Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C30: Polymers and Organic Systems |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: John Cressman, George Mason University Room: 338 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C30.00001: Immobilization of polymer microgels containing metal nanocatalysts onto inorganic surfaces L. Papoutsakis, M.A. Frysali, M. Kaliva, M. Vamvakaki, S.H. Anastasiadis This study is concerned with the attachment of electrostatically and sterically stabilized polymer microgel particles containing either amino (poly(2-(diethylamino)ethyl methacrylate), PDEA) or carboxylic acid (poly(acrylic acid), PAA; poly(methacrylic acid), PMMA) functional groups onto inorganic surfaces. The microgels are prepared by emulsion radical polymerization and are utilized as nanoreactors for the synthesis of metal nanoparticles to be used as nanocatalysts; Pd and Ru nanoparticles have been synthesized. The attachment of the microgel particles onto the various surfaces, which can potentially be used as the walls of microfluidic reactors, is studied; glass, silicon and alumina were used as substrates. We investigated the effects of parameters such as concentration of the suspension, substrate orientation whereas we utilized various recipes for ``trapping'' the microgel particles within interfaces to achieve their deposition onto the inorganic surfaces. The durability of the microgel particles attached onto the surfaces against hydration and shear forces was tested utilizing repeated immersion of the surfaces into water undergoing mechanically-generated hydrodynamic flow. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C30.00002: Predicting Universal Pattern Formation on Spheres with Application to Self-Assembly of Patchy Colloids Erik Edlund, Oskar Lindgren, Martin Nilsson Jacobi Patchy colloids, colloidal particles with attractive or repulsive patches, serve as a central example of building blocks for self-assembly [1]. The patches can be created using e.g. glancing angle deposition, but recently interest has turned towards using self-assembly for creating the patterns themselves [2]. We present theory for predicting pattern formation of isotropically interacting particles on spheres, based on a relaxation of a Potts-like model who's interactions can be diagonalized (a generalization of the approach in [3]). We give a simplified model of immiscible surfactants on gold nanoparticles [4,5] which we use to demonstrate the theory. We use the theory to design patchy particles for self-assembly of specific geometric structures.\\[4pt] [1] E. Bianchi, R. Blaak, and C. N. Likos, Phys. Chem. Chem. Phys. 13 (2011)\\[0pt] [2] A. M. Jackson, J. W. Myerson, and F. Stellacci, Nat. Mater. 3 (2004)\\[0pt] [3] E. Edlund and M. Nilsson Jacobi, Phys. Rev. Lett. 105 (2010)\\[0pt] [4] C. Singh et al., Phys. Rev. Lett. 99 (2007)\\[0pt] [5] I. C. Pons-Siepermann and S. C. Glotzer, Soft Matter 8(23) (2012) [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C30.00003: Temperature dependent depletion interaction from PEO and other polymers Bezia Laderman, Lang Feng, Stefano Sacana, Paul Chaikin We have found and tested a depletion mechanism, in which the inter-colloidal attractive interaction can be tuned in an easily accessible temperature range. Usually depletion is considered as a concentration dependent, temperature independent interaction, except when a thermo-sensitive depletent, like Poly-NIPA is used. Our system consists of water, NaCl, micron-size colloids and a polymer depletent. With such solutions colloidal crystals form at room temperature, but as the temperature is increased above a critical point ($T_c$), we observe the crystals melt and the colloids disperse. The process is thermo-reversible since crystals reform in a few minutes after the temperature is tuned below $T_c$. We studied the dependence of the critical temperature $T_c$ on factors such as the ionic strength, component/surface chemistry of the particle, type of depletent and additional non-ionic surfactants. Since the gyration radius of the depletent used in this study does not vary significantly with temperature, we argue that a temperature dependent adsorption of polymer depletent on the colloidal surface is responsible for the observed phase transition. Given the generality of the components used, our finding is useful for directed or self-assembly on the colloidal scale. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C30.00004: Modeling two-dimensional materials self-assembly: from Honeycomb to Kagome lattices Simiso K. Mkhonta, Ken R. Elder, Zhi-Feng Huang Novel two-dimensional materials of graphene-type and beyond have been of great interest in both fundamental research and a wide range of applications. In this work we study the self assembly properties of these 2D structures via the development of a phase-field-crystal model. The free energy functional in the model is designed to favor self assembly in crystals commensurate with a triangular symmetry, leading to a range of complex phases including honeycomb, kagome, and oblique, in addition to the simple triangular phase. We also examine the elastic properties of these novel crystalline structures, and the nonequilibrium evolution processes of these systems which are governed by diffusive time-scale dynamics. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C30.00005: Stochastic self-assembly of incommensurate clusters Maria D'Orsogna, Greg Lakatos, Tom Chou We examine the classic problem of homogeneous nucleation and self-assembly by deriving and analyzing a fully discrete stochastic master equation. We enumerate the highest probability steady-states, and derive exact analytical formulae for quenched and equilibrium mean cluster size distributions. Upon comparison with results obtained from the associated the mass-action Becker-D\"{o}ring (BD) equations, we find striking differences between the two corresponding equilibrium mean cluster concentrations. These differences depend primarily on the divisibility of the total available mass by the maximum allowed cluster size, and the remainder. When such mass ``incommensurability'' arises, a single remainder particle can ``emulsify'' the system by significantly broadening the equilibrium mean cluster size distribution. This discreteness-induced broadening effect is periodic in the total mass of the system but arises even when the system size is asymptotically large, provided the ratio of the total mass to the maximum cluster size is finite. Our findings define a new scaling regime in which results from classic mass-action theories are qualitatively inaccurate, even in the limit of large total system size. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C30.00006: Ultra-soft 100 nm thick zero Poisson's ratio film with 60{\%} reversible compressibility Chieu Nguyen, Steve Szalewski, Ravi Saraf Squeezing films of most solids, liquids and granular materials causes dilation in the lateral dimension which is characterized by a positive Poisson's ratio. Auxetic materials, such as, special foams, crumpled graphite, zeolites, spectrin/actin membrane, and carbon nanotube laminates shrink, i.e., their Poisson's ratio is negative. As a result of Poisson's effect, the force to squeeze an amorphous material, such as a viscous thin film coating adhered to rigid surface increases by over million fold as the thickness decreases from 10 $\mu$m to 100 nm due to constrain on lateral deformations and off-plane relaxation. We demonstrate, ultra-soft, 100 nm films of polymer/nanoparticle composite adhered to 1.25 cm diameter glass that can be reversibly squeezed over 60{\%} strain between rigid plates requiring (very) low stresses below 100 KPa. Unlike non-zero Poisson's ratio materials, stiffness decreases with thickness, and the stress distribution is uniform over the film as mapped electro-optically. The high deformability at very low stresses is explained by considering reentrant cellular structure found in cork and the wings of beetles that have Poisson's ratio near zero. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C30.00007: Inferring elastic properties of an fcc crystal from displacement correlations: sub-space projection and statistical artifacts Asad Hasan, Craig Maloney We compute the effective dispersion and density of states (DOS) of two-dimensional sub-regions of three dimensional face centered cubic (FCC) crystals with both a direct projection-inversion technique and a Monte Carlo simulation based on a common Hamiltonian. We study sub-regions of both (111) and (100) planes. For any direction of wavevector, we show an anomalous $\omega^2\sim q$ scaling regime at low $q$ where $\omega^2$ is the energy associated with a mode of wavenumber $q$. This scaling should give rise to an anomalous DOS, $D_\omega$, at low $\omega$: $D_\omega \sim \omega^3$ rather than the conventional Debye result: $D_\omega\sim \omega^2$. The DOS for the (100) sub-region looks to be consistent with $D_\omega \sim \omega^3$, while the (111) shows something closer to the Debye result at the smallest frequencies. Our Monte Carlo simulation shows that \emph{finite sampling} artifacts act as an effective disorder and bias the $D_\omega$ in the same way as the \emph{finite size} artifacts, giving a behavior closer to $D_\omega \sim \omega^2$ than $D_\omega \sim \omega^3$. These results should have an important impact on interpretation of recent studies of colloidal solids where two-point displacement correlations can be obtained in real-space via microscopy. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C30.00008: Measuring colloidal osmotic compressibility of a polymer-crowded colloidal suspension by optical trapping Jinxin Fu, Vural Kara, H. Daniel Ou-Yang Particle interactions determine the stability of nanoparticle suspensions and the phase separation of particle-polymer mixtures. However, due to the small sizes of the dispersed nanoparticles, it is not easy to directly measure interaction forces between particles in a colloidal suspension. In this paper, we propose an ``Optical Bottle'' approach to quantify these particle interactions in a suspension by measuring the colloidal osmotic compressibility of the nanoparticles. Virial expansion of the colloidal osmotic compressibility yields virial coefficients of different orders. The second order virial coefficient of aqueous suspensions of colloidal polystyrene nanospheres in the presence of high-salt (KCl) and polyethylene glycol (PEG) is found to decrease with increasing PEG concentration, suggesting an attractive depletion interaction between the PEG-crowed polystyrene particles. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C30.00009: The impact of surface properties on particle-interface interactions Anna Wang, David Kaz, Ryan McGorty, Vinothan N. Manoharan The propensity for particles to bind to oil-water interfaces was first noted by Ramsden and Pickering over a century ago, and has been attributed to the huge reduction in surface energy when a particle breaches an oil-water interface and straddles it at its equilibrium height. Since then materials on a variety of length scales have been fabricated using particles at interfaces, from Pickering emulsions to Janus particles. In these applications, it is simply assumed that the particle sits at its hugely energetically favourable equilibrium position. However, it was recently shown that the relaxation of particles towards their equilibrium position is logarithmic in time and could take months, much longer than typical experiments. Here we investigate how surface charge and particle 'hairiness' impact the interaction between micron-sized particles and oil-water interfaces, and explore a molecular kinetic theory model to help understand these results. We use digital holographic microscopy to track micron-sized particles as they approach an oil-water interface with a resolution of 2 nm in all three dimensions at up to thousands of frames per second. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C30.00010: Wavefront Kinetics of Plamsa Oxidation of Polydimethylsiloxane: Implications for Micropatterning Size Limits by Wrinkling Angus Bayley, Joao Cabral, Joanne Lingling Liao, Arnaud Chiche, Paul Stavrinou We investigate spontaneous wrinkling of bilayers under compressive strain as a means of producing highly ordered micropatterns that span macroscopic areas. Our focus is a fast track wrinkling method, involving plasma oxidation of pre-stretched elastomeric polydimethylsiloxane (PDMS), which when subsequently relaxed forms one-dimensionally aligned sinusoidal surface undulations. For the first time, we evaluate this micropatterning method in terms of the range of geometries of 1D wrinkles it can produce. Our investigation reveals the presence of an apparent minimum wrinkling wavelength for a given value of prestrain (approximately 600nm for a prestrain of 10{\%}), offering clues regarding the kinetics of glassy film formation on the surface of PDMS during plasma oxidation, which is subsequently investigated. X-ray reflectometry and analysis of wrinkling behavior for a selection of PDMS samples exposed to a range of plasma doses yields evidence that this transient film growth process is not dissimilar to the process of frontal photopolymerization. With the benefit of this finding, a route to further minimization of wrinkle periodicity - increasing processing pre-strain - is identified and subsequently implemented, allowing us to access periodicities as low as 140nm. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C30.00011: Novel low temperature phase transitions in short grafted chains as a model for monolayers of amphiphile molecules with ionic heads Carlos Gonzalez-Castro, Guillermo Ramirez-Santiago We have carried out extensive Monte Carlo simulations in the NPT ensemble of a model for Langmuir monolayers of amphiphile molecules with ionic heads deposited on an interface. We considered a previously proposed coarse--grained model [1] in which the molecules are represented as short chains made up of beads with one slightly larger head confined at the interface. By analyzing the behavior of several order parameters as a function of temperature and pressure we obtained a liquid expanded phase and various ordered condensed phases with different molecular tilts. More importantly, we found a novel ``untilted'' to ``collective tilted'' to ``small correlated tilted'' phase transition at low temperatures, and different pressure values, as suggested by the behavior of two order parameters. One that measures the average molecular inclination and the other that measures the average projection on the x--y plane of the vector that joins the head center with the last monomer center of each molecule. The latter yields information about the correlation of the molecular tilt. \\[4pt] [1] C. Stadler, H. Lange and F. Schmid, Phys. Rev. E, Vol. 59, (1999). [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C30.00012: Optical conveyors: Active tractor beams for colloids, emulsions and aerosols David Ruffner, David Grier A tractor beam is a travelling wave that transports material back to its source. We experimentally demonstrate such a beam by coherently superposing coaxial Bessel beams. These optical conveyors have periodic intensity variations along their axes that act as highly effective optical traps for micrometer-scale objects. Varying the Bessel beams' relative phase shifts the traps axially and thereby selectively transports trapped objects either downstream or upstream along the length of the beam. The same methods used to project a single optical conveyor can project arrays of independent optical conveyors, allowing bidirectional motion. This opens up new possibilities for three dimensional transport of colloids, emulsion droplets and aerosol particles with sub-micrometer resolution over ranges extending to 50 micrometers and potentially beyond. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C30.00013: Robust thermosensitive colloidal photonic crystals Jin-Gyu Park, William Rogers, Sofia Magkiriadou, Young-Seok Kim, Vinothan Manoharan Photonic structures made of colloidal nanoparticles that show dynamic switching have tremendous potential applications including tunable lasers, biological/chemical sensors, and optical devices. As a building block, hydrogel nanoparticles made of poly(N-isopropylacrylamide)(pNiPAm) are particularly interesting due to their tunability in size with response to temperature. Uses of pNiPAm as a 3-dimensional building block in colloidal arrays, however, are strictly limited because the structures are easily destroyed by increased thermal fluctuations around their volume phase transition temperature. Here we demonstrate a simple and robust way to assemble photonic crystals made of soft pNiPAm colloidal particles. Our particles consist of a polystyrene core and transparent p(NiPAm-co-Acrylic acid) shell. The scattering is therefore dominated by the polystyrene core, yet the inter-scatterer distance is tunable with temperature change. We use depletion attraction to assemble the colloidal particles into 3D photonic crystals. The resulting structures show dynamic modulations of stop-bands from 24C to 70C without losing the structural features. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C30.00014: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 5:18PM - 5:30PM |
C30.00015: On Ulam's packing conjecture: is the ball the worst shape for packing? Yoav Kallus The question of which convex shapes leave the most empty space in their densest packing is the subject of Reinhardt's conjecture in two dimensions and Ulam's conjecture in three dimensions. In two dimensions, a regular octagon whose corners have been smoothed to arcs of hyperbolas is known to be a local minimum of the optimal packing fraction and the circle is known to not be a local minimum. In three dimensions, we show that the ball is a local minimum: it is the worst packing shape among shapes of sufficiently low asphericity. We also discuss related results in higher dimensions and for the worst shape for other optimal arrangement problems. [Preview Abstract] |
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