Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D14: Colloids II: Structure and Rheology |
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Sponsoring Units: DFD Chair: David Pine, New York University Room: 315 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D14.00001: Lock and Key Colloids Stefano Sacanna, William Irvine, Paul Chaikin, David Pine We demonstrate a recognition mechanism between microscopic (colloidal) particles based on a simple ``lock-and-key" principle that relies only on the complementary morphology of the particles involved. The system we developed consists of charge-stabilized spherical silica colloids (keys) and specially designed polymeric particles with spherical cavities (locks). The assembly of locks with keys is driven by depletion interactions between the particles and an uncharged water soluble polymer (poly-ethylene oxide). We show that by balancing electrostatic repulsion and depletion attraction, we induce a selective and reversible lock-and-key self-assembly. Moreover, we can design the lock and key single units to have separate functionalizable chemistries, such that the resulting composite particle (lock+key) will exhibit anisotropic surface properties. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D14.00002: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 2:54PM - 3:06PM |
D14.00003: Pairwise Additivity in Colloidal Electrostatics Jason W. Merrill, Sunil K. Sainis, Eric R. Dufresne We present a method for measuring electrostatic and hydrodynamic interactions between colloidal particles based on observations of short-time trajectories. We use this method to explore whether forces between colloidal particles can be considered pairwise additive by comparing the force between a pair of particles to forces between groups of several particles. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D14.00004: Structure and dynamics of suspensions of nanoparticles in nematic liquid crystals Brian Gettelfinger, Gary Koenig, Jose Moreno-Razo, Juan Hernadez-Ortiz, Nicholas Abbott, Juan de Pablo A hierarchical modeling approach has been adopted to examine the structure and dynamics of nanoparticles suspended in liquid crystals. A molecular model is used to predict the defects that arise in nanoparticle assemblies, as well as their response to applied flow fields. The model is solved by resorting to a radial basis function based technique. The validity of the model and our numerical results are established by direct comparison to results from molecular dynamics simulations of nanoparticles in nematic and isotropic liquid crystals. Results for particle diffusion and aggregation at equilibrium and in flowing systems are then used to interpret our experimental data for a variety of systems. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D14.00005: Monodisperse polymethyl methacrylate (PMMA) spheres in organic media: synthesis update Andrew Hollingsworth, Mark Elsesser, William Irvine, David Pine, Paul Chaikin Since the publication of Antl, et al. [Colloids and Surfaces 17 (1986) 67--78] more than 20 years ago, several research groups in the soft condensed matter area have attempted the dispersion polymerization of sterically-stabilized PMMA particles. Most have found that success of this particular synthesis depends critically on the quality of the comb-graft stabilizer, poly(12-hydroxystearic acid)-g-PMMA. More recent work has extended the particle synthesis to include the incorporation of covalently attached, fluorescent dyes in the particle interior. Our goal has been to reproduce some of these results-- a challenging task-- and to improve the process, leading to a reliable method for preparing core-dyed PMMA particles. We will report on several important findings related to this research goal, and demonstrate that our particles can be used to make colloidal clusters via the recently published emulsion encapsulation and shrinkage technique [Science 301, 483--487 (2003)]. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D14.00006: A model nanocolloidal rod system to explore structural transitions in networks and bundles Georgina Wilkins, Patrick Spicer, Michael Solomon We introduce a model system consisting of self-assembled polyamide anisotropic colloids suspended in aqueous surfactant solutions. The colloidal particles are formed by precipitation from an amorphous polyamide powder that is dispersed with mechanical agitation in an aqueous surfactant phase at temperatures from 59 to 100$^\circ$C. The aspect ratio increases monotonically with temperature: at $\textrm{T} = 59^\circ \textrm{C}$, short rods with aspect ratio r $= 8 \pm1$ form. At $\textrm{T} = 100^\circ \textrm{C}$, rope like structures with r $= 306 \pm14$ form. By confocal laser scanning microscopy (CLSM) and dynamic light scattering (DLS) as volume fraction is increased we show a structural transition from dilute rod behaviour with diffusive dynamics to a homogeneous network structure with increasingly slow dynamics. Furthermore, increasing the aspect ratio of rods induces the same structural transition from dilute rod behaviour to a network structure. Finally, we vary the interaction potential between the rods by a polymer induced depletion interaction and observe an unexpected quiescent network to bundle transition. The bundles are several rod diameters wide and 1 - 2 rod lengths long. The rods appear to be ordered nematically within each bundle. The bundling transition leads to an order of magnitude decrease in the storage modulus of the suspensions. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D14.00007: Polymer grafted particles: Architectural effects on the dynamics Panayiotis Voudouris, Jihoon Choi, Hong Dong, George Fytas, Michael Bockstaller, Kris Matyjaszewski We present a combined static and dynamic light scattering study of two polystyrene PS{\&}SiO$_{2}$ particle solution systems in which tuning of the grafting density and molecular weight of the surface bound PS afford intermediate (0.5nm$^{-2})$ and concentrated (0.84nm$^{-2})$ brush densities. The different packing environment of PS chains give rise to distinctively different rich dynamic response above a threshold volume fraction that yields insight into the role of polymer grafts on the structure formation of hairy particles. This work is the first report on the missing dynamics of hybrid core-shell nanoparticles with distinct behavior intermediate between ultra soft multiarm star polymers and hard sphere colloids. With increasing grafting density of PS ligands the dynamic properties approach those of hard sphere systems while retaining some of the polymer-specific dynamic characteristics. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D14.00008: Structure of Quasi-One Dimensional Ribbon Colloid Suspensions Binhua Lin, Stuart A. Rice, Thomas Stratton, Bianxiao Cui We report the results of an experimental study of a colloid fluid confined to a quasi-one dimensional (q1D) ribbon channel. Our findings confirm the principal predictions of previous theoretical studies of such systems. These are (1) that the density distribution of the liquid transverse to the ribbon channel exhibits stratification and (2) that even at the highest density the order along the strata, as measured by the longitudinal pair correlation function, is characteristic of a liquid. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D14.00009: Dynamics of Internal Stresses and Scaling of Strain Recovery in Aging Colloidal Gels Ajay Singh Negi, Chinedum Osuji On cessation of flow, dilute suspensions of carbon black particles undergo rapid gelation and display instantaneous residual or internal stresses which relax slowly with time. We monitor the evolution of these stresses (under zero strain) and find a weak power law decay, $\sigma _{i}\sim $t$_{w}^{\alpha }$ over 5 decades of time where $\alpha \approx $0.1. The system exhibits aging, with the elastic modulus scaling as a weak power law of elapsed time, G'$\sim $t$^{\beta }$, with $\beta \approx \alpha $. Imposition of zero stress conditions after waiting time t$_{w}$, at internal stress $\sigma _{i}$(t$_{w})$, results in strain recovery as the system relaxes without the zero strain constraint. Older systems exhibit less recovery than younger ones. Remarkably, strain recoveries at different t$_{w}$ can be shifted to construct a single master curve in which the magnitude of the recovery is shifted vertically according to $\sigma _{i}$(t$_{w})^{-1}$ and horizontally simply with elapsed time. The scaling of the strain recovery with internal stress suggests that the internal stress state is characteristic of the age of the system and of the manner in which the system will continue to evolve. This result has important implications for our understanding of glassy behavior in soft materials. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D14.00010: Synthesis and Self-assembly of Janus and Patchy Particles by Lift-up Microcontact Printing Shan Jiang, Steve Granick Janus and patchy particles were synthesized by a simple and novel lift-up microcontact printing method. The geometry of the particles is revealed by both optical fluorescence microscopy and scanning electron microscopy. It is demonstrated that the Janus balance (geometry) of the particles can be easily fine tuned. Interesting and unique cluster structures were self-assembled from particles synthesized by this method. The method allows particles not only of divalent but also of trivalent geometry to be formed in large quantity. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D14.00011: Thermal Properties of Particulate Suspensions Rebecca Christianson, Jessica Townsend It has been known since the 1800's that addition of solid phase particles to a liquid can improve the thermal conductivity of the liquid. However, the instability of such suspensions made them impractical for cooling applications. With the advent of affordable technology for synthesizing nanometer scale particles, it became possible for stable suspensions with improved thermal properties to be created. Initial investigations of nanoparticle suspension coolants (termed nanofluids) seemed to indicate an anomalous enhancement of the thermal conductivity above that predicted by conventional theories. However, subsequent experimental work showed issues with the reproducibility of these early results, which has been attributed by some sources to aggregation within the suspensions. I will present work from our group studying the properties and practical application of stable nanoparticle suspensions, as well as our initial findings on the effects of aggregation on the measured thermal properties of particulate suspensions. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D14.00012: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:54PM - 5:06PM |
D14.00013: Adiabatic Intramolecular Movements for Water Systems in Ab initio simulations Luana Pedroza, Antonio J.R. da Silva The detailed description of hydrogen bonds in water is essential to understand the great variety of processes that occur in this system. Clearly the most appropriate way to do that description would be to treat all the degrees of freedom quantum mechanically. Another possibility is to treat the nuclei classically and the electrons quantum mechanically. A very commom approximation in these simulations is to consider each water molecule as a rigid body, which clearly imposes a limitation on the real description of the molecules and their interactions. We here present an effective treatment of the intramolecular degrees of freedom of water, where these modes are decoupled from the intermolecular one, adiabatically allowing these coordinates to be positioned at their local minimum of the PES. This decoupling is performed combining an AIMC simulation using the rigid bodies approximation with an intramolecular optimization. As an application of our methodology we have studied small water clusters. We show that even in the case of the water dimer the sampling of phase space is significantly modified when intramolecular optimization is included (J. Chem. Phys., 128, 104311 (2008)). As a result, there are clear changes in features such as the dipole moment and structural properties. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D14.00014: New look of fractional exclusion statistics Dragos-Victor Anghel I discuss the concept of fractional exclusion statistics and I show that it leads to inconsistencies in the calculation of the particle distribution that maximizes the partition function. These inconsistencies appear when mutual exclusion statistics is manifested between different subspecies of particles in the system. In order to eliminate these inconsistencies, I introduce new mutual exclusion statistics parameters, which are proportional to the dimension of the Hilbert sub-spaces on which they act. These new definitions lead to properly defined particle distributions and thermodynamic properties. I also show that fractional exclusion statistics is manifested in general interacting systems and I calculate the exclusion statistics parameters. Most importantly, I prove that indeed, the mutual exclusion statistics parameters are proportional to the dimension of the Hilbert space on which they act. \\[4pt] {\bf Related publications:}\\[0pt] [1] D. V. Anghel, J. Phys. A: Math. Theor. {\bf 40}, F1013 (2007).\\[0pt] [2] D. V. Anghel, Phys. Lett. A {\bf 372}, 5745 (2008).\\[0pt] [3] D. V. Anghel, arXiv:0804.1474. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D14.00015: Capillary forces on nanowires Jun Ma, Shengfeng Cheng, Jay Wallace, Patricia McGuiggan, Mark Robbins The capillary forces on nanowires have been measured by attaching them to the cantilever of an Atomic Force Microscope (AFM). The nanowires are immersed and retracted from a liquid/air interface. The entire capillary force curve is compared to continuum theory and molecular simulations. Nanowires with different diameters and chemistry and various liquids are investigated. Surface tension, contact angle hysteresis, and dynamical contact angles can be extracted under reasonable assumptions about how the contact line moves along the nanowires. [Preview Abstract] |
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