Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y43: Focus Session: Fluids Under Confinement, Colloids and Liquid Crystals |
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Sponsoring Units: DPOLY GSOFT DCMP Chair: Alberto Fernandez-Nieves, Georgia Institute of Technology Room: 214C |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y43.00001: Dripping Cylindrical Double Emulsions Jiawei Yang, Laura Adams, David Weitz Not all drops drip from the orifice of a faucet or capillary as spherically shaped drops. By encapsulating water drops inside an ultra thin sheath of oil, cylindrically shaped drops emerge. A stability theory is presented which describes not only the volume of the ultra thin sheath, but also the flow velocity conditions under which cylindrical drop formation is possible. We compare our theoretical model to time-dependent dynamics of drop formation that is captured experimentally with a fast camera and imaged through a microfluidic device. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y43.00002: Hydrodynamic damping of dense colloidal packings under confinement Michael Ryan, Tim Still, Matthew Waite, Arjun Yodh, Kevin Aptowicz We experimentally study hydrodynamic damping of collective motion in dense colloidal crystals confined in a 1 micrometer channel. Particle diameters are on the order of the channel width resulting in quasi-two-dimensional entropic crystals. The packing fraction of the crystals, formed from soft thermo-responsive spheres, is varied with temperature. Digital video-microscopy is utilized to explore the phonon dynamics of the colloidal crystals. Friction coefficients along high symmetry directions in q-space are extracted and provide insight about the hydrodynamic forces at play. As expected, damping of collective motion increases with increasing packing fraction. Preliminary results suggest the friction coefficient decreases with increasing phonon wavelength, but it does not appear to vanish. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y43.00003: Simulation of blade printing of colloidal morphologies Alexander Wagner, Alan Denton, Erik Hobbie We present a new four-component multi-phase lattice Boltzmann simulation method and its application to blade printing of colloidal morphologies. We consider a solution of colloids and polymers that is applied as a thin film on a substrate. As the mixture is exposed to the surrounding air the solvent evaporates leaving the colloid-polymer mixture unstable to phase-separation. Our new method predicts that as a function of the application speed, initial concentrations, and film thickness different morphologies can be generated. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 9:12AM |
Y43.00004: Dynamics of Colloids in Nematic Liquid Crystals Invited Speaker: Oleg Lavrentovich Dynamics of small particles in fluids have fascinated scientist for centuries. Phenomena such as Brownian motion, sedimentation, and electrophoresis continue to inspire cutting-edge research and innovation. The fluid in which the particles move is typically isotropic, such as water or a polymer solution. Recently, our group started to explore what would happen if particles are placed in an anisotropic fluid: a liquid crystal. The study reveals that the liquid crystal changes dramatically the dynamic behavior, leading to levitation of the particles, their anomalous Brownian motion and new mechanisms of eletrokinetics. The new phenomena are rooted in anisotropy of the liquid crystal properties, such as surface tension and elasticity, different electric conductivity in the directions parallel and perpendicular to the average molecular orientation. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y43.00005: Disc-shaped colloids interacting in a nematic liquid crystal Alena Antipova, Colin Denniston We examine the behavior of (ferromagnetic) micron-sized structures such as disc-shaped colloidal particles in a nematic liquid crystal using Lattice Boltzmann algorithm. Without any external forces the position of the disc with respect to the liquid crystal director minimizes the free energy of the system and no distortion of the director field is observed. When the rotating magnetic field is present, the torque on the disc with homeotropic surface anchoring should change with analogy to electrostatic energy, which implies the disc continues turning following the field. However, when the disc reaches some critical position and the director field around it is highly distorted, the disc suddenly flips to minimize the free energy. Position and motion of pairs of such discs under similar conditions can be controlled by the angular velocity of magnetic field, it's magnitude and initial configuration of the system. As a result of analysis of discs' dynamics, a new way to control self-organization of disc particles was produced. We also will demonstrate some results on ferromagnetic torus micro-colloidal particle in nematic with more complicated boundary conditions. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y43.00006: Liquid Crystals Confined in Micro and Nanochannels Yu-Bing Guo, Jie Xiang, Oleg Lavrentovich, Qi-Huo Wei Geometrical confinements cause frustration, topological defects in liquid crystal molecular orientations, altering behaviors of phases and phase transitions. In this paper, we developed microfabrication processes for assembling cells of glass slides with various well-defined confinement geometries such as microfluidic channels and nanofluidic channels, and will present experimental studies on the structures and phase transitions of nematic and cholesteric liquid crystals under these confinements. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y43.00007: Defects in liquid crystals in confined geometries: simulation studies Sajedeh Afghah, Andrew Konya, Robin Selinger Using numerical simulations in three dimensions, we study the formation of defect structures in liquid crystals in confined geometries. We model a cholesteric in a microchannel with homeotropic anchoring on four sides and periodic boundary conditions along the channel length. We find that channel aspect ratio and cholesteric pitch control resulting defect structures, and in some cases produce evenly spaced bubble domains. We also performed simulation studies of a nematic liquid crystal confined in a cell with a periodic array of pillars with homeotropic anchoring on all surfaces, and examine formation of a periodic array of defects. To simulate temperature-driven microstructural evolution, we include in our model the temperature dependence of the Frank elastic coefficients and cholesteric pitch, fitted from experimental studies. Computational speed is improved by implementation of our simulation algorithm in CUDA. Simulation results are compared to recent experimental studies by the group of Qi-Huo Wei at Kent State. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y43.00008: Fluctuating Hybrid lattice Boltzmann method for nematic liquid crystals Ganna Piatkovska, Colin Denniston As one goes from micron to nanometer lengths scales in a liquid crystal, thermal fluctuations become increasingly important and can have significant impact on colloids and polymers immersed in the liquid crystal. We develop and test numerically a Hybrid lattice Boltzmann model for liquid crystal hydrodynamics that incorporates the thermal fluctuations of tensor order parameter. It is shown that a good equilibration is obtained over a wide range of length scales using uncorrelated noise. We find the condition on the system's correlation lengths when the use of correlated noise would be necessary to achieve an equilibration. In particular, in simulations without electric field, the mesh size should be chosen in such a way that the correlation length defined by the elastic constant is less than 2 in lattice units. When the electric field is present, the situation is more complex since the correlation length associated with the electric field comes into play. Some applications of the introduced noise are considered. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y43.00009: Tetratic and smectic liquid crystals on a sphere: defects, patterns and cubes Oksana Manyuhina, Mark Bowick We construct the elastic free energy for tetratic order and find a closed form solution for +1/4 disclinations. Confined to a sphere we expect tetratic order to manifest itself in eight +1/4 disclinations, giving the total charge of +2. Within the one elastic constant approximation for the tetratic free energy, their equilibrium positions define the vertices of a cube, rather than the twisted cube, found earlier within the XY-model. We show that it is energetically favorable for the sphere to deform to a rounded cube with flattened faces and locally high Gaussian curvature at the eight vertices. Motivated by experimental observations of smectic shells, we apply our analytic results to study the relative stability of defect configurations and the formation of periodic texture for thick smectic shells. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y43.00010: Visualization of the Flow Field induced by an Oscillating Post in a Freely Suspended Smectic Liquid Crystal Membrane Zhiyuan Qi, Kyle Ferguson, John Papaioannou, Yancey Sechrest, Tobin Munsat, Cheol Park, Matt Glaser, Joe Maclennan, Noel Clark, Tatiana Kuriabova, Thomas Powers Thin fluid membranes immersed in a less viscous, bulk fluid are of fundamental interest as approximations of true two-dimensional (2D) fluids and as models of biological membranes. Many previous studies of such fluid membranes have focused on 2D macroscopic hydrodynamic effects such as the diffusion and interaction of inclusions, with fewer experimental investigations of microscopic properties such as the flow field. We have measured the 2D flow field generated by a rigid, oscillating post inserted in a freely suspended smectic liquid crystal film surrounded by air by analyzing the motion of tracer particles in the film. Our experiments confirm Saffman's prediction that the far-field flow velocity decays as 1/r (where r is the distance from the post) in the longitudinal direction, and as 1/r\textasciicircum 2 in the tangential direction. The measurements are in good agreement with flow fields computed using a model that generalizes the Levine/MacKintosh point-force response functions. We have also investigated confinement effects when the post is located near the film boundary. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y43.00011: Liquid crystal-enabled electro-osmosis through spatially separated charges in photo-patterned surface alignment Chenhui Peng, Yubing Guo, Sergij Shiyanovskii, Qihuo Wei, Oleg Lavrentovich Electrically-controlled dynamics of fluids and particles at microscales is a fascinating area of research with applications ranging from microfluidics and sensing to sorting of biomolecules. We demonstrate that anisotropic conductivity of liquid crystals in combination with photopatterned surface alignment enables highly efficient electro-osmosis (LCEO) rooted in space charging of regions with distorted orientations. LCEO velocities grow with the square of the field, which allows one to use an AC field to drive steady flows and to avoid electrode damage. By controlling the director patterns, one can dramatically change the nature of LCEO flows, for example, trigger a pumping effect in dipolar configuration and reverse the flow direction in quadrupolar patterns. Ionic currents in liquid crystals that have been traditionally considered as an undesirable feature in displays, offer a broad platform for versatile applications such as liquid crystal enabled electrokinetics, micropumping and mixing. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y43.00012: Elastic Response of Liquid Crystalline Mixtures Jonathan Whitmer Liquid crystalline (LC) materials comprised of multiple mesogenic species, or mesogenic species and dopants, are widely used industrially to obtain materials having specific viscous, optical, or elastic properties. While commonly used materials exhibit additive elastic constants in the homogenous liquid phase, it is less clear how these materials respond to inhomgenous applied stresses often occurring in confinement. Here we utilize coarse-grained LC models and a recently developed formalism for free-energy calculations to investigate the elastic coefficients of LC mixtures and their behavior under asymmetric stress. [Preview Abstract] |
Friday, March 6, 2015 10:48AM - 11:00AM |
Y43.00013: Viral nematics in confined geometries Kyle Lawlor, Oksana Manyuhina, Mark Bowick, Cristina Marchetti Motivated by recent experiments on the rod-like virus bacteriophage fd confined to circular and annular regions, we present a theoretical study of confined nematic liquid crystals in such two-dimensional geometries. It is well known that a dense suspension of the fd-virus exhibits nematic order. Recent experimental work and comparison with numerical modeling predicts that in this system the ratio of the bend and splay elastic constants is close to unity. Using the one-elastic-constant approximation for the Frank free energy of nematic liquid crystals, we examine the competition between bulk elasticity and surface anchoring in controlling confined director configurations. We show that many of the observed configurations can be described in terms of bulk and surface topological defects. A similar effect is known to occur in 3D nematic droplets, where a change in anchoring conditions can drive the splitting of a bulk defect into surface defects. In contrast, in our 2D systems, such a splitting is driven by changes in the size and geometry of the system. We show that the continuum theory is capable of accounting for many of the observed configurations. [Preview Abstract] |
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