Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B12: GSNP Graduate Student Prize and Liquid Crystals: Smectics |
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Sponsoring Units: GSNP DFD Chair: Susan Coppersmith, University of Wisconsin, and Luz J. Martinez-Miranda, University of Maryland Room: B110-B111 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B12.00001: Random Packing Density of Platonic Solids Jessica Baker, Arshad Kudrolli Motivated by the relation between particle shape and packing, we investigate the volume fraction occupied by faceted particles as a function of number of particle sides. Such particles are arguably better representative of natural sand than spheres. For simplicity, we focus on the highly symmetric five Platonic solids which are polygons with congruent sides, vertices and angles, and experimentally measure their packing densities. Plastic dice with 4, 6, 8, 12, and 20 sides are fluidized or shaken randomly to find configurations corresponding to the loosest stable packing and densest packing, respectively. We find that the packing fraction obtained by both protocols peak at the cube and then monotonically decrease below the corresponding values obtained for spheres. Interestingly, the overall trend is similar but systematically lower that the maximum volume fractions reported for frictionless platonic solid particles. The effect of friction of the particles and the shape of the boundary shape on observed packing fractions is also investigated. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B12.00002: A Bayesian Approach to Detecting Amino Acid Covariance in Multiple Sequence Alignments Lucy Colwell, Michael Brenner, Andrew Murray Determining which residues of a protein control its biological function is a classical question in molecular biology. In particular, proteins can change their structure or function by mutating just a small set of residues. An attractive idea is that distinct sets of residues are responsible for different phenotypic properties, so that one property can be changed while another is not. Members of such a set mutate at similar points in a multiple sequence alignment and so are correlated. It has long been proposed that analysis of correlations in the mutation patterns of protein sequences may provide an important means of extracting functional information about proteins from sequence alignments. Here, we propose a methodology for incorporating functional and structural annotations of the sequences analyzed to improve the efficacy of algorithms at detecting such residue sets. We provide a Bayesian framework in which known biological properties of the sequences are used to define a prior probability that quantifies our belief that sequence positions with different conservation levels are associated with the phenotype of interest. Recent experimental data is used to demonstrate that applying these principles results in improved detection ability, allowing us to distinguish between pairs that demonstrate similar levels of correlation but are not of equal relevance to the phenotypic purpose being addressed. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B12.00003: Coarse-grained computational studies of the assembly of viral capsids around a flexible polymer Oren Elrad, Michael Hagan During the replication of many viruses, hundreds to thousands of protein subunits assemble around the viral nucleic acid to form a protein shell called a capsid. Recent electron microscopy experiments on small ssRNA viruses have shown that their enclosed RNA adopts the icosahedral symmetry of the overall capsid structure. The process that leads to this ordered encapsulation of the RNA is unknown. In this talk, we will explore dynamical simulations of coarse grained models that represent capsid proteins assembling around a flexible polymer, which shed light on the mechanisms by which icosahedral order emerges. We will discuss geometric and kinetic factors that control assembly, including the limits on the length of RNA that can be efficiently packaged. We will also report on several forms of cooperative polymer-protein motions that contribute to efficient and robust assembly. Finally, we will discuss how the simulation predictions can be tested with imaging experiments, bulk assembly kinetics measurements, and recently developed single molecule techniques that monitor the assembly of individual capsids. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B12.00004: Controlling Elastic Instabilities: From Complex Pattern Formation to Functionality Elisabetta Matsumoto, Randall Kamien Exploiting elastic instability in thin films has proven a robust method for creating complex patterns and structures across a wide range of lengthscales. Even the simplest of systems, an elastic membrane with a lattice of pores, under stress, generates a plethora of complex patterns featuring long-range orientational order. Harnessing the underlying elastic instability allows for the rational design of materials with highly desirable properties: from a film with a switchable photonic bandgap to a material with a negative Poisson ratio. Within the framework of linear elasticity, we model the system as a lattice of interacting deformation elements, or ``dislocation dipoles,'' which captures the configuration and orientational order present in any conceivable deformation of the system. In addition, when we promote this system to a curved surface, a novel set of features, patterns and broken symmetries appears. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B12.00005: Extending the Scaling Laws of Plasticity Georgios Tsekenis, Karin Dahmen Crystalline materials are known to deform in an intermittent way with avalanches. Power laws govern the statistics of the avalanche sizes, energies and times between avalanches. In this work we are studying the universal aspects of plasticity and dislocation dynamics. We employ a discrete dislocation dynamics simulation, which allows us to reproduce the distributions of avalanche sizes and energies of previous works. In addition, our model accounts for time explicitly. Thus we are able to extract distributions of dislocation slip avalanche durations and interevent times, which compare quite well with the experimental findings. We are also able to extract the power spectra of the dislocation activity that exhibit power law behavior as well. Furthermore, finite stress rate forces avalanches to occur concurrently in time and/or space and appears to lead to similar effects as previously studied for spin systems driven by an increasing magnetic field. The study of larger system sizes and slower stress rates and comparison to new experiments will give us deeper insight into the problem of plasticity as a nonequilibrium critical phenomenon. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:39PM |
B12.00006: BREAK
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Monday, March 15, 2010 12:39PM - 12:51PM |
B12.00007: Investigation of B7 Liquid Crystal Undulation Texture D. Chen, M.A. Glaser, J.E. Maclennan, N.A. Clark, E. Korblova, D.M. Walba In bent-core liquid crystals, the strong local preference for layering, coupled with the bent shape of the molecules, leads to two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering and molecule tilt. These instabilities combine to produce chiral layered phases such as the B2 and B7 phases. In the B7 phase, layer undulation arise due to the formation of periodic polarization splay stripes [D. A. Coleman, \textit{et al. Science} \textbf{301}, 1204 (2003)]. We have studied the topological defects in the B7 undulation texture obtained by freeze fracture transmission electron microscopy, and have observed dislocations and disclinations in the undulation texture analogous to those in the layered systems. Investigation of these defects gives insight into the B7 polarization splay stripe structure. A model is proposed to calculate the free energy of the defects. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B12.00008: Discovery of a novel smectic-C* liquid crystal phase with six-layer periodicity Shun Wang, LiDong Pan, Ronald Pindak, Zengqiang Liu, Tinh Nguyen, Cheng-Cher Huang We report the discovery of a new SmC*$_{d6 }$liquid crystal phase with six-layer periodicity by resonant x-ray diffraction. Upon cooling, the new phase appears between the SmC*$_{\alpha }$ phase having a helical structure and the SmC*$d4$ phase with four-layer periodicity. This SmC*$_{d6}$ phase was identified in two mixtures which have an unusual reversed SmC*d4-SmC$* $phase sequence. The SmC*$_{d6 }$phase shows a distorted clock structure. The existence of phases having liquid-like in-plane ordering with a ``long-range'' lock-in periodicity (now being extended to six layers) is one of the long-lasting questions in condensed matter. Major efforts are required to address the physical origin of long-range interactions and novel physical properties of the SmC*$_{d6 }$phase. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B12.00009: ABSTRACT WITHDRAWN |
Monday, March 15, 2010 1:15PM - 1:27PM |
B12.00010: Polarization modulated orthogonal smectic phases Chenhui Zhu, Renfan Shao, Joseph Maclennan, Dong Chen, Yongqiang Shen, Matthew Glaser, Noel Clark*, R. Amaranatha Reddy, David Walba, Per Rudquist Polar orthogonal phases of bent-core materials have long been predicted, and a field-induced SmAP$_{F}$ has been reported$^{1}$. We have recently observed a stable SmAP$_{F}$ phase, obtained by design for an asymmetric bent-core mesogen with only one tail$^{2}$. Here we report studies on a similar single-tail bent-core mesogen, W596, which suggest the new material possesses an I-SmAP$_{F}$'-SmAP$_{F}$-Crystal phase sequence on cooling, where the SmAP$_{F}$' phase is similar to the SmAP$_{F}$ phase except that the SmAP$_{F}$' phase is polarization modulated, leading to the formation of layer undulations, evidenced from the 1D periodic line patterns in freeze fracture transmission electron microscopy images, and x-ray reflections in addition to the main smectic layering reflection. Results of polarizing optical microscopy, electro-optic studies, and differential scanning calorimetry will also be presented. [1] Y. Shimbo, et al. PRL 97, 113901 (2006). [2] D. M. Walba, et al. 11th FLC Conference, 2007, p31. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B12.00011: Helical nanofilaments and the high chirality limit of smectics-A Gareth Alexander, Elisabetta Matsumoto, Randall Kamien Motivated by recent experiments on chirality in smectic systems of achiral bent core molecules [1], I shall describe our recent work on the theory of chiral smectic-A liquid crystals and argue that at sufficiently large chiralities the traditional twist grain boundary phase is augmented by a new texture [2]. This bulk texture is characterized by an array of parallel, coherently rotating helical nanofilaments, which represent the local optimal configuration for chiral smectics, laced together by a lattice of defects, in a fashion akin to the cholesteric blue phases. A mean field analysis of the properties of this nanofilament phase shows good qualitative agreement with the germaine features of the experiment, which can be improved upon by including the layer curvature energy, as well as bringing up several subtle aspects of the familiar analogy between smectics and superconductors. [1] L. E. Hough et al, Science 325, 456-460 (2009). [2] E. A. Matsumoto, G. P. Alexander, and R. D. Kamien, arXiv:0909.3529 [cond-mat.soft] (2009). [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B12.00012: Orientational order and topological defects in smectics Bryan Chen, Gareth Alexander, Randall Kamien The homotopy theory of topological defects in liquid crystals fails to completely characterize disclinations in smectics, due to the broken translational symmetry. We provide an approach to enumerating topological defects that correctly accounts for the interaction between translations and rotations. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B12.00013: Chromonic liquid crystals and their dispersion in polymers Jung Park, Xuxia Yao, Mohan Srinivasarao Chromonic liquid crystals can self-assemble into an ordered complex fluid, potentially applicable for biosensor, polarizers, optical compensetors and organic solar cells. Different from common amphiphilic lyotropic mesophases, aggregation of the chromonic liquid crystals is thought to be isodesmic and without optimum aggregation size. We studied the aggregation behavior by Vis-spectroscopy, and the phase behavior by polarizing optical microscopy and differential scanning calorimetry. We also used capillary flow to achieve uniform planar alignment in a flat capillary, and measured polarized Raman scattering, from which the temperature and concentration dependence of order parameters, both $<$P200$>$ and $<$P400$>$, and the orientation distribution were deduced. Order parameters increase as concentration increases and decrease as temperature increases. Polymer dispersed chromonic droplets with different director configurations were obtained by using different water soluble polymers and those anchoring phenomena were compared. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B12.00014: Topological defects of lyotropic chromonic liquid crystals Xuxia Yao, Alejandro Rey, Jung Park, Mohan Srinivasarao Lyotropic chromonic liquid crystals (LCLCs), an interesting and relatively poorly studied class of lyotropic liquid crystals, have gained increasing attention from 1980s. The studies of topological defects of LCLCs have been rarely reported in literature. We found LCLCs actually provide a good model system to study the defects, due to their unique properties. Defects, such as loops and point defects distributed on disclination line, etc., were created and controlled through symmetry-breaking phase transitions in some cells with special geometries. The dynamics of these defects was studied and more elastic properties of LCLCs were deduced. [Preview Abstract] |
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