Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W35: Colloidal Clusters, Interactions, and Synthesis |
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Sponsoring Units: GSOFT Chair: Robert Hoy, University of South Florida Room: 210B |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W35.00001: Structure and dynamics of model colloidal clusters with short-range attractions Krystle Quinn, Robert Hoy We examine the structure and dynamics of small isolated $N$-particle clusters interacting via short-ranged Morse potentials. ''Ideally prepared ensembles'' obtained via exact enumeration studies of sticky hard sphere packings serve as reference states allowing us to identify key statistical-geometrical properties as well as to quantitatively characterize how nonequilibrium ensembles prepared by thermal quenches at different rates $\dot{T}$ differ from their equilibrium counterparts. Our results provide a theoretical framework for extending recent experimental studies of small colloidal clusters to examine nonequilibrium phenomena. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W35.00002: Ground States and Folding Dynamics of Colloidal Clusters Ellen Klein, W. Benjamin Rogers, Vinothan N. Manoharan We experimentally study colloidal clusters consisting of $N $\textless 100 spherical particles with short range, isotropic interactions. These clusters are a model system for understanding colloidal self-assembly and dynamics, since the positions and motion of the particles can be observed in real space. For $N \le $ 10 the ground states are degenerate; previous work\footnote{ \textit{The Free-Energy Landscape of Clusters of Attractive Hard Spheres}, G. Meng, et. al. Science \textbf{327} 563 (2010).} has shown that the probabilities of observing specific clusters depend primarily on their rotational entropy, which is determined by symmetry. Thus, less symmetric structures are more frequently observed. However, for large $N$ the ground state should be a highly symmetric close-packed lattice. We seek to understand how this transition occurs as a function of $N. $To do this, we coat \quad colloidal particles with complementary DNA strands that induce a short-range, temperature dependent interparticle attraction. We then assemble and anneal an ensemble of clusters with N $\ge $ 10. We characterize the number of apparent ground states, their symmetries, and their probabilities as a function of $N$. We also observe how these clusters fold into minimal-energy configurations by subjecting them to an electric field that we then relax. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W35.00003: Re-shaping colloidal clusters Daniela Kraft Controlling the geometry and yield of anisotropic colloidal particles remains a challenge for hierarchical self-assembly. I will discuss a synthetic strategy for fabricating colloidal clusters by creating order in randomly aggregated polymer spheres using surface tension and geometrical constraints. The technique can be extended to a variety of charge-stabilized polymer spheres and offers control over the cluster size distribution. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W35.00004: Counterion-Mediated Assembly of Spherical Nucleic Acid-Au Nanoparticle Conjugates (SNA-AuNPs) Sumit Kewalramani, Liane Moreau, Guillermo Guerrero-Garc\'Ia, Chad Mirkin, Monica Olvera de la Cruz, Michael Bedzyk Controlled crystallization of colloids from solution has been a goal of material scientists for decades. Recently, nucleic acid functionalized spherical Au nanoparticles (SNA-AuNPs) have been programmed to assemble in a wide variety of crystal structures. In this approach, the assembly is driven by Watson-Crick hybridization between DNAs coating the AuNPs. Here, we show that counterions can induce ordered assembly of SNA-AuNPs in bulk solutions, even in the absence of base pairing interactions. The electrostatics-driven assembly of spherical nucleic acid-Au nanoparticle conjugates (SNA-AuNPs) is probed as a function of counterion concentration and counterion valency [$+$1 (Na$^{+})$ or $+$2 (Ca$^{2+})$] by \textit{in situ} solution X-ray scattering. Assemblies of AuNPs capped with single-stranded (ss-) or double-stranded (ds-) DNA are examined. SAXS reveals disordered (gas-like) $\to $ face-centered-cubic (FCC) $\to$ glass-like phase transitions with increasing solution ionic strength. These studies demonstrate how non-base-pairing interactions can be tuned to create crystalline assemblies of SNA-AuNPs. The dependence of the inter-SNA-AuNP interactions on counterion valency and stiffness of the DNA corona will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W35.00005: Measuring colloidal charges in low polar media from statistics of particle trajectories Daniel Evans, Andrew Hollingsworth, David Grier We present a method for characterizing the surface charge on a pair of interacting poly(methylmethacrylate) spheres suspended in low polar media. This system does not rely on charge-stabilizing agents such as the well-known AOT or OLOA dispersants. The spheres undergo an overdamped thermal motion measured at millisecond time intervals with blinking optical tweezers and a high-speed camera. By examining particle trajectory statistics with kernel density estimators, we can calculate the drift velocity, diffusivity and hence the force as a function of particle separation. This interaction exhibits the characteristics of a screened-Coulomb force on the piconewton scale. These measurements imply a particle charge distribution much broader than that observed in aqueous dispersions of monodisperse spheres. Implications of this polydispersity for charge-stabilized colloidal crystals will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W35.00006: Does Suspension Crowding Screen Hydrodynamic Interactions? Yu Su, Roseanna N. Zia, James W. Swan Resistance and mobility functions describe linear couplings between moments of the hydrodynamic traction on a suspended particle and the motion of that or other particles. For two isolated spheres, these functions are well known and have been applied directly in the solution of many important problems for dilute colloidal dispersions. We have devised a new stochastic technique to calculate an analogous set of functions for two spheres immersed in a suspension that are then used to model the near-equilibrium dynamics of concentrated dispersions, including viscoelasticity and long-time diffusion. Of interest is the degree of screening of hydrodynamic interactions by the intervening medium. We find that the mobility is unscreened at the pair level, even in suspensions of high concentration, confirming that hydrodynamic interactions are an essential part of the dynamics of crowded systems and cannot be neglected in favor of simple renormalization schemes. We compare our results for the hydrodynamic interactions between suspended particles to predictions from two-point microrheology. This technique can be used to infer the complex viscosity from long-ranged decay of the pair mobility in viscoelastic materials. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W35.00007: Pair Interactions of Superhydrophobic Colloids at an Oil-Aqueous Phase Interface Colm Kelleher, Anna Wang, Ivan Guerrero, Bhaskar Krishnatreya, Andrew Hollingsworth, David Grier, Vinothan Manoharan, Paul Chaikin Superhydrophobic PMMA colloids, dispersed in oil, can become highly charged. In the presence of an interface with a conducting aqueous phase, image charge effects lead to strong binding of colloidal particles to the interface, despite the fact that the equilibrium contact angle $\theta_{c} > 170^{\circ}$. We present the results of a series of experiments designed to probe the attraction of individual colloids to the interface, and the repulsion between pairs of interfacially bound colloids. We show that these interactions are relatively uniform, reproducable, and time-independent, and can be described by a simple model in which the only parameters are the particle charge and the Debye screening length in the oil phase. These factors make this system a good candidate for studying various equilibrium and non-equilibrium phenomena in 2D condensed matter physics, for example defect formation and dynamics in 2D colloidal crystals, and structural rearrangements in sheared colloidal glasses. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W35.00008: Precise measurement of surface plasmon forces at a metal-dielectric interface using a calibrated evanescent wave Lulu Liu, Alex Woolf By observing the motion of an optically trapped microscopic colloid, sub-piconewton static and dynamical forces have been measured using a technique called photonic force microscopy. This technique, though potentially powerful, has in the past struggled to make precise measurements in the vicinity of a reflective or metallic interface, due to distortions of the optical field. We introduce a new in-situ, contact-free calibration method for particle tracking using an evanescent wave, and demonstrate its expanded capability by the precise measurement of forces of interaction between a single colloid and the optical field generated by a propagating surface plasmon polariton on gold. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W35.00009: Osmotic equilibrium of colloidal nanoparticles transiently confined in an optical trap Jinxin Fu, H. Daniel Ou-Yang Equilibrium number density profile of colloidal particles in a potential force field depends on the particle number density, the force field and interactions between the particles. Einstein described the particle number density profile by an osmotic equilibrium equation relating colloidal osmotic pressure and the potential force in his 1905 paper on the Brownian motion. For a dilute suspension of colloids, when particle interactions are negligible, the osmotic equilibrium equation can be used to determine unknown potential energy profiles from the Boltzmann distribution of the particle number density. Using a known potential energy profile, one can determine the colloidal osmotic pressure as a function of particle density, i.e., the osmotic equation of state, from the density profiles of interacting colloids. We use particle density profiles determined by confocal imaging of fluorescent polystyrene nanoparticles transiently confined in an optical trap to determine the colloidal osmotic equation of state for colloids in the presence of KCl and neutral polymers. The osmotic compressibility and chemical potentials of the colloids are calculated from the osmotic equation of state to predict colloidal stability and phase transitions. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W35.00010: Determination of colloidal osmotic equation of state by dielectrophoresis Jacob Mazza, Hao Huang, H. Daniel Ou-Yang Osmotic equation of state [P(N,T)] describes both the mechanical properties and phase behavior of a colloidal suspension. As an alternative to sedimentation, we propose a new approach to determine P(N,T) by dielectrophoresis (DEP). Using fluorescence confocal microscopy, we obtain particle density profiles in order to determine the DEP force distribution when the particle concentration is low and the inter-particle interactions are negligible. From the known force distribution and Einstein's osmotic equilibrium equation, we can calculate P(N,T) from the particle density profile of interacting, charge-stabilized polystyrene latex particles under different salt concentrations and added neutral polymers. The osmotic equation of state for colloidal suspensions can then be crosschecked by sedimentation equilibrium. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W35.00011: Differential Dynamic Microscopy of Weakly Scattering and Polydisperse Protein Rich Clusters Jacinta Conrad, Mohammad Safari, Peter Vekilov Biological objects often scatter light weakly and are frequently smaller than the diffraction limit, complicating measurements of their dynamics. Differential dynamic microscopy (DDM) is a recently developed method used to quantify dynamics of sub-$\mu$m particles in solutions from fluctuations in intensity in optical micrographs. DDM is well established for monodisperse particles but has not been applied to polydisperse biological nanoparticles. Here, we used DDM to measure dynamics of polydisperse nanoscale objects, protein-rich liquid clusters in protein solutions, whose size ranged from tens to hundreds of nanometers and whose total volume fraction was less than 10$^{-5}$. For solutions of two proteins, lysozyme and hemoglobin A, we measured the dynamics of clusters using DDM and evaluated their diffusion coefficients from the dependence of the diffusion lag time on the scattering wave vector. The average diffusion coefficient of clusters measured using DDM was consistently smaller than that obtained from dynamic light scattering at 90$^\circ$. The apparent discrepancy between results was explained by Mie scattering theory, which indicates that larger clusters preferentially scatter more light in the forward direction. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W35.00012: Fast Holographic Characterization of Dimpled Spheres Mark Hannel, Christine Middleton, David Grier We present a method for quickly analyzing the radii and refractive indexes of dimpled spheres using Holographic Video Microscopy. Our method utilizes an azimuthal median to suppress the perturbation caused by the dimple on an otherwise radially symmetric hologram. The resulting one-dimensional radial profile is fit to Lorenz-Mie theory using Support Vector Machines (SVM).We then discuss the limitations of this method as well as the use of SVMs trained on different scattering geometries or theories. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W35.00013: Highly uniform polyhedral colloids formed by colloidal crystal templating Yifan Wang, James McGinley, John Crocker We seek to create polyhedral solid particles by trapping oil droplets in a colloidal crystal, and polymerizing them in situ, resulting in polyhedral particles containing spherical dimples in an ordered arrangement. Specifically, highly monodisperse, micron-sized droplets of 3-methacryloxypropyl trimethoxysilane (TPM) were first prepared through a poly condensation reaction, following well established methods. The droplets were mixed with an excess of polystyrene(PS) particles (diameter in 2.58 $\mu $m), which formed close packed (FCC or HCP) colloidal crystals by natural sedimentation and compression under partial drying to an extent, with TPM oil droplets trapped into their tetrahedral and octahedral interstitial sites and wet PS particles. Depending on the initial particle volume fraction and extent of drying, a high yield of dimpled particles having different shapes including tetrahedra and cubes were obtained after oil initiated polymerization and dissolution of the host PS particles, as seen under SEM. The effects of TPM to PS particles size ratio, drying time, and other factors in relation to the yield of tetrahedral and cubic dimpled particles will be presented. Finally, fractionation techniques were used to obtain suspensions of uniform polyhedral particles of high purity. [Preview Abstract] |
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