Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W21: Liquid Crystals III |
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Sponsoring Units: DFD Chair: P. Collings, Swarthmore College Room: Baltimore Convention Center 318 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W21.00001: Dielectric Dispersion Effects in Liquid Crystals. Oleg Lavrentovich, Ye Yin, Mingxia Gu, Sergij Shiyanovskii As the switching speed in practical LC devices is pushed from the currently common 10 ms to sub-millisecond levels, it is important to take into account the effects associated with the finite rate with which the electric displacement changes in the external electric field. We discuss two important general consequences of the dielectric relaxation phenomenon: (1) Non-local time relationship between the electric displacement and the electric field [1]. In a quickly changing electric field, orientation of the liquid crystal depends not only on the instantaneous value of the electric field, but also on the previous values of the field and previous orientations of the material. (2) Dielectric heating. [1] Y. Yin, S.V. Shiyanovskii, A.B. Golovin, and O. D. Lavrentovich, \textit{Phys. Rev. Lett.} \textbf{95}, 087801 (2005) . [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W21.00002: Using micro-focus synchrotron X-ray diffraction to probe textured liquid crystal samples Ronald Pindak, Brandon Chapman, Ruiting Wang, Ishtiaque Syed, Giovanni Carbone, Charles Rosenblatt, Michi Nakata, Christopher Jones, Noel Clark, Shin-Woong Kang, Satyendra Kumar, Julie Cross 16 KeV X-rays from a bend magnet source at Sector 20 of the Advanced Photon Source were micro-focused by Kirkpatrick-Baez mirrors to a 14$\mu $m x 14$\mu $m cross-section and used in conjunction with an in-situ polarizing optical microscope to measure the diffraction from select areas in textured liquid crystal samples between thin glass plates. The technique will be described and its utility illustrated by three examples: (1) measuring the orientational deformation of smectic-A liquid crystal layers under the bend strain imposed by an AFM-scribed polymer alignment film, (2) mapping the concentration dependence of the liquid crystal phases exhibited by suspensions of short DNA oligomers of 6 to 16 base-pairs, and (3) selecting local monodomain regions from a globally unaligned conducting porphyrin-derivative sample for structural determination of its liquid crystal phases. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W21.00003: Impurity Induced Cross-over from Continuous to First-order Nematic-to-Smectic A Phase Transitions in a Liquid Crystal. Jan Thoen, Katleen Denolf, Bert Van Roie, Christ Glorieux We used adiabatic scanning calorimetry (ASC) to study the impact of adding small amounts of cyclohexane on the N-SmA transition of the liquid crystal octylcyanobiphenyl (8CB). The transition remains continuous upto a mole fraction of cyclohexane near 0.05, where at a tricritical point the transition becomes first- order with latent heats increasing with mole fraction of cyclohexane. Along the continuous part of the N-SmA transition line the effective specific heat capacity critical exponent increases from 0.31 for 8CB to 0.50 at the tricritical point. Ongoing experiments with other non-mesogenic impurities will also be reported. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W21.00004: Phase morphology of a disk-sphere dyad molecule. Li Cui, Jeffrey Collet, Lei Zhu A disk-sphere dyad molecule was synthesized by attaching a discotic triphenylene molecule to a spherical polyhedral oligomeric silsesquioxane (POSS) molecule via esterification reaction. The self-assembly behavior of the dyad molecule was studied by differential scanning calorimetry, polarized light microscopy, X-ray diffraction (XRD), and transmission electron microscope. Two-dimensional (2D) XRD results showed the dyad molecules self-assembled into a lamellar structure, which composed of a crystalline POSS layer and a discotic-nematic triphenylene double-layer. The POSS layer consisted four layers of ABCA-stacked spherical molecules. The liquid crystalline triphenylene molecules were parallel and staggered in the double-layer. Computer simulation of the XRD intensity confirmed the proposed structural model. Compared with that of the POSS crystal in bulk (melting point at ca 220 $^{o}$C), the melting temperature of POSS crystal was dramatically decreased to 67 $^{o}$C, possibly due to effects of the asymmetry molecular shape and plasticization of the discotic triphenylene moieties between POSS layers. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W21.00005: Excitation-Enhanced Optical Reorientation in Pure Liquid Crystalline Materials Thai V. Truong, Yuen-Ron Shen Electronic excitation with polarized light necessarily creates complementary orientational anisotropies from the excited and ground-state molecules. If the intermolecular interaction with the surrounding experienced by the excited-state molecules is different from that experienced by the ground-state molecules, a net excitation-induced orientational anisotropy will develop, enhancing the molecular reorientation provided directly by the optical field. This effect is analogous to that observed in dye-doped liquid crystals (LC) when dye molecules are excited. We report here the study of the effect in a pure isotropic LC medium. We use an optical pump-probe method to observe the excitation-induced reorientational dynamics. As the system relaxes back from picosecond pulse excitation, an increase in the orientational anisotropy of the ground-state molecules is observed, signifying the enhanced optical reorientation due to the state-dependent intermolecular interaction. The observed dynamics is well predicted by a mean-field model describing the intermolecular interaction between LC molecules. This work was supported by NSF [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W21.00006: Effect of disorder on a nematic-smectic A phase transition Simon Larochelle, Mehmet Ramazanoglu, Robert J. Birgeneau Using X-ray scattering, we studied the nematic to smectic A phase transition of the liquid crystal butyloxybenzilidene-octylaniline (4O.8) confined in an aerosil gel. The aerosil particles introduce quenched randomness in the system by providing pinning centers to the liquid crystal molecules. We find that the introduced disorder destroys the long range nature of the phase transition, and that the transition becomes similar to a transition in a finite-size system. Finite low temperature correlation lengths of the ordered moments are measured and the order parameter follows a power law behavior with respect to the reduced temperature in a limited temperature range. We also show evidence for a shift of the effective order parameter critical exponent $\beta$ with increasing disorder. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W21.00007: Observation of polarization current accompanying smectic A electroclinic reorientation Renfan Shao, Lixing Wang, Christopher D. Jones, David A. Coleman, Duong Nguyen, Michi Nakata, Joseph E. Maclennan, Per Rudquist, David M. Walba, Noel A. Clark We have been studying the liquid crystalline material W530, and report observations of polarization current of the field-induced molecular reorientation in the SmA phase. W530 exhibits the following phase diagram on cooling: isotropic -- SmA -- uncharacterized Sm'X' -- metastable SmC -- crystal. The temperature range of the SmA and SmX phases is $\sim $50\r{ }C, and x-ray diffraction (XRD) shows very little layer spacing change throughout the width of these two phases, while the SmC fractional layer compression is $\sim $5{\%}. The SmX is nearly identical in appearance to the SmA phase under depolarized light microscopy (DPLM). However, when measuring polarization current while cooling from SmA to SmX, two polarization peaks appear throughout the range of the SmX phase. By adapting the Langevin model for deVries SmA, we are able to explain the two polarization peaks. Through a combination of DPLM cone angle and birefringence measurements, dielectric spectroscopy measurements, aligned sample and powder XRD experiments, and freely suspended film observations, we are able to show that the previously uncharacterized phase is a deVries SmA. Work supported by NSF MRSEC Grant DMR-0213918. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W21.00008: Defects in liquid crystal nematic shells A. Fernandez-Nieves, A.S. Utada, V. Vitelli, D.R. Link, D.R. Nelson, D.A. Weitz We generate water/liquid crystal (LC)/water double emulsions via recent micro-capillary fluidic devices [A. S. Utada, et.al. Science 308, 537 (2005)]. The resultant objects are stabilized against coalescence by using surfactants or adequate polymers; these also fix the boundary conditions for the director field n. We use 4-pentyl-4-cyanobiphenyl (5CB) and impose tangential boundary conditions at both water/LC interfaces by having polyvinyl alcohol (PVA) dispersed in the inner and outer water phases. We confirm recent predictions [D. R. Nelson, NanoLetters 2, 1125 (2002)] and find that four strength s=+1/2 defects are present; this is in contrast to the two s=+1 defect bipolar configuration observed for bulk spheres [A. Fernandez-Nieves, et.al. Phys. Rev. Lett. 92, 105503 (2004)]. However, these defects do not lie in the vertices of a tetrahedron but are pushed towards each other until certain equilibration distance is reached. In addition to the four defect shells, we observe shells with two s=+1 defects and even with three defects, a s=+1 and two s=+1/2. We argue these configurations arise from nematic bulk distortions that become important as the shell thickness increases. Finally, by adding a different surfactant, sodium dodecyl sulphate (SDS), to the outer phase, we can change the director boundary conditions at the outermost interface from parallel to homeotropic, to induce coalescing of the two pair of defects in the four defect shell configuration to yield two defect bipolar shells. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W21.00009: Water as a Wetting Agent for Liquid Crystal Films Ergys Subashi, Rafael Garcia The dewetting of nCB liquid crystals from silicon wafer surfaces was first observed in 1999 [1] and has since grown into a subject of great fascination. The dewetting behavior occurs within a narrow coexistence region just below the nematic-to-isotropic phase transition temperature. When a wetted film is brought within this coexistence region, the film splits into two film thicknesses that are in apparent equilibrium with each other. A tentative but highly controversial explanation for this phase diagram has been proposed van Effenterre [2] in terms of mean field forces acting within the film. In our laboratory, we have undertaken a high-resolution measurement of the shape of this dewetting region for 5CB on silicon in search of evidence for the existence of fluctuation-induced forces that affect the thickness of these films. We have found, to our surprise, that ambient humidity affects the wetting behavior. Based on preliminary evidence taken thus far, water appears to act as a wetting agent that promotes the wetting of 5CB on silicon. We will present measurements showing how water affects the two-film thickness coexistence region. [1] F Vandenbrouck et al., Phys. Rev. Lett. 82, 2693 (1999). [2] D. Van Effenterre et al., Phys. Rev. Lett. 87, 125701 (2001). [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W21.00010: Surface morphology of SiO deposited substrates and alignment of nematic LC* Leela Joshi, Satyendra Kumar, Riccardo Barberi Glass substrates with thin film of SiO are known to align nematic liquid crystals homogeneously for oblique deposition. X-ray reflectivity was employed to probe the surface morphology of approximately 150{\AA} thick SiO films deposited at different landing angles. The interfacial roughness and morphological anisotropy was determined along the two orthogonal in-plane directions and the average electron density profile of the film calculated. The results show that the homogeneous and planar aligning films consists of SiO film with different roughness anisotropy and film thickness. The results will be discussed in light of previous reflectivity and AFM results on SiO [1] and other [2] surfaces. [1]. R. Barberi, Giocondo, G.V. Sayko, AK. Zvezdin, Phys. Lett. \textbf{A213}, 293 (1996). [2]. S. Kumar, J.-H. Kim, and Y. Shi, Phys. Rev. Lett. \textbf{94}, 077803 (2005). [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W21.00011: Crystal Nucleation behavior near gas-liquid spinodal line Limei Xu, Giancarlo Franzese, Sergey V. Buldyrev, H. E. Stanley The complex problem of crystal nucleation is currently at stage. Using molecular dynamics simulations, we study the crystal nucleation behavior of colloids modeled by hard-core particles with narrow square well attractive potential. For this system the liquid gas critical point lies below the gas-crystal equilibrium line. We investigate how the nucleation rate depends on the pressure and density, in particular in the vicinity of the liquid-gas spinodal. We find that there is a correlation between nucleation rate and spinodal line. We interprete our results using classical nucleation theory. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W21.00012: Entropy driven formation of a chiral liquid crystalline phase of helical rods Zvonimir Dogic, Edward Barry, Zach Hensel, Michael Shribak, Rudolf Oldenbourg We study the liquid crystalline phase behavior of a concentrated suspension of helical flagella isolated from Salmonella typhimurium. With increasing concentration, a suspension of helical flagella undergo an entropy driven first order phase transition to a liquid crystalline state having a novel chiral symmetry. Flagella are prepared with different polymorphic states, some of which have a pronounced helical character while others assume a rod-like shape. We show that the static phase behavior and dynamics of chiral helices are very different when compared to simpler achiral hard rods. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W21.00013: Dynamics of Director Fluctuations in Confined and Filled Liquid Crystals. Edwin Arroyo, Sarmistha Basu, Fouad Aliev Dynamic light scattering was applied to study the influence of randomness as well of boundary conditions (planar-axial and homeotropic-radial) and layer thickness (at nanoscale) of 5CB and 8CB confined to random porous matrices, to cylindrical pores and filled with Aerosil particles (hydrophilic and hydrophobic) on phase transitions and relaxation of director orientational fluctuations. For confined 8CB in the nematic phase two well-defined relaxation processes were for confined liquid crystals. The first process is associated with bulk-like nematic director fluctuations. The second relaxation process (with relaxation time slower than the first one) is most likely due to the fluctuations in layers nearest the wall surface. We found that for homeotropic boundary conditions of confined liquid crystal, the pore wall-liquid crystal interactions influence on the properties of the surface layer is stronger than in the case of axial orientation, particularly, and the influence of boundary conditions on N-Sm-A phase transition in confined 8CB is stronger than on isotropic- nematic phase transition. The separation between the first and the second (slow) process is clearer for thinner layers and the amplitude of slow process is greater for thinner layers. This suggests that the slow process is surface related relaxation. This relaxation was observed in filled liquid crystals as well. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W21.00014: Micro-focus synhrotron X-ray diffraction study of novel mesomorphic porphyrin derivatives Shin-Woong Kang, Lanfang Li, Quan Li, Michi Nakata, Brandon Chapman, Ronald Pindak, Satyendra Kumar The mesophase structures of three novel mesomorphic porphyrin derivatives were examined using polarized optical microscopy and microfocus synchrotron X-ray diffraction at various temperatures using a beam with a 14 $\mu $m$\times $14$\mu $m cross-section at the bending magnet beamline of Sector 20 at the Advanced Photon Source. The x rays were diffracted from microscopic monodomains in thin glass cells while simultaneously observing the optical textures. The results confirmed a hexagonal arrangement of discotic columns in the liquid crystalline phase. At a lower temperature, highly ordered plastic crystal phase was obtained. The results of the microdiffraction experiment and promising properties of these compounds as a carrier transporting material will be presented. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W21.00015: Stochastic Rotation Dynamics: generalizations and applications for non-ideal fluids, binary mixtures and colloids Thomas Ihle, Erkan Tuzel, Daniel Kroll A particle-based algorithm for the coarse-grained modeling of a fluctuating Solvent, namely Stochastic Rotation Dynamics (SRD), was recently introduced by Malevanets and Kapral[1]. This algorithm describes a fluid with an ideal gas equation of state and has been successfully applied to study polymers, colloids, and vesicles in flow. Here, we present generalizations of SRD for modeling fluids with non-trivial equations of state[2]. In particular, we show how to model a simple liquid with a non-ideal equation of state by incorporating excluded volume effects. We show the thermodynamic consistency of the model by independently measuring the pressure, density fluctuations and the speed of sound and compare with analytical results. This idea is extended to model binary mixtures with a miscibility gap; and the phase diagram of such a mixture will be presented. Furthermore, colloids are included in the SRD solvent and results for colloidal suspensions driven by external forces will be shown. [1] A. Malevanets, R. Kapral, J. Chem. Phys. 110, 8605 (1999). [2] T. Ihle, E. Tuzel, D. M. Kroll, cond-mat/0509631. [Preview Abstract] |
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