Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B18: Focus Session: Liquid Crystals, Nano to Meso Scale Structure in Ordered Matter and Liquid Crystal I: Nanocomposites and Smectics |
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Sponsoring Units: DCMP GSNP DPOLY Chair: Joseph MacLennan, University of Colorado, Boulder Room: 403 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B18.00001: Dendritic Patterns in Nematic Liquid Crystal Nanocomposites Sebastian Gurevich, Alejandro Rey Liquid crystal (LC) mixtures with nanoparticles (NP) are of fundamental interest in the development of advanced materials. Of particular interest is developing means to direct the assembly of the NPs. The interactions in LC-NP mixtures are still under active research, although important tendencies have been established. Little attention has been given to morphological instability patterns, and those mediated by diffusion of NPs are yet to be explored. Using the continuum model of Soule et al [1], we explore numerically the growth of nematic droplets in an isotropic liquid under conditions that lead to a variety of dendritic like morphologies controlled by a diffusive instability mediated by the NPs and the anisotropy of the nematic field. The numerical implementation of the model, which represents a mixture of calamitic nematic LC (cylinders) and NPs (hard spheres) of comparable size, is based on the adaptive mesh refinement scheme developed by Provatas et al, allowing access to realistic time and length scales. Our work lays the ground to developing new means to direct the assembly of NPs over large areas by exploiting the morphological instabilities at nematic-isotropic interfaces. The diversity of morphologies may also allow estimating the value of material parameters that are otherwise difficult to obtain experimentally. [1] Soft Matter, 2012, 8, 2860 [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B18.00002: Liquid Crystalline Orientational Control via the Electric Field of Localized Surface Plasmons Makiko Quint, Linda Hirst, Sayantani Ghosh We probe the effect of induced electric fields of localized surface plasmons (LSPs) generated by self-assembled gold nanoparticles (AuNPs) on the directional orientation of a thin film of nematic liquid crystal, 4-cyano-4$'$-pentylbipenyl (5CB). We excite the composite AuNP-LC with excitation tuned on and off resonance with the LSP absorption peak, and track the birefringence of the LC. Our results demonstrate re-orientation of the director of the LC when the LSPs are excited resonantly, and that this effect is temperature dependent. Our studies indicate LSP generated electric fields may offer an all-optical protocol to locally control LC orientation. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B18.00003: Behavior of anisotropic particles at air/nematic interface Iris B. Liu, Mohamed A. Gharbi, Randall D. Kamien, Shu Yang, Kathleen J. Stebe Colloidal particles with non-spherical shapes or deliberate patchiness can create capillary interactions that direct assembly in well-defined orientations. These effects have been considered only recently at simple fluid interfaces, and are largely unexplored at complex fluid interfaces. Anisotropic particles immersed in liquid crystals can also generate strong directed interactions. In this work, we explore the influence of particle geometry in colloidal interactions at nematic interfaces. We use particles of cylindrical shape, with controlled surface chemistry (anchoring and wetting properties) and report their behavior at an air/nematic interface. We study the interactions and self-assembly of these particles as a function of their aspect ratios. When cylinders are captured at the nematic interface, they induce deformation of the interface to satisfy wetting properties at particle surface. In addition, their presence induces distortion of the uniform director filed at the air/nematic interface to satisfy anchoring properties. Elastic and capillary interactions compete with each other and the resulting potential drives assembly of particles into novel structures. Recent progress in understanding colloidal interaction of anisotropic particles is presented. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B18.00004: True colloids of ferroelectric nanoparticles in liquid crystals: grand challenges and recent breakthroughs Yuriy Garbovskiy, Anatoliy Glushchenko Recent publications in the field of colloids of ferroelectric nanoparticle in liquid crystals revealed a high variety of physical effects. At the same time, there are still many inconsistencies between the results reported by different scientific groups. This fact reflects the complexity of such colloids. Particularly, inherent ferroelectricity of the nanoparticles or particles aggregation in some cases are among the important factors causing the variation of the reports. In this presentation we report experimental results based on the system which is free of the above mentioned challenges. We show how to prepare the true colloid of ferroelectric nanoparticles in liquid crystals; demonstrate impact of the ferroelectricity on the physical properties of the colloid; and present new electro-optical effects observed in these systems. We cover also a variety of possible applications of liquid crystals doped with ferroelectric nanoparticles. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B18.00005: Increasing Dispersion of Quantum Dots in Liquid Crystal Using Mesogenic Ligands Zachary Nuno, Andrea Rodarte, Blessing Cao, Ronald Pandolfi, Makiko Quint, Sayantani Ghosh, Jason Hein, Linda Hirst High concentrations of quantum dots (QDs) dispersed in liquid crystal materials will tend to aggregate together. One reason for this is the elastic cost of local liquid crystal alignment with the ligands on the surface of the QDs. We use mesogenic ligands with a flexible arm to allow the ligands to align with the director axis of the liquid crystal, thereby reducing aggregation and promoting QD dispersion in the host material. The dispersion of CdSe (core only) and CdSe/ZnS (core/shell) QDs with isotropic and mesogenic ligands is compared using fluorescence microscopy, x-ray scattering, and scanning confocal microscopy. The results from these techniques demonstrate that the mesogenic functionalized QDs do not aggregate into dense clusters as observed with the isotropic functionalized QDs. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B18.00006: Ordered and disordered colloidal particle monolayers at liquid crystal interfaces Wei-Shao Wei, Matthew Lohr, Mohamed Amine Gharbi, Kathleen Stebe, A.G. Yodh In this work, we investigate ordered colloidal particle monolayers at the air/liquid-crystal (LC) interface. Specifically, silica microparticles are treated with DMOAP to create homeotropic anchoring of LC mesogens at their surfaces. These particles are then spread on an air-exposed interface of the LC 5CB. Macroscopic ordered patterns of these microparticles form due to long-range interactions between particles that are mediated by elastic deformations of the underlying LC. Different confinement conditions lead to various self-assembled patterns ranging from hexagonal lattices to chain-like dipole formations. Using dark-field video microscopy, we track and analyze the dynamics of the colloidal particles in the hexagonal crystal packing, deriving mean squared displacements, phonon modes and density of states, etc., under several conditions. Further, heating of the nematic LC into its isotropic phase enables us to observe melting dynamics of this unusual quasi-2D crystal. The investigations provide insight into crystalline packings controlled by liquid-crystal mediated colloidal interactions. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B18.00007: Electromechanical memory effect in a ferroelectric nanoparticle-suspended liquid crystal Rajratan Basu A small quantity of BaTiO$_{\mathrm{3}}$ ferroelectric nanoparticles (FNP) was doped in a liquid crystal (LC), and the LC$+$FNP hybrid was found to exhibit an electromechanical memory effect in the isotropic phase. The permanent dipole moment of the FNPs causes the LC molecule to form short-range order surrounding the FNPs. This FNP-induced short-range order becomes more prominent in the isotropic phase when the global nematic order is absent. These short-range domains, being anisotropic in nature, interact with the external electric field. When the field goes off, these domains stay oriented due to the absence of the long range order in the isotropic phase, showing a hysteresis effect. The area under the hysteresis graph shows a significant pretransitional behavior on approaching the nematic phase from the isotropic phase. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B18.00008: Liquid Crystal properties of Silver (Ag) Nanowires as a Function of Flow Luz J. Martinez-Miranda, Liangbing Hu, Colin D. Preston We study the liquid crystal properties of nanowires of silver (Ag) as a function of flow of the solvent. Specifically, we are interested in finding the flow-concentration point where the electrical properties (IV curve) are: 1. Along one direction; and, 2. At a maximum along that particular direction. We are interested in the structure intermediate between the liquid crystal phase and the isotropic phase (the heterogenous phase) and how ``ordered'' this phase becomes with flow. Flow is varied in our case by having a substrate with gratings of varying depth in them. The flow due to the grating and the thickness of the film, plus the size of the nanowires will dictate the degree of order in the heterogeneous phase. This order dictates how the electrical properties orient in the resulting film. These studies can be expanded to include other semiconducting and/or metallic nanowires. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B18.00009: Memory effects of nematic liquid crystals in porous network: the role of geometry Francesca Serra, Shane Eaton, Marco Buscaglia, Roberto Cerbino, Giulio Cerullo, Roberto Osellame, Tommaso Bellini We exploit here the bistability of nematic liquid crystals (NLC) induced by their confinement into bicontinuous porous networks. In such a confined liquid crystal, the application of a strong external field induces the reconfiguring of topological defects, which then become locked as they entangle with the porous material. In this sense, the system has a memory of the applied field and it retains its orientation also when the field is removed. Computer simulations already showed that this effect depends on the geometry and the topology of the porous material. Incorporating liquid crystals in laser-microfabricated structures, made with two-photon polymerization, allows us to experimentally test this concept. We compare networks with different geometry and measure the memory of liquid crystals: we show that, as computer simulations predict, the cubic geometry yields the biggest memory effect. Both experiments and simulations also show that defects and anisotropies in the porous structure are important parameters that can substantially affect the memory. The small size of the scaffold (50-100 microns) and the large memory of the liquid crystals in cubic scaffold make this system promising for applications. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B18.00010: Nanoparticle Diffusion and Aggregation in 2D Smectic Membranes Kyle Meienberg, Greg Smith, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark We observe directly the diffusion and aggregation of nanoparticles embedded in thin, freely-suspended smectic films using reflected light microscopy in order to better understand the hydrodynamics of inclusions in finite, two-dimensional fluids. The Saffman-Delbr\"{u}ck (SD) model has been used previously to describe the diffusion of micron-sized objects in thin membranes in a variety of experimental systems. Nano-sized Buckyball aggregates embedded in smectic A films are observed to have much faster diffusion rates than predicted by SD theory. These experiments suggest that SD theory breaks down in the limit of nanometer-scale inclusions where molecular diffusion processes are dominate. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B18.00011: Two-Dimensional Diffusion of a Droplet near the Rigid Boundary of a Fluid Smectic Film Zhiyuan Qi, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark, Tatiana Kuriabova, Thomas Powers Homogeneous, freely-suspended fluid smectic A liquid crystal films in vacuum provide an ideal system for studying two-dimensional (2D) hydrodynamics in the absence of additional drag from the surrounding air. The 2D Brownian motion of a single oil droplet embedded in such a film and diffusing near a rigid wall was captured using digital video microscopy. Analysis confirms that the diffusion is anisotropic, with different mobilities for droplet motion parallel and perpendicular to the wall, in qualitative agreement with calculations based on the Oseen tensor assuming non-slip boundary conditions. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B18.00012: Dynamics of point defects in free-standing smectic films Kirsten Harth, Ralf Stannarius Ordering phenomena that break certain symmetries often involve the dynamics of defects. In liquid crystals, such processes are easily visualized in polarized light microscopy. Point defect annihilation has been investigated experimentally and numerically in thermotropic nematics, lyotropics and active nematics. Free-standing smectic films are ideal model systems to probe two-dimensional hydrodynamics. Films in the smectic C phase represent the simplest anisotropic fluid in two dimensions. Within elastic one-constant approximation and neglect of hydrodynamic effects, the description of defect dynamics is identical to that of point charges in electrodynamics. Whereas the equations for two point charges are easily solvable analytically, the case of three or more charges is equivalent to the classical three-body problem. We present new methods to create defect pairs and groups of defects of topological strength +1 in a controlled way. The annihilation process, the repulsion of sets of +1 defects in several geometrical configurations, and repulsion of +1 defects from larger positive net topological charges are analyzed. We discuss the influence of material flow and compare the experimental results to theory and simulations. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B18.00013: Coalescence of fluid droplets in freely-suspended smectic liquid crystal films Cheol Park, Zhiyuen Qi, Joseph Maclennan, Matthew Glaser, Noel Clark We have studied the coalescence dynamics of the lens-shaped fluid droplets embedded in freely suspended smectic A liquid crystal films. The early time expansion of the bridge connecting the merging domains as a function of the time from the onset of coalescence is measured using high speed video microscopy. Optical interference is used to extract the shape evolution of the coalesced droplet with the measured thickness profiles giving fundamental insights into the transport processes within the droplets before and after coalescence. We compare the early-time growth of the bridge between the droplets with 2D and 3D theoretical predictions and propose a model of the fluid flow between the droplets to explain the coalescence dynamics. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B18.00014: Brownian Motion of Topological Point Defects in Smectic C Films Kate Wachs, Cheol Park, Zhiyuen Qi, Joseph Maclennan, Noel Clark We observe the diffusive motion of topological point defects in the c-director field of freely-suspended smectic C liquid crystals films using reflected polarized light microscopy. Racemic films two layers thick were created and placed within a vacuum chamber. The diffusion of defects confined to islands of different thickness and diameter was measured as a function of air pressure. As the pressure decreases, the diffusion coefficient increases until it reaches the limit that results from pure 2D confinement. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B18.00015: Mutual Diffusion of Inclusions in Freely-Suspended Smectic Liquid Crystal Films Tatiana Kuriabova, Zhiyuen Qi, Zoom Nguyen, Cheol Park, Matthew Glaser, Joseph Maclennan, Noel Clark, Thomas Powers We study experimentally and theoretically the hydrodynamic interaction of pairs of circular inclusions in two-dimensional, fluid smectic membranes suspended in air. By analyzing their Brownian motion, we find that the radial mutual mobilities of identical inclusions are independent of their size but that the angular coupling becomes strongly size-dependent when their radius exceeds a characteristic hydrodynamic length. The observed dependence of the mutual mobilities on inclusion size is described well for arbitrary separations by a model that generalizes the Levine/MacKintosh theory of point-force response functions and uses a boundary-element approach to calculate the mobility matrix. [Preview Abstract] |
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