Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A37: Liquid Crystals I |
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Sponsoring Units: DFD Chair: Antal Jakli, Kent State University Room: LACC 512 |
Monday, March 21, 2005 8:00AM - 8:12AM |
A37.00001: Spatially Periodic Alignment of Liquid Crystals by Patterned Photopolymerization Jian Zhou, David Collard, Jung Park, Mohan Srinivasarao We demonstrate an electrically switchable diffraction grating based on periodically patterning the anchoring conditions of a nematic liquid crystal (NLC) within a polymer matrix via a patterned photopolymerization. We used two comonomers with opposite tendency to align the NLC and different reactivity ratio, which lead to definition of the areas with alternating homeotropic and planar alignment of the NLC through a UV irradiation with a photomask. The photopolymerization-induced diffusion of the monomers accounts for the spatial concentration distribution of these monomers. The LC diffraction gratings we made are switchable under low electric fields, and also have structural stability offered by the polymer matrix. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A37.00002: Quantitative study of the colloidal interaction forces and defect line tension in liquid crystals using optical trapping of polymer particles and defects Ivan Smalyukh, Oleg Lavrentovich, Andrey Kuzmin, Aleksandr Kachynski, Paras Prasad We demonstrate optical trapping and manipulation of defects and transparent microparticles in a thermotropic nematic liquid crystal with low birefringence. The three-dimensional director fields and positions of the particles manipulated by laser tweezers are visualized using the Fluorescence Confocal Polarizing Microscopy. The disclination lines are manipulated using tightly-focused linearly-polarized laser beams and optically trapped colloidal particles. We employ the particle manipulation to measure line tension of a topologically stable disclination line and to determine colloidal interaction of particles with perpendicular surface anchoring of the director at their surfaces. We show that the laser trapping of particles and defects in liquid crystals opens new possibilities for their fundamental studies as well as for new applications, such as assembling of colloidal structures and photonic crystals in the liquid crystal medium. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A37.00003: Effect of Magnetic Nanoparticles, Their Size and Functionalization on Liquid Crystal Order Luz J. Martinez-Miranda, Kevin McCarthy, Jr, Lynn K. Kurihara, Jason J. Harry, Robert L. Bruce We have observed the effects of adding magnetic nanoparticles with a different surface termination to smectic A 8CB liquid crystals by examining the liquid crystals by X-ray scattering. Adding the magnetic nanoparticles improves the liquid crystal's response to a magnetic field by at least one to two orders of magnitude. We have performed the experiments with four types of organic compounds covering the nanoparticles, using 11 nm and 2 nm FeCo nanoparticles, and have varied the applied magnetic field from 225 mT to 362 mT. There is a variation on the effect due to the size of the nanoparticles and also to the concentration of the particles in the mixture. As a function of magnetic field, the 11 nm and 2 nm particles terminated in polyethelyne glycol 3000 exhibit the largest rotation with the magnetic field. The liquid crystal rotates in opposite directions depending on the concentration of particles. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A37.00004: Pattern formation by liquid crystal of colloidal gold nanorods Kyoungweon Park, Mohan Srinivasarao To utilize the properties of nanoparticles to make nanoscale devices, large-scale spatial organization of NRs is required. Colloidal NRs can self assemble to form lyotropic liquid crystals(LC). The formation of lyotropic liquid crystals is a unique way to assemble NRs in solution since metal NR liquid crystals can combine the properties of liquid crystals with the electronic properties of metal component. We observed LC phase of gold NRs by resorting to an evaporating aqueous NR solution. The convective flow caused by the solvent evaporation carries the NRs from the bulk solution to solid-liquid-air interface, which makes the solution locally very concentrated, and therefore phase transition of NRs occured. By changing the aspect ratio, concentration and polydispersity of NR, and evaporation rate, we observed various pattern formation by LC phase similar to Liesegang ring. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A37.00005: Generalized Order Parameters for Systems of Orientationally Ordered Anisometric Particles Michael Shelley, Sharon C. Glotzer, Peter Palffy-Muhoray A variety of nanoparticle systems, such as semiconductor nanowires, mineral liquid crystals, colloidal clays and ferrofluids can exhibit orientationally ordered phases. These systems are therefore liquid crystals, where the relevant constituents are nanoparticles rather than molecules. We introduce generalized order parameters, based on the symmetry of the constituents, to describe orientational order in such systems. We describe the procedure to identify the relevant order parameters, discuss the connection between these and experimental observables and present the results obtained from simulations. Some mathematical issues related to representations are also addressed. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A37.00006: Liquid crystals in random porous media: Disorder is stronger in low--density aerosils Dima Feldman, Robert Pelcovits The nature of glass phases of liquid crystals in random porous media depends on the effective disorder strength. We study how the disorder strength depends on the density of the porous media and demonstrate that it can increase as the density decreases. We also show that the interaction of the liquid crystal with random porous media can destroy long--range order inside the pores. This work was supported in part by NSF DMR- 0131573. \vskip .2in \noindent [1] D. E. Feldman and R. A. Pelcovits, Phys. Rev. E {\bf 70}, 040702(R) (2004). [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A37.00007: The Effect of Aerosil Network on Liquid Crystal (4O.8) Phase Transition Mehmet Ramazanoglu, Simon Larochelle, Robert J. Birgeneau We report a high resolution x-ray diffraction study of the nematic (N) to smectic-$A $(Sm$A) $transition in the single-layer smectic (Sm$A_{m})$ liquid crystal butyloxybenzylidene octylaniline (4O.8) within aerosil dispersion. Aerosils are dispersed in liquid crystal material with a broad range concentration. They dramatically affect the phase transition properties in different liquid crystals [1]. These effects were studied in the view of random field theory introduced by quenched randomness of the silica gel network. The second order N-Sm$A$ phase transition and strong first order Sm$A$-CrB freezing transitions are shifted to lower temperatures. Sm$A$ line-shape is broadened indicating a short-range order. Correlation lengths and power-law fits show behavior similar to bilayer Sm$A_{d}$ liquid crystals. The present work enables us to test our understanding of random field effects introduced by dispersed aerosils forming a network in Sm$A_{m}$ material. [1] S. Park, R.L. Leheny, R.J. Birgeneau, J.-L. Gallani, C.W. Garland and G.S. Iannacchione, Phys. Rev. E 65 050703(R) (2002) [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A37.00008: Rf/ac-calorimetry on 8CB+aerosil dispersions Saimir Barjami, Germano Iannacchione Using a new high-resolution AC calorimetric technique employing RF (dielectric) heating, both the heat capacity and permittivity of a sample may be simultaneously probed. Relative resolutions of better than 0.06\% in the ratio of heat capacity to applied heating power, and 0.03\% in the phase shift measurements are easily obtained. This new and powerful technique has been applied to aerosil dispersions in the liquid crystal octylcyanobiphenyl (8CB) through the \textit{I}-\textit{N} and \textit{N}-Sm\textit {A} phase transitions. The temperature dependence of the applied heating power, proportional to the permittivity and hence the orientational order, shows no change through the \textit{N}- Sm\textit{A} for $\rho_S = 0.1$~g/cc. This indicates that at the point where sharp $C_p$ features are lost, the Sm\textit{A} and \textit{N} phases are effectively decoupled, consistent with recent NMR studies. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A37.00009: Structure and Phase Behavior of Smectic Liquid Crystals in Anisotropic Disorder Dennis Liang, Robert Leheny We present x-ray scattering studies of the smectic liquid crystals octylcyanobiphenyl (8CB) and 4-n-pentylphenylthiol-4-n-octyloxybenzoate ($\bar {8}$S5) confined in strained colloidal silica gels. The gels possess anisotropy that stabilizes long-range nematic order in the liquid crystals while introducing random field effects that disturb the smectic order. The strong azimuthal focusing of the scattering enables detailed characterization of the smectic correlations. The short-range smectic order that forms in this environment is inconsistent with a topologically ordered state predicted for 3D random field \textit{XY} systems and is quantitatively like the correlations of smectics confined by isotropic gels. The quenched disorder modifies the nematic -- smectic-A critical behavior, for example, suppressing the anisotropic scaling of the correlation lengths observed in the pure liquid crystals. The smectic-A and smectic-C scattering indicates that the behavior of smectics confined in gels is dictated by random fields coupling directly to the smectic order parameter while fields coupling to the nematic director play a subordinate role. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A37.00010: Ordered Patterns of Liquid Crystal Toroidal Defects by Microchannel Confinement Youli Li, Myung Chul Choi, Thomas Pfohl, Jacob N. Israelachvili, Cyrus R. Safinya, Zhiyu Wen, Mahn Won Kim We present the first experimental results demonstrating a novel approach to controlling the size as well as the spatial patterning of defect domains in a smectic liquid crystal by geometric confinement in surface modified microchannels. By confining the liquid crystal 8CB (4'-octyl-4-cyanobiphenyl) in micron sized rectangular channels with controlled surface polarity, we were able to generate defect domains that are not only nearly uniform in size but also arranged in quasi-two-dimensionally ordered patterns. Atomic force microscopy measurements revealed that the defects have a toroidal topology, which we argue is dictated by the boundary conditions imposed by the walls of the microchannel. We show that the defects can be considered as colloidal objects, which interact with each other to form ordered patterns. This method opens the possibility to exploit the unique optical and rheological properties associated with LC defects to making new materials. For example, the control of the shape, size and spatial arrangement of the defects at the mesoscale suggests applications in patterning, templating, and when extended to lyotropic liquid crystals, a process leading to uniform size spherical particles for chemical encapsulation and delivery (Work supported by NIH GM-59288, NSF DMR-0203755 and ONR N00014-00-1-0214) [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A37.00011: Coarsening dynamics of a liquid crystal biosensor: Effects of adsorbed nanoparticles, flow and thermal fluctuations Orlando Guzman, Manan Chopra, Nicholas Abbott, Juan de Pablo We explore the coarsening dynamics of a model for experimental liquid-crystal (LC) biosensors for viral particles: a thin film of LC confined between two parallel, aligning substrates, after a quench into the nematic phase. In the absence of particles, the LC undergoes a coarsening process that ultimately produces a uniform orientation state. By contrast, when the surface coverage c exceeds a critical value, the dynamics is slowed down and the system exhibits multidomain behavior, characterized by a finite correlation length for the tensor order parameter. In this work, we use (a) a dynamic field theory to study the dependence of the critical coverage c with operational parameters of the biosensor, such as the scalar order parameter, the separation between substrates, and the adsorption pattern; and (b) a fluctuating LC lattice Boltzmann method that allows the exploration of the effects that hydrodynamics and thermal fluctuations have on the coarsening process and the critical concentration in 3D. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A37.00012: A Light Driven Artificial Goldfish J. Neal, H. Finkelmann, P. Palffy-Muhoray, M. Shelley, T. Toth-Katona Liquid crystalline elastomers (LCEs) undergo large and rapid shape changes when illuminated by light. We have immersed an azo-dye doped LCE in a fluid and alternately illuminate either side of the LCE with light to create an artificial goldfish of sorts. These light induced deformations allow the LCE to interact with a fluid environment in novel ways. We use a fluid flow visualization technique to attempt to understand the dynamics of these interactions. We describe our experimental setup, the LCE drive scheme used, and our observations of induced motion in both the LCE sample and the surrounding fluid. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A37.00013: Real-time microbe detection based on director distortions and light transmittance around growing immune complexes in lyotropic chromonic liquid crystals Oleg Lavrentovich, Sergei Shiyanovskii, Tod Schneider, Ivan Smalyukh, Tomohiro Ishikawa, Gary Niehaus, Kathy Doane, Christopher Woolverton We describe director distortions in the nematic liquid crystal (LC) caused by a spherical particle with tangential surface orientation of the director and show that light transmittance through the distorted region is a steep function of the particle's size. The effect allows us to propose a real-time microbial sensor based on a non-toxic lyotropic chromonic LC (LCLC) that detects and amplifies the presence of immune complexes. A cassette is filled with LCLC, antibody, and antigen-bearing particles. Small and isolated particles cause no macroscopic distortions of the LCLC. Upon antibody-antigen binding, the growing immune complexes distort the director and cause detectable optical transmittance between crossed polarizers. The work was supported by NSF/ITIC DMR-0346348. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A37.00014: Dielectric relaxation in confined liquid crystal: molecular and collective modes Manuel Rivera, Fouad Aliev Dielectric spectroscopy was used to investigate the influence of confinement of the liquid crystals on phase transitions and the dynamics of molecular reorientations via rotation of molecules around their short axis. The pore size was varied from 200 nm then the system shows behavior close to the behavior of three-dimensional (3D) bulk liquid crystal - to extreme narrow confinement that may be considered a quasi-one-dimensional (1D) system. We did not observe either the N-I or nematic-solid crystal phase transition under quasi-1D-confinement: in contrast, in pores of larger pore sizes, these transitions are observed with a shift and broadening of the phase transition as compared to bulk LC. We observed that, instead of undergoing the N-I phase transition in the quasi-1D-system, liquid crystal inside the narrow pores solidifies. Relaxation of molecular origin freezes out upon cooling the sample from the isotropic phase. Molecular reorientations were dielectrically active at temperatures much above the bulk N-I transition temperature but with the relaxation rate much slower and dielectric spectrum much broader than in the bulk. The relaxation due to cooperative dynamics of fluctuations of director orientations (investigated in complementary dynamic light scattering experiments) was not observed in quasi-1D-system. We suggest that the slowing down of the mode, which is molecular in the bulk material, is may be due to enhancement of the effective viscosity liquid crystal under such narrow confinement and resulting glass-like dynamics. [Preview Abstract] |
Monday, March 21, 2005 10:48AM - 11:00AM |
A37.00015: Dynamics of director relaxation in nanoconfined liquid crystal: dynamic light scattering investigation Sarmistha Basu, Fouad Aliev Dynamic light scattering was applied to study the boundary conditions (planar-axial and homeotropic-radial) and layer thickness (at nanoscale) of 8CB confined to cylindrical pores influence on phase transitions and relaxation of director orientational fluctuations. For confined 8CB in the nematic phase two well-defined relaxation processes were observed for both axial and radial orientations of the liquid crystal. The first process is qualitatively 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. In samples with homeotropic boundary conditions we observed the onset of smectic-A phase order forming on the pore wall even though the rest of the liquid crystal could be in the nematic phase. 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. [Preview Abstract] |
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