Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D21: Colloids III: Particles at Interfaces and in Confined Geometries |
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Sponsoring Units: DFD Chair: Charles Reichhardt, Los Alamos National Laboratory Room: Baltimore Convention Center 318 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D21.00001: Phase behavior of standing disks in 2D Kun Zhao, Christopher Harrison, Matthew Sullivan, Thomas Mason, David Huse, William Russel, Paul Chaikin We use photolithography to fabricate plate-like colloidal PMMA disks (diameter$\sim $5.3micron, thickness$\sim $0.8micron). Using an electric field normal to the cover slip, we can get a monolayer of disks standing on their edges. The system resembles a 2D set of colloidal rectangles. We study the phase behavior of this system and find that there is K-T transition from isotropic to nematic (quasi-smectic). Between these two phases, we find a regime where tetratic correlations are longer range than nematic. By studying the disclinations and domain walls, we suggest that the tetratic phase is driven by the nearby nematic and exists on a length scale larger than the nematic domain wall spacing but smaller than the interdisclination distance. [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D21.00002: Attraction between charged silica spheres at a water-air interface Penger Tong, Wei Chen, Susheng Tan, Warren T. Ford Charged colloidal particles at aqueous interfaces are found to experience attractive interactions but the origin of such attraction is not well understood. Here we report an experimental study of attractive interactions between micron- sized charged silica spheres at a water-air interface. Atomic force microscopy is used to examine the charge distribution of the silica surface. Digital video microscopy is used to measure the equilibrium pair potential between the interfacial silica spheres over varying salt concentrations in the aqueous phase. It is found that the measured interaction potential U (r) has an energy barrier with height $\sim0.15 ~k_BT$ at large particle separation r. For smaller separations, U(r) has an attractive well of order $0.3 k_BT$. At even smaller separations, the usual Coulomb repulsion dominates. The experimental observation can be explained in terms of a balance between the screened Coulomb repulsion and unscreened dipole interactions. [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D21.00003: Like-charge attraction originated from intrinsic charge inhomogeneity Yi Zhou, Zhoushen Huang, Tai Kai Ng We study attractions between charged colloidal spheres in a solution and at a water-air interface. Both intrinsic inhomogeneous charge distribution and induced charge fluctuations may result in like-charge attraction. The intrinsic inhomogeneous charge distribution comes from the inhomogeneous surfaces of colloidal spheres, characterized by the number of patchy domains, while charge fluctuations occur in a homogenous background of charges. As the number of patchy domains increases, it will change from the intrinsic charge inhomogeneity to the induced charge fluctuation case. This crossover is studied theoretically and the results are compared with the recent experiments on polystyrene and silicon particles. A simple mechanism is presented to explain the enhancement of the attraction between two charged colloidal particles when they are suspended near a wall. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D21.00004: Attraction between charged colloidal spheres at oil-water interface Chuan Zeng, Hugo Bissig, Anthony Dinsmore The behavior of micron-sized, charged-stabilized colloidal spheres confined at oil-water interface was studied using microscopy. Aggregates of colloidal particles were observed, suggesting an attractive capillary force arising from electrostatic stress on the interface. We report measurements of a long-range attraction between carboxyl-modified polystyrene spheres (radius $\sim $ 1 micron) at the interface between 1,1,1-trifluoroheptan-2-ol and water using image analysis and particle tracking. The interaction between two isolated spheres was measured and compared to recent theoretical models. We also measured the interaction of single particles with large clusters as well as the interactions between clusters. We found acceleration due to the capillary attraction and a complex angular dependence owing to the anisotropy of the meniscus around a cluster. We acknowledge support from NASA through the Fluid Physics program (NRA 02-OBPR-03-C). [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D21.00005: The elasticity of nanoparticle networks on liquid droplets Young-Hsyang Chen, Hsuan-Yi Chen, Alex Levine Inspired by recent experiments on the formation of linked nanoparticle networks [A.D. Dinsmore et al Science, {\bf 298}, 1006, (2002)] on the surface of oil-in-water droplets, we study the modification of the droplet surface elasticity due to the formation of a percolating network of linked nanoparticles. What coverage of nanoparticles is required to modify the elasticity of the droplet? Using Brownian dynamics simulations to model the DLCA (diffusion limited cluster aggregation) of nanoparticles on the surface we construct these networks and monitor the appearance of an elastic contribution due to the nanoparticles at a critical nanoparticle area density. This transition is a type of rigidity percolation on a compact surface. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D21.00006: Crystalline Particle Packings on Spheres Yaping Jing, Alex Travesset The problem of packing particles on spheres appears in several soft condensed matter systems such as the building of PMMA cages (Pickering emulsions or colloidosomes), the micropatterning of colloidal particles relevant for photonic crystals or the geometric strucures of Clathrin cages responsible for the vesicular transport of cargo in cells, just to name a few. In this talk we show how the structural and mechanical properties of spherical crystals can be described analytically from continuum elastic models and discuss how the results are extended to describe other geometries as well. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D21.00007: A 3d View of Spherical Crystals and Grain Boundary Scars Mark Bowick, Thomas Einert, Peter Lipowsky, Jorg Schilling, Andreas Bausch We present an experimental system suitable for producing spherical crystals and for observing the distribution of lattice defects (disclinations and dislocations) on a significant fraction (50{\%}) of the sphere. The introduction of fluorescently labeled particles enables us to determine the location and orientation of grain boundary scars. We find that the total number of scars and the number of excess dislocations per scar agree with theoretical predictions and that the geometrical centers of the scars are roughly positioned at the vertices of an icosahedron. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D21.00008: Formation of nanoparticle-coated liquid metal droplets and measurement of their electronic properties. Kan Du, C. Knutson, Rui Hong, M. Tuominen, T. Emrick, T. Russell, A. Dinsmore We form stable droplets of molten metal, investigate their stability, and demonstrate their potential for forming electronic devices. Droplets of liquid Ga, 0.1-100 microns in diameter, were stabilized by surfactants and by insulating, conducting, and semiconducting nanoparticles. We investigate electronic transport through the nanoparticle-coated droplets. Here we use silica, gold and CdSe nanoparticles which spontaneously form a layer on the droplets. A few droplets form junctions between two platinum wires; we apply a bias voltage to the wires and measure the current before and after evaporation of the solvent. Improved understanding of the electrical characteristics may allow inexpensive assembly of a large number of devices with controlled size, symmetry and function. We acknowledge support from the Center for UMass/Industry Research on Polymers (CUMIRP). [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D21.00009: MR Colloid Self-Assembly in Confined Geometries Patrick Doyle, Ramin Haghgooie The characteristic length scales found in microfluidic devices have been shrinking drastically over the past several years. As a result it is becoming increasingly important to study the effects of this tight confinement. We have used the Brownian Dynamics simulation technique to study the self-assembly of magnetorheological (MR) colloids under confinement. To compliment these simulations, we have used particle tracking to study micron-sized colloids assembling in fluidic channels. For quasi-two dimensional systems, we report a seemingly contradictory response of the system to confinement between parallel hard walls. In contrast to previous circular geometries, we see re-entrant melting with respect to changing channel width and not with respect to field strength. As the channel height is increased (in the range of a few particle diameters), we observe oscillations in the mean cluster spacing with respect to gap height. These oscillations and the transition to the large gap scaling regime will be discussed. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D21.00010: Confocal Microscopy of Hard Sphere Crystal Growth Matthew Sullivan, Kun Zhao, Andrew Hollingsworth, P.M. Chaikin, William B. Russel Classical crystal growth is determined by the competition between chemical potential differences and surface tension. We use confocal microscopy to measure three dimensional crystal growth in a density matched suspension of PMMA-PHSA colloidal particles. The crystal is nucleated from a surface template that forces growth of face-centered-cubic crystals. The growing crystal surface is rough on the scale of several particle diameters, but the average growth rate is well described by the classical Wilson-Frenkel growth law. The local growth rate does depend on the roughness of the surface, however, and this growth rate variation provides a measure of interfacial surface tension. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D21.00011: Simulated Crystallite Melting Kinetics in Two Dimensions D. W. Blair, J. R. Savage, A. D. Dinsmore, J. Machta, R. A. Guyer, A. J. Levine We report on results of numerical simulations of the melting of two-dimensional crystallites. Recent experiments in colloidal systems demonstrate that colloidal crystallites undergo a two-stage melting process. Initially large crystallites melt at a constant rate until reaching a critical size at which there is a dramatic increase in the melting rate. Throughout the initial melting stage the crystallite interior maintains bond orientational order; this order is abruptly lost at the critical size. Using Brownian dynamics simulations of particles interacting via a variety of short-range central potentials, we find that in two dimensions small crystallites generically melt in two-stages characterized by a sudden increase in melting rate that coincides with an abrupt loss of bond orientational order. The critical size, particle number ${\cal O}(20)$, is in agreement with experimental data and appears insensitive to details of the short-range interparticle potential. We discuss a possible mechanism for this change in melting dynamics at a critical crystallite size. This work is supported in part by NSF (DMR-0242402) and NASA (NAG8-1659). [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D21.00012: Influence of hydrodynamic coupling on pair-diffusion in a quasi-one-dimensional colloid system Binhua Lin, Xinliang Xu, Stuart Rice, Haim Diamant The effect of hydrodynamic interaction on the separation dependence of the center of mass and relative pair diffusion coefficients of colloid particles in a quasi-one-dimensional system, including the influence of proximate walls, has been calculated using the method of reflections. There is excellent agreement between the theoretical predictions and the experimental data. We show that the separation dependence of the relative pair diffusion coefficient has oscillatory structure on the scale length of the correlation length in the system, and we directly relate that oscillatory structure to the pair correlation function of the system. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D21.00013: Short-time dynamics of a Brownian particle Branimir Lukic, Sylvia Jeney, Zeljko Sviben, Christian Tischer, Ernst-Ludwig Florin, Andrzej J. Kulik, Laszlo Forro We record the thermal position fluctuations of a $single$ micron- sized sphere immersed in a fluid by optical trapping interferometry with nanometer spatial and microsecond temporal resolution. On the shortest time scales investigated, the sphere's inertia has a small, but measurable, effect. We find, in accord with the theory of Brownian motion including hydrodynamic memory effects, that the transition from ballistic to diffusive motion is delayed to significantly longer times than predicted by the standard Langevin equation. This delay is a consequence of the inertia of the fluid. When the particle is confined by a harmonic potential with a depth on the order of $k_{B}T$, we find that these inertial effects determine the particle's motion at the similar time scale as the potential. Surprisingly, we don't observe the free diffusive behavior in such confined system. [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D21.00014: Transport of nanoparticles in a temperature gradient Shawn Putnam, David Cahill Thermodiffusion, mass transport in a temperature gradient, is commonly characterized by either the thermodiffusion coefficient $D_T$ or the Soret coefficient $S_T$; e.g., at low particle concentration $c$, the particle flux of a colloidal suspension subjected to a temperature gradient $\nabla T$ is $\textbf{J}=- cD_T\nabla T-D_c\nabla c$, where $D_c$ is the diffusion coefficient and the Soret coefficient is $S_T = D_T/D_c$. We present our measured $D_T$ data for aqueous suspensions of charged polystyrene spheres, alumina nanoparticles, and globular proteins of lysozyme. Special emphasis is given to our published work on charged polystyrene spheres with different surface functionalities. For example, in solutions with large concentrations of monovalent salts, $\agt$ 100 mM, $D_T$ for 26 nm spheres with carboxyl functionality can be varied within the range $-0.9 \times 10^{- 7}$ cm$^{2}$ s$^{-1}$ K$^{-1} < D_T < 1.5 \times 10^{-7}$ cm$^{2}$ s$^{-1}$ K$^{-1}$ by changing the ionic species in solution; in this case $D_T$ is the product of the electrophoretic mobility $\mu_E$ and the Seebeck coefficient of the electrolyte $S_e=(Q^*_{\mathrm{C}}-Q^*_{\mathrm{A}})/2 e T$, $D_T = -S_e\ \mu_E$, where $Q^*_{\mathrm{C}}$ and $Q^*_{\mathrm{A}}$ are the single ion heats of transport of the cationic and anionic species respectively. On the contrary, in low ionic strength solutions of LiCl, $\alt$ 5 mM, $D_T$ for the 26nm carboxyl spheres is negative, independent of particle concentration, and independent of the Debye length; $D_T = -0.73\pm0.05 \times 10^{-7}$ cm$^{2} $ s$^{-1}$ K$^{-1}$. The temperature dependence of $D_T$ is also discussed with results from our current work with polystyrene spheres, alumina nanoparticles, and protein solutions of lysozyme. [Preview Abstract] |
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