearities in the strains strongly affect the energetics and interactions between defects, thus complicating their analysis. By exploiting the properties of Jacobi elliptic functions, we construct a triply-periodic surface locally composed of screw dislocations, called Schnerk's surface, which has the structure of a series of ninety degree twist-grain boundaries. This is a candidate structure for the recently observed large-angle twist-grain boundary phases. Because of the analytic tractability of our construction, we compute that the grain boundaries interact exponentially at long distances through both the compression and bending energies, and that there is a preferred grain boundary spacing. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U21.00013: Calorimetric study of aligned liquid crystal + aerosil F. Cruceanu, G. S. Iannacchione, D. Liang, R. L. Leheny A high-resolution ac-calorimetric study was performed on magnetically aligned colloidal dispersions of 8CB and aerosil spanning the weakly first-order \textit{I}-\textit{N} and second order \textit{N}-Sm\textit{A} phase transitions. Stable aligned samples were prepared by repeated cycling between the isotropic and nematic phase in the presence of a $2$~T magnetic field. Zero-field measurements were carried out on six aligned conjugate density samples ranging from $0.03$ to $0.15$~g~cm$^{-3}$ (mass of aerosil per volume of liquid crystal). For comparison, two unaligned samples from the same batch ($0.05$ and $0.13$~g~cm$^{-3}$) were also studied. The unaligned samples reproduce very closely previous studies on this system. The magnetically aligned samples, exhibits lower transition temperatures for the same aerosil density sample and a shift to higher aerosil density of the non-monotonic $T_c$ evolution. The clear differences between aligned and unaligned sample indicate the ``memory'' of the magnetic field even after heating deep into the isotropic phase. The origin of this ``memory'' remains unexplained. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U21.00014: Disordering Effects in Smectic -- Aerosil Gels* Vishal Pandya, Daniele Finotello We studied quenched disorder effects on the 12CB liquid crystal upon dispersion of silica nano particles (type A-300): hydrophilic silica spheres of diameter 7nm and surface area S = 300 m$^{2}$g$^{-1}$, with hydroxyl groups covering the surface. The LC-aerosil dispersions form a gel if aerosil density exceeds the percolation threshold. For low densities of aerosil dispersions and in cooling the sample, the LC director in the void volume is parallel and follows the external NMR field; a well defined and stable LC configuration forms. When a complete silica network forms and the sample orientation in the field is changed, a few silica links are broken by the field, re-aligning only a few Sm layers; the aerosil locks in the LC configuration which follows a P$_{2}$ (Cos$\Theta )$ dependence. In contrast, if the dispersion is cooled from isotropic phase outside the field, the spectra in the Sm phase is a powder pattern. The field anneals the aerosil-induced random disorder up to a certain density beyond which, disordering effects dominate; for aerosil densities greater than $\rho _{S} \quad \approx $ 0.055~g/cm$^{3}$ spectral patterns are consistent with an isotropic distribution of smectic domains. The quenching of the 12CB Sm-A phase at $\rho _{S} \quad \approx $ 0.055~g/cm$^{3}$, is one order of magnitude less than that in 8CB [1]. The aerosil induced disorder, quantified by the percent of LC molecules in a powder pattern, depends linearly on the density. [1] T. Jin and D. Finotello, \textit{Phys. Rev}. E 69, 041704 (2004); \textit{Phys. Rev Lett}. 86, 818 (2001). *Supported by NSF-INT 03-06851, OBR B-7844 and B-7845. [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U21.00015: The Effect of Aerosil Network on Smectic A-Reentrant Nematic Liquid Crystal Mehmet Ramazanoglu, Simon Larochelle, Robert J. Birgeneau We report on a high resolution x-ray scattering study of aerosil dispersion effects on nematic-smectic A and smectic A-reentrant nematic phase transitions in 6OCB (hexyloxycyanobiphenyl) and 8OCB (octyloxycyanobiphenyl) liquid crystal mixtures. Dispersed aerosil particles introduce quenched randomness to the liquid crystal phases, which destroys the long range smectic order [1]. The experiment was conducted on mixtures with different 6OCB:8OCB concentrations and aerosil densities. The parabolic smectic A phase boundary is found to be slightly distorted in the presence of the aerosil network, with shifted transition and median (T$_{M})$ temperatures. Above T$_{M}$, the order parameter, susceptibility and parallel correlation lengths for the thermal and random parts of the structure factor show behaviors similar to those observed in non-reentrant nematic-smectic A second order phase transitions [2]. At T$_{M}$, where the order parameter has its maximum value, the scattering peaks are only defined by the random part of the structure factor. The smectic order parameter decreases with a further decrease in temperature, while the susceptibility and thermal correlation length increasingly show nematic-like behavior. Finally, at the lowest temperature, the mixtures are found in the reentrant nematic phase. [1] P.S. Clegg et. al. PRE 67,021703 (2003) [2] S. Larochelle et. al. in preparation [Preview Abstract] |
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