Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V15: Liquid Crystals I |
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Sponsoring Units: DFD Chair: Jonathan Selinger, Kent State University Room: 316 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V15.00001: Interaction and Response of a Smectic-A liquid crystal to a 2 nm Nanometer Particle: Phase transition due to the Functionalization Compound Luz J Martinez-Miranda, Lynn K Kurihara We have studied the in-plane (parallel to the magnetic field) alignment of 8CB mixed with FeCo nanoparticles covered with different funtionalization compounds. The functionalization compounds are Polyethelene glycol (PEG (3000)), hydroxyl succinimide (NHS), aminopropyl tri-ethoxy silane (APTS) and mercapto hexa-decanoic acid (MHDA). We have studied them using X-ray scattering. We have found that the inverse integrated intensity of the X-ray scans in the plane of the magnetic field is a good measure of how much energy the system (liquid crystal, nanoparticles, functionalization compound) will need to reorient the liquid crystal in the magnetic field. In addition, we have observed that the orientation the liquid crystal adopts with respect to the nanoparticles can result in a phase transition that takes the liquid crystal to a more disordered and symmetric phase that favors the rotation, as happens in the smectic-nematic transition, observed in the sample with APTS. We relate the disordering to the changes observed in the transition for the liquid crystal and this termination to recent heat capacity measurements by Cordoyiannis et al. [1]. \textbf{References} [1] Cordoyiannis, G., Kurihara, L.K., Martinez-Miranda, L. J, Glorieux, C., Thoen, J., submitted to PRE (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V15.00002: Theory of Ferroelectric Nanoparticles in Nematic Liquid Crystals Lena Lopatina, Jonathan Selinger Many recent experiments have reported that ferroelectric nanoparticles have drastic effects on nematic liquid crystals. Low concentrations of such particles increase the isotropic-nematic transition temperature by over 10 C, and greatly increase the sensitivity of the nematic phase to applied electric fields. To understand these effects, we develop a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals. In this theory, the key issue is the distribution of orientations for the electrostatic dipole moments of the nanoparticles. This distribution is characterized by an orientational order parameter, which interacts with the orientational order of the liquid crystals and stabilizes the nematic phase. We estimate the coupling strength and calculate the resulting enhancement in the transition temperature, in good agreement with experiments. We also predict the response to applied electric fields, showing that the Kerr effect is enhanced above the isotropic-nematic transition. These predictions apply even when the electrostatic interactions are partially screened by moderate concentrations of ions. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V15.00003: Unconventional dimerization in mesogenic semi-phasmidic type carboxylic acid* Shin-Woong Kang, Seung-Yeon Jeong, Dena Mae Agra-Kooijman, Satyendra Kumar, Veena Prasad, Sanjay Varshney The nematic and columar mesophases formed by a semi-phasmidic type carboxylic acid are investigated by DSC, X-ray scattering, FTIR spectroscopy, and polarizing optical microscopy. FTIR spectroscopy and X-ray results confirm that two semi-phasmidic type carboxylic acid molecules form a mesogenic dimer through inter-molecular hydrogen bonding. X-ray diffraction results obtained under \textit{in-situ} magnetic field reveal unique characteristics that set them apart from conventional linear dimers formed via hydrogen bonding. These dimers possess negative dielectric and diamagnetic anisotropies. Values of the length scales corresponding to the diffraction peaks and their orientation relative to the magnetic field strongly suggest the formation of a ``bent-core-like'' mesogenic dimers rather than conventional coplanar linear dimers. *Supported by grant NSF/DMR-086991. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V15.00004: Rheological Properties of T-Shaped Liquid Crystals Nicholas Diorio, Christopher Bailey, Carsten Tschierske, Antal J\'akli The rheological properties of ``T-shaped'' liquid crystal molecules are investigated. These T-shaped molecules show novel liquid crystal phases with a variety of lamellar and columnar structures [1,2,3]. We examined the viscoelastic behavior of these materials over varying temperatures and shear rates. Because of the limited quantities of these materials, a home- made nanoliter rheometer [4] is used that only requires a few nanoliters of material. [1] M. Prehm, X.H. Cheng, S. Diele, M. K. Das, and C. Tschierske; J. AM. CHEM. SOC. 2002, 124, 12072-12073 [2] X.Cheng, M. K. Das, U. Baumeister, S. Diele, and C. Tschierske; J. AM. CHEM. SOC. 2004, 126, 12930-12940 [3] M. Prehm, F. Liu, U. Baumeister, X. Zeng, G. Ungar, and C. Tschierske; Angew. Chem. Int. Ed. 2007, 46, 7972 –7975 [4] C. Bailey, A. J\'akli, ``Broad range nanoliter rheometer'', Provisional patent , KSU 325 (2008) [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V15.00005: The shape and conformation of the mesogenic group in tetrapodic liquid crystals Hyungguen Yoon, Shinwoong Kang, George H. Mehl, Satyendra Kumar The nematic phases formed by tetrapodic mesogens based on Si- or Ge- core have been investigated by various methods [1]; deuterium NMR, polarized IR spectroscopy, and light scattering. In these studies, biaxiality of the nematic phase has been the central issue. The average statistical shape that such a complex molecule adopts and how the four mesogens attached to Si/Ge atom are oriented in the nematic and lower temperature phases are naturally important questions. However, these have not been sufficiently discussed. We performed synchrotron x-ray diffraction experiments on magnetic aligned Ge-tetrapodes, augmented by conoscopy, capacitance, and electro-optical experiments. The results of these studies and the inferences drawn for the mesogenic group formation will be presented. [1] K. Neupane, S.W. Kang, S. Sharma, D. Carney, T. Meyer, G. H. Mehl, D.W. Allender, S. Kumar, and S. Sprunt, \textit{Phys. Rev. Let.} \textbf{97}, 207802 (2006), and references there in. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V15.00006: Quantum dot dispersion in nematic liquid crystal J. Kirchhoff, R.H. Inman, D.S. Chandhok, S. Ghosh, L.S. Hirst Optical and electrical properties of quantum dots can be~significantly altered~by aligning the dots in a linear chain. ~Dispersing quantum dots in liquid crystals can lead to the formation of linear chains due to the partial ordering of the liquid crystal molecules. ~Typically, this results in a red shift in the emission spectrum of the dots as the induced order leads to enhanced~dipolar interactions, resulting in electronically coupled states.~Dispersions of quantum dots are studied as a function of the concentration, size, and shape of the dots in a nematic rod-like liquid crystal material. ~The quantum dots~are seen to aggregate if the concentration of the dots~is too high, leading to little correlation between the quantum dot dispersion and liquid crystal texture. ~In decreasing the quantum dot concentration the aggregates lessen in size and are more uniformly distributed within the liquid crystal. ~Spherical, rod-like, and disc-like quantum dots with emission peaks ranging from 490 nm to 640 nm were studied using polarized optical microscopy and scanning microscopy photoluminescence measurements. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V15.00007: Dielectric and Optical Properties of Nematic ODBP-Ph-C7 Bohdan Senyuk, Hugh Wonderly, Sergii Shiyanovskii, Oleg Lavrentovich, Victor Pergamenshchik Thermotropic biaxial nematic liquid crystals are promising for application in fast switching electro-optical devices.$^{1}$ In the present work, we study the optical and dielectric properties of the nematic phase of thermotropic LC material 4,4'(1,3,4{\-}oxadiazole{\-}2,5{\-}diyl)~di{\-}$p${\-}heptylbenzoate (ODBP-Ph-C7) with boomerang-like molecules, reported to exhibit the biaxial nematic phase$^{2}$. We study ODBP-Ph C7 in well-aligned ``monocrystalline'' states. The experiments show unusual optical and dielectric properties of the material, such as high dielectric permittivity and dielectric relaxation time that increases with temperature. We also analyze defect structures formed by ODBP-Ph- C7 in different geometries of confinement and boundary conditions. $^{1 }$G. R. Luckhurst, Thin Solid Films \textbf{393}, 40 (2001); $^{2}$ B. R. Acharya, A. Primak, T.J. Dingemans, E.T. Samulski and S. Kumar, Pramana J.Phys. \textbf{61}, 231 (2003). [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V15.00008: Highly Oriented Neurofilament Liquid Crystalline Gels for Imaging and Scattering Studies H.C. Holger, R. Beck, C. Ding, J.B. Jones, J. Deek, N.C. MacDonald, Y. Li, C.R. Safinya The neuronal cytoskeleton is composed of a variety of filamentous proteins including, neurofilaments (NFs), microtubules (MTs) and actin. These components form networks that maintain the cell's structure and shape. At high filament concentration, the proteins self-assemble in-vitro and form liquid crystalline phases maintained by their spatial anisotropy and interfilament interactions. Under physical confinement in microchannel with widths less than few persistence lengths, both MT and actin bundles align parallel to the surface$^{1}$. AFM imaging shows that despite being shorter and more flexible filamentous protein, NF networks maintain larger oriented domains over much longer length scales and unexpectedly align perpendicular to the microchannel walls$^{2}$. We discuss the extended orientation order in NFs in terms of the inter-filaments interactions originating from their polyampholyte side-chains. Supported by DOE DE-FG-02-06ER46314, NSF DMR-0803103, and the Human Frontier Science Program organization. [1] N.F.Bouxsein \textit{et al.}, APL \textbf{85} (2004) 5775; L.S.Hirst \textit{et al.} Langmuir \textbf{21} (2005) 3910 [2] H.C.Hesse, R.Beck, J.Deek \textit{et al.} Langmuir \textbf{24} (2008) 8397 [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V15.00009: Theory and simulation of two-dimensional nematic and tetratic phases Jun Geng, Jonathan V. Selinger Recent experiments and simulations have shown that two-dimensional systems can form tetratic phases with four-fold rotational symmetry, even if they are composed of particles with only two-fold symmetry. To understand this effect, we propose a model for the statistical mechanics of particles with almost four-fold symmetry, which is weakly broken down to two-fold. We introduce a coefficient $\kappa$ to characterize the symmetry breaking, and find that the tetratic phase can still exist even up to a substantial value of $\kappa$. Through a Landau expansion of the free energy, we calculate the mean-field phase diagram, which is similar to the result of a previous hard-particle excluded-volume model. To verify our mean-field calculation, we develop a Monte Carlo simulation of spins on a triangular lattice. The results of the simulation agree very well with the Landau theory. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V15.00010: Quadrupolar particles in a nematic liquid crystal: Effects of particle size and shape Francisco Hung We investigate the effects of particle size and shape on the quadrupolar (Saturn ring-like) defect structures formed by a nematic liquid crystal (NLC) around nm- and micron-sized particles with spherical, spherocylindrical and cubic shapes. Our calculations, based on a Landau-de Gennes expansion in terms of the tensor order parameter Q, indicate that for pairs of nanoparticles in close proximity, the most stable defect structure is the ``entangled hyperbolic'' [1]. For pairs of micron-sized particles the NLC forms entangled 'figure of eight' defects [1] around pairs of spheres and spherocylinders. In contrast, we only observed unentangled defect structures around pairs of micron-sized cubic particles. For pairs of spherical and spherocylindrical particles, the transition between ``entangled hyperbolic'' and ``figure of eight'' structures occurs when the particle diameter is between 100 nm and 1 micron. Our calculations suggest that the NLC-mediated interactions between the nanoparticles are fairly strong (up to 5700 kT). These interactions can bind the particles together at specific locations, and thus could be used to assemble the particles into ordered structures with different morphologies. [1] M. Ravnik et al., Phys. Rev. Lett. 99, 247801 (2007) [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V15.00011: A Model Liquid Crystalline System Based on Rodlike Viruses with Tunable Chirality Daniel Beller, Edward Barry, Zvonimir Dogic Filamentous bacteriophages such as the \textit{fd} virus have long been used as ideal model systems to investigate the phase behavior of suspensions of rodlike particles. We study the structure and phase behavior of a mutant, \textit{fd} Y21M, and compare them to the properties of conventional \textit{fd} wild-type (\textit{wt}). These two viruses exhibit dramatically different phase behavior despite differing only by a single amino acid of the major coat protein pVIII. We find that this is attributable to significant differences in the flexibility of the viruses. Using the more rigid \textit{fd} Y21M, we are able for the first time to quantitatively test the Onsager description of the isotropic-nematic phase transition of rigid rods. Even more surprising are the differences in the behavior of the cholesteric phase of \textit{fd }Y21M and \textit{fd wt}. While \textit{fd} \textit{wt} forms a cholesteric pitch with a left-handed helix, \textit{fd} Y21M forms a cholesteric pitch with the opposite handedness. In addition, the magnitude of the cholesteric pitch changes by almost fivefold. Using mixtures of the two viruses, we are able to create liquid crystal systems with tunable control over the macroscopic chiral behavior. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V15.00012: Apparent Broken Reciprocity in Chiral Liquid Crystals Michele Moreira, Nithya Venkataraman, Peter Palffy-Muhoray, Lorenzo Marrucci Reciprocity in optics is predicated on bounded scattering media with symmetric and linear permittivity, conductivity and permeability. Due to their anisotropy and chirality, cholesteric liquid crystals (CLCs) form periodic dielectric structures. If the periodicity is comparable to the wavelength of light, these phases are self-assembled photonic band gap structures. There appear in the permittivity odd powers of the wave vector resulting from nonlocality and broken inversion symmetry. Evidence of non-reciprocity has been found in optically active crystals by Bennett [1] and in stacks of cholesteric and nematic liquid crystal cells by Takezoe [2]. We investigate experimentally and theoretically the possibility of a violation of optical reciprocity of a hetero-photonic-bandgap structure made of two CLC cells of diferent pitch. We have observed a significant apparent violation, but we show that the effect is due to light scattering, and in fact these choleseric structures are reciprocal. \\[3pt] [1] P.J. Bennett et al .Opt. Lett. 21, 1955 (1996) \\[0pt] [2] J. Hwang; et al. Nat. Mat. 4, 383 (2005). [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V15.00013: Raman scattering study of orientation order parameters in thermotropic biaxial nematic LC Min Sang Park, Bum Jin Yoon, Jung Ok Park, Veena Prasad, Mohan Srinivasarao There has been considerable interest in the liquid crystalline phases formed by bent-core molecules, since these molecules were considered to have a biaxial nematic phase. In an effort to understand the orientation behavior and the degree of phase biaxiality in bent-core mesogen, Raman spectroscopy has been used to measure the scattering intensities for orthogonal linear polarization. This straightforward methodology allows the values of both uniaxial, $<$P$_{200}>$ and $<$P$_{400}>$, and biaxial order parameters, $<$P$_{220}>$, $<$P$_{420}>$, and $<$P$_{440}>$ to be quantified. From experimentally derived order parameters, the most probable orientation distribution functions are constructed. The results of these measurements will be presented in the context of experimental evidence of phase biaxiality obtained by other methods including X-ray diffraction and NMR [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V15.00014: Liquid crystalline behaviors of H-bonded dimer formed from the semi-phasmidic type carboxylic acid* Seung-Yeon Jeong, Shin-Woong Kang, Satyendra Kumar, Veena Prasad, Sanjay Varshney Liquid crystalline properties of acid-functionalized semi-phasmidic azo-compound were characterized by DSC, polarizing optical microscopy, and electro-optical measurements. The results suggested that this unconventional mesogenic dimer has a non-typical effective ``bend'' angle where two monomers form the hydrogen bond. To confirm this we performed electro-optical experiments in the nematic phase with strong external electric and magnetic fields. Cells with different surface geometries and treatments we used. The results reveal a behavior that is very distinct from that expected of conventional linear mesogenic dimers formed by a hydrogen bonding. The results confirm negative values of dielectric and diamagnetic anisotropies. Our observations indicate that ``bent-core-like'' dimeric mesogens are formed through an unconventional inter-molecular association. *Work supported by grant NSF/DMR-086991. [Preview Abstract] |
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