Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A29: Focus Session: Colloids I |
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Sponsoring Units: DFD Chair: David Weitz, Harvard University Room: Colorado Convention Center 303 |
Monday, March 5, 2007 8:00AM - 8:36AM |
A29.00001: How confinement modifies the colloidal glass transition Invited Speaker: We study concentrated colloidal suspensions, a model system which has a glass transition. These are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. We view the motion of these colloidal particles in three dimensions by using an optical confocal microscope. This allows us to directly study the microscopic behavior responsible for the macroscopic viscosity divergence of glasses. In particular, we study how confinement changes the particle dynamics. We confine a colloidal suspension between two parallel walls, and find that in thin sample chambers the particle motion is greatly slowed. This suggests that confinement causes the onset of the glass transition to happen ``sooner,'' at particle concentrations which are not normally glassy. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A29.00002: Periodic Stresses and Shear Thickening in an Attractive Colloidal Gel Chinedum Osuji, David Weitz We report on the observation of periodic stresses in a colloidal gel at rest and under minute shear deformation. Dilute suspensions of carbon black colloidal particles in hydrocarbon oil with an attractive Van der Waals interaction are found to shear thicken in two distinct regimes. The first, low shear rate regime is ascribed to network elongation and the high shear regime to hydrodynamic clustering, akin to that observed in concentrated hard sphere systems. Due to the attractive interaction between particles, the shear thickened state persists long after cessation of flow and in the high shear rate regime gives rise to high modulus, compacted networks. These gels display residual stresses and exhibit a peculiar time dependent aging in which the normal force exerted by the stationary gel as well as the shear modulus display low frequency long lived oscillations. Simple tensile deformation of the gel results in comparatively higher frequency periodic normal forces. We propose a simple mechanism to account for the observed data. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A29.00003: Plastic restructuring in compressed colloidal glasses Daniel Blair, David Weitz We report the observation of localized plastic restructuring in compressed colloidal glasses. By placing a expanding bulk hydrogel in contact with the colloidal glass, we can drive the system above the glass transition volume fraction, $\phi=0.58 \to 0.64$. We measure the local strain tensor using three dimensional confocal microscopy and particle tracking techniques. The increase in volume fraction exhibits a smooth exponential increase. However, local irreversible transformations exhibit strong fluctuations that are correlated to the local free volume. We will elucidate the mechanisms for these localized relaxation events, and make comparisons to recent models of sheared amorphous solids. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A29.00004: Ideal Glass Transitions, Barrier Hopping and Dynamic Heterogeniety in Suspensions of Nonspherical Colloids G. Yatsenko, K.S. Schweizer The slow translational dynamics and nongaussian fluctuation effects of glassy isotropic fluids composed of nonspherical objects is studied based on a nonlinear stochastic Langevin equation of motion that includes activated barrier hopping. Suspensions of homonuclear diatomic and linear triatomic shaped colloids of variable bond length have been studied. The ideal glass transition boundary (crossover to activated dynamics) is predicted to be a nonmonotonic function of particle aspect ratio and surprisingly occurs at a nearly unique value of the dimensionless compressibility. The magnitude and volume fraction dependences of the entropic barrier, localization length and shear moduli for different aspect ratio systems collapse well onto master curves based on a reduced volume fraction variable that quantifies the distance from the ideal glass transition. Calculations for long polyatomic rods have also been performed. The ideal glass boundary decreases with aspect ratio slower than the nematic phase transition boundary. Solution of the nonlinear Langevin equation via Brownian trajectory simulation have also been performed. Results for the mean square displacement, decoupling of relaxation and diffusion, nongaussian parameter and other measures of dynamic heterogeneity have been determined for different colloidal shapes. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A29.00005: Experimental Measurement of Freezing Kinetics in Two-Dimensional Colloidal Crystals J.R. Savage, A.D. Dinsmore We study the freezing kinetics of two-dimensional colloidal crystals formed by a short-range attractive potential. We use aqueous suspensions of micron-sized latex spheres mixed with surfactant micelles, which create a depletion attraction among the spheres. The depletion attraction between the spheres and the coverslip enables us to create a two-dimensional system. Upon uniformly heating or cooling the sample, the micelles grow or shrink and the depletion attraction changes in magnitude. Optical microscopy is used to track the motions of thousands of colloidal spheres in the process of freezing or melting . By varying the density (area fractions of 17-34{\%}) and the amount of supercooling, we can measure the dynamics of nucleation and growth of crystallites. A two-stage nucleation process can be seen in samples with density of 30{\%} in which a meta-stable liquid droplet is first formed; then the crystallite is nucleated from within. At higher and lower densities the crystals nucleate in the typical fashion with large 6-fold orientational symmetry at small cluster size. We will present results on the evolution of the orientational order of crystallites and their degree of crystallinity as a function of both time and cluster size. We will also compare and contrast these density dependent freezing results to earlier work done on the melting process. This work is supported by the NSF-DMR 0605839. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A29.00006: Particle dynamics near the re-entrant glass transition Andrzej Latka, Ahmed Alsayed, Yilong Han, Arjun Yodh, Piotr Habdas Colloidal suspensions are a model system for studying the glass transition. At the volume fraction $\phi $g $\approx $ 0.58 a hard sphere colloidal glass is formed. The formation of a hard sphere glass is attributed to the caging effect, in which the particles form cages around each other that restrict their movement. Introducing an attractive depletion force between the particles causes the hard sphere glass to melt and the system becomes a liquid. Through further increase of the attractive force an attractive glass is formed. Our system is a suspension of nearly hard-sphere colloidal particles and nonadsorbing linear polymer which induces a depletion attraction between the particles. Using microscopy techniques, we study how the dynamics of the particles change as the attractive potential is increased and attractive glass is approached. In particular, we examine the mean square displacement and frequency of particle jumps over a range of attraction strengths. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A29.00007: Translation-rotation coupling in dense colloidal suspensions Minsu Kim, Stephen Anthony, Steve Granick Single-particle tracking has been used to contrast translational and rotational diffusion in colloidal suspensions. Not enough is known from prior study about the rotation of colloids, owing perhaps to the paucity of suitable measurements techniques, but this is now remedied by using Modulated Optical Nanoparticles(MOONs), which are fabricated by capping one hemisphere with a thin layer of reflective metal. Density of the suspensions is varied and both translational and rotational mean squared displacement are quantified. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A29.00008: Single liposome tracking in dense suspensions of stabilized liposomes Yan Yu, Stephen Anthony, Liangfang Zhang, Angelo Cacciuto, Steve Granick Methods developed to stabilize phospholipid vesicles against fusion, up to volume fraction around 80{\%}, enable one to perform single-particle tracking on these soft, flexible, hollow objects. Stabilization is accomplished by studding the outer leaflet with charged nm-sized particles. Image analysis of time trajectories, obtained using epifluorescence imaging, was performed at sub-pixel resolution. This talk will emphasize aspects of curiously heterogeneous dynamics and also quantification of ``cage'' size in this system. Taken together, this system of charged, polydisperse, flexible objects displays rich dynamics that contrasts acutely with known behavior for hard-sphere dense particle systems. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A29.00009: Yield stress of stearically stabilized colloids Suresh Ahuja, Terry Bluhm The bulk property, yield stress has been modeled by Larson in the past for spherical colloidal particles with dependence on volume fraction of solids particle diameter and interaction potential (sum of van der Waals potential and electrostatic potential. In our organic pigment dispersions polymer stabilized followed Herschel-Bulkley equation with yield stress which was non-linearly dependent on pigment surface area measured by BET. Stability of dispersions changed with time in terms of particle size and yield stress as well as on the type of deformation, shear applied to the dispersion. The results of yield stress are compared with models in terms of interaction potential, particle size and zeta potential.. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A29.00010: Janus Colloids Assemble into Cluster Shapes Liang Hong, Angelo Cacciuto, Erik Luijten, Steve Granick We explore the assembly of two types of micron-sized, spherical Janus particles: those with opposite electric charge on both hemispheres (``bipolar'') and those hydrophobic on one hemisphere and hydrophilic on the other (``amphiphilic''). Bipolar particles form clusters, not strings, as the particle diameter exceeds the electrostatic screening length. The cluster shapes are analyzed by combined epifluorescence microscopy and Monte Carlo computer simulations with excellent agreement, indicating that the particles assemble in aqueous suspension to form equilibrated aggregates. The simulations show that charge asymmetry of individual bipolar particles is preserved in the clusters. The assembly of amphiphilic particles presents analogies to the self-assembly of molecular surfactants, forming monolayers at the air-water interface and micelles in the aqueous suspension. By tuning the salt concentration, different phases of micelle can be imaged in real space. Computer simulations confirm the geometries of these micelles and reveal possible formation mechanisms. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A29.00011: Wetting layer dynamics in colloid polymer mixtures by evanescent wave dynamic light scattering Benoit Loppinet, Pangiotis Voudouris, Giorgos Petekidis Evanescent wave obtained at the total internal reflection can be used as the incident beam of a dynamic light scattering experiment where its short penetration depth allow to selectively probe fluctuations close to a hard wall. Colloid concentration fluctuation in gas-liquid phase separated colloid-polymer mixtures obtained with PMMA hard spheres (R=120nm) and polystyrene polymer in index match cis/trans decalin were investigated in the vicinity of a vertical hard wall with in particular the dense colloidal layer wetting the hard wall in the top (gas) phase. There, the q-dependent collective dynamics reveal a liquid like behaviour similar to the one observed in the bottom phase dynamics, both marginally slower than the dynamics measured in the bottom (liquid) phase bulk and very different from the dilute like dynamics observed in the bulk top phase. Results are discussed in terms of hydrodynamic interactions. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A29.00012: Gravitational collapse of depletion gels Juan Jose Lietor-Santos, Alberto Fernandez-Nieves, Chanjoong Kim, Peter J. Lu, David A. Weitz We study how colloidal gels collapse under the presence of a gravitational stress. We do so macroscopically, monitoring the time dependence of the creaming or sedimentation front, and microscopically, using confocal microscopy. Our system consists of fluorescently labeled spheres that are index matched to the surrounding solvent. Temperature allows fine control of the density mismatch, further enabling fine tuning of the gravitational stress. Addition of non-adsorbing polymer induces an attraction whose range and strength can also be tuned. We will present results pertaining macroscopic studies for different particle volume fractions and interaction energies and preliminary microscopic results aiming to locally describe the structure collapse. [Preview Abstract] |
Monday, March 5, 2007 10:48AM - 11:00AM |
A29.00013: Depletion Interaction: Effect of Depletant's Non-ideality Dzina Kleshchanok, Remco Tuinier, Peter R. Lang Depletion interaction is one of the central issues of colloidal stability; it arises between colloidal bodies suspended in a solution of non-adsorbing polymers, micelles, spheres, rods etc. Recently depletion of ideal non-ionic monodisperse polymers, monodisperse hard spheres and rods was extensively studied using various theoretical methods [1]. These cases enable a detailed theoretical analysis and serve as a model for other more complicated systems. However, in many experimental cases the depletants deviate from the requirements of the theories, for example, one has to deal with polydisperse, charged or (partly) adsorbing depletants. Another problem can arise when it is not possible to use the Derjaguin approximation to compute the depletion potential (e.g. the size of depletant is comparable with the size of colloids). All these effects can lead to the crucial deviations from the idealizing theories. We experimentally studied depletion interaction induced by non-ideal depletants between a charged colloidal sphere and a charged solid wall using Total Internal Reflection Microscopy (TIRM). Here we discuss the influence on the depletion potential due to the polymer size polydispersity (dextran), polymer's adsorption (polyethylene oxide (PEO)) and the colloid/ depletant's size ratio (fd-viruses). .1. Tuinier, R. et al., \textit{Adv. Colloid Interface Sci }\textbf{2003,} 103, 1. [Preview Abstract] |
Monday, March 5, 2007 11:00AM - 11:12AM |
A29.00014: Direct observation of dynamical heterogeneity near the colloidal gel transition Maria Kilfoil, Yongxiang Gao We use confocal microscopy to probe the microscopic dynamics near the colloidal gel transition where the dynamics shows spatial heterogeneity. We are able to separate fast and slow particles independently from self part of van Hove density-density correlation function. The distinct part of van Hove correlation function shows clearly a signature of dynamical heterogeneity and the behavior is dominated by the fast particles. We further observe intermittent dynamics for these particles: the motion is not continuous. This provides the first microscopic picture of intermittent dynamics in colloidal gels. [Preview Abstract] |
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