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At least 1,405 records · Page 78Linked to original sources
PubMed2006

Evolution of the fractal-like aggregate system in colloids.

The evolution of a colloidal fractal-like aggregate system is studied theoretically with account for the reduction of the single-particle concentration in the space between these growing aggregates. Two different mechanisms of the initial aggregate formation are considered. They are heterogeneous aggregation, when the initial aggregates appear on foreign nuclei (the biggest particles, pollution, etc.), and homogeneous nucleation, when the initial clusters appear due to a random meeting and combination of single Brownian particles. The principles of colloid supersaturation decrease and the aggregate distribution in size are obtained in both cases as well as the internal structure of the clusters. Unlike the usual condensation-type phase transition, the system of discrete clusters is formed at the final stage of the aggregation process. The internal particle distribution in these clusters does not obey the universal scaling power law, which differs from the model case of separate cluster growth in an infinite colloidal medium with permanent concentration of free particles.

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PubMed2006

Formation and evolution of sediment layers in an aggregating colloidal suspension.

The coupled aggregation and sedimentation of colloidal particles with short-range attractions are investigated. Nonadsorbing polymer is used to induce depletion interactions between the hard-sphere particles. Gravitational forces, caused by a density mismatch between the particles and the suspending fluid, result in the sedimentation of particles and aggregates, as well as the compaction and rearrangement of the final sediment layer. At low polymer concentrations CP, or low initial volume fractions phio, clusters formed during the period of fast sedimentation are small, and the structure of the final sediment is dense. Conversely, at high CP, or high phio, large clusters form during sedimentation, and the resulting sediment structure is significantly less dense, with large void volumes. The size of the presediment aggregates depends on CP and phio with a functional form that resembles other thermally activated barrier hopping processes in colloidal systems, such as the delayed sedimentation of colloidal gels. Finally, when the particles are weakly attractive, gravitational stresses are found to induce compaction of the sediment over long periods of time. However, sediments composed of particles that are strongly attractive resist rearrangements and compaction, even when the sediment layers have a relatively large amount of free volume.

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PubMed2006

Hydrodynamic interactions and Brownian forces in colloidal suspensions: coarse-graining over time and length scales.

We describe in detail how to implement a coarse-grained hybrid molecular dynamics and stochastic rotation dynamics simulation technique that captures the combined effects of Brownian and hydrodynamic forces in colloidal suspensions. The importance of carefully tuning the simulation parameters to correctly resolve the multiple time and length scales of this problem is emphasized. We systematically analyze how our coarse-graining scheme resolves dimensionless hydrodynamic numbers such as the Reynolds number Re, which indicates the importance of inertial effects, the Schmidt number Sc, which indicates whether momentum transport is liquidlike or gaslike, the Mach number, which measures compressibility effects, the Knudsen number, which describes the importance of noncontinuum molecular effects, and the Peclet number, which describes the relative effects of convective and diffusive transport. With these dimensionless numbers in the correct regime the many Brownian and hydrodynamic time scales can be telescoped together to maximize computational efficiency while still correctly resolving the physically relevant processes. We also show how to control a number of numerical artifacts, such as finite-size effects and solvent-induced attractive depletion interactions. When all these considerations are properly taken into account, the measured colloidal velocity autocorrelation functions and related self-diffusion and friction coefficients compare quantitatively with theoretical calculations. By contrast, these calculations demonstrate that, notwithstanding its seductive simplicity, the basic Langevin equation does a remarkably poor job of capturing the decay rate of the velocity autocorrelation function in the colloidal regime, strongly underestimating it at short times and strongly overestimating it at long times. Finally, we discuss in detail how to map the parameters of our method onto physical systems and from this extract more general lessons-keeping in mind that there is no such thing as a free lunch-that may be relevant for other coarse-graining schemes such as lattice Boltzmann or dissipative particle dynamics.

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PubMed2002

Colloidal dynamics on disordered substrates.

Using Langevin simulations, we examine driven colloids interacting with quenched disorder. For weak substrates the colloids form an ordered state and depin elastically. For increasing substrate strength, we find a sharp crossover to inhomogeneous depinning and a substantial increase in the depinning force, analogous to the peak effect in superconductors. The velocity versus driving force curve shows criticality at depinning, with a change in scaling exponent occurring at the order to disorder crossover. Upon application of a sudden pulse of driving force, pronounced transients appear in the disordered regime which are due to the formation of long-lived colloidal flow channels.

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PubMed2002

Re-entrant glass transition in a colloid-polymer mixture with depletion attractions.

Performing light scattering experiments we show that introducing short-ranged attraction to a colloid suspension of nearly hard spheres by addition of a free polymer produces new glass-transition phenomena. We observe a dramatic acceleration of the density fluctuations amounting to the melting of a colloidal glass. Upon increasing the strength of the attractions the system freezes into another nonergodic state sharing some qualitative features with gel states occurring at lower colloid packing fractions. This re-entrant glass transition is in qualitative agreement with recent theoretical predictions.

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PubMed2002

Entropic wetting and many-body induced layering in a model colloid-polymer mixture.

We develop an efficient simulation scheme to study a model suspension of equally sized colloidal hard spheres and nonadsorbing ideal polymer coils, both in bulk and adsorbed against a planar hard wall. The many-body character of the polymer-mediated effective interactions between the colloids yields a bulk phase diagram and adsorption phenomena that differ substantially from those found for pairwise simple fluids; e.g., we find an anomalously large bulk liquid regime and, far from the bulk triple point, three layering transitions in the partial wetting regime prior to a transition to complete wetting by colloidal liquid.

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PubMed2003

Elastic properties of 2D colloidal crystals from video microscopy.

Elastic constants of two-dimensional (2D) colloidal crystals are determined by measuring strain fluctuations induced by Brownian motion of particles. Paramagnetic colloids confined to an air-water interface of a pendant drop are crystallized under the action of a magnetic field, which is applied perpendicular to the 2D layer. Using video microscopy and digital image processing we measure fluctuations of the microscopic strain obtained from random displacements of the colloidal particles from their mean (reference) positions. From these we calculate system-size dependent elastic constants, which are extrapolated using finite-size scaling to obtain their values in the thermodynamic limit. The data are found to agree rather well with zero-temperature calculations.

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PubMed2003

Confinement-induced colloidal attractions in equilibrium.

The Poisson-Boltzmann theory for colloidal electrostatic interactions predicts that charged colloidal spheres dispersed in water should repel each other, even when confined by charged surfaces. Direct measurements on highly charged polystyrene spheres, however, reveal strong, long-ranged confinement-induced attractions that have yet to be explained. We demonstrate that anomalous attractions also characterize the equilibrium pair potential for more weakly charged colloidal silica spheres sedimented into a monolayer above a glass surface. This observation substantially expands the range of conditions for which mean-field theory incorrectly predicts the sign of macroions' interactions, and provides new insights into how confinement induces long-ranged like-charge attractions.

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PubMed2004

Nature of an electric-field-induced colloidal martensitic transition.

We study the properties of a solid-solid close-packed to body-centered tetragonal transition in a colloidal suspension via fluorescence confocal laser scanning microscopy, in three dimensions and in real space. This structural transformation is driven by a subtle competition between gravitational and electric dipolar field energy, the latter being systematically varied via an external electric field. The transition threshold depends on the local depth in the colloidal sediment. Structures with order intermediate between close-packed and body-centered tetragonal were observed, with these intermediate structures also being stable and long lived. This is essentially a colloidal analogue of an "atomic-level" interfacial structure. We find qualitative agreement with theory (based purely on energetics). Quantitative differences can be attributed to the importance of entropic effects.

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PubMed2004

Local melting and drag for a particle driven through a colloidal crystal.

We numerically investigate a colloidal particle driven through a colloidal crystal as a function of temperature. When the charge of the driven particle is larger or comparable to that of the colloids comprising the crystal, a local melting can occur, characterized by defect generation in the lattice surrounding the driven particle. The generation of the defects is accompanied by an increase in the drag force on the driven particle, as well as large noise fluctuations. We discuss the similarities of these results to the peak effect phenomena observed for vortices in superconductors.

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PubMed2004

Configurational temperature of charge-stabilized colloidal monolayers.

Recent theoretical advances show that the temperature of a system in equilibrium can be measured from static snapshots of its constituents' instantaneous configurations, without regard to their dynamics. We report the first measurements of the configurational temperature in an experimental system. In particular, we introduce a hierarchy of hyperconfigurational temperature definitions, which we use to analyze monolayers of charge-stabilized colloidal spheres. Equality of the hyperconfigurational and bulk thermodynamic temperatures provides previously lacking thermodynamic self-consistency checks for the measured colloidal pair potentials, and thereby casts new light on anomalous like-charge colloidal attractions induced by geometric confinement.

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PubMed2004

Gravity-induced aging in glasses of colloidal hard spheres.

The influence of gravity on the long-time behavior of the mean squared displacement in glasses of polydisperse colloidal hard spheres was studied by means of real-space fluorescent recovery after photobleaching. We present, for the first time, a significant influence of gravity on the mean squared displacements of the particles. In particular, we observe that systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) show anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. We show that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explains the observation that colloidal hard sphere systems which are a glass on Earth rapidly crystallize in space.

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PubMed2004

Stabilization of colloidal suspensions by means of highly charged nanoparticles.

We employ a novel Monte Carlo simulation scheme to elucidate the stabilization of neutral colloidal microspheres by means of highly charged nanoparticles [Proc. Natl. Acad. Sci. U.S.A. 98, 8950 (2001))]]. In accordance with the experimental observations, we find that small nanoparticle concentrations induce an effective repulsion that prevents gelation caused by the intrinsic van der Waals attraction between colloids. Higher nanoparticle concentrations induce an attractive potential which is, however, qualitatively different from the regular depletion attraction. We also show how colloid-nanoparticle size asymmetry and nanoparticle charge can be used to manipulate the effective interactions.

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PubMed2004

Dynamic colloidal stabilization by nanoparticle halos.

We explore the conditions under which colloids can be stabilized by the addition of smaller particles. The largest repulsive barriers between colloids occur when the added particles repel each other with soft interactions, leading to an accumulation near the colloid surfaces. At lower densities these layers of mobile particles (nanoparticle halos) result in stabilization, but when too many are added, the interactions become attractive again. We systematically study these effects--accumulation repulsion, reentrant attraction, and bridging--by accurate integral equation techniques.

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PubMed2005

Elasticity and critical bending moment of model colloidal aggregates.

The bending mechanics of singly bonded colloidal aggregates are measured using laser tweezers. We find that the colloidal bonds are capable of supporting significant torques, providing a direct measurement of the tangential interactions between particles. A critical bending moment marks the limit of linear bending elasticity, past which small-scale rearrangements occur. These mechanical properties underlie the rheology and dynamics of colloidal gels formed by diffusion-limited cluster aggregation, and give critical insight into the contact interactions between Brownian particles.

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PubMed2005

High-order mode-coupling theory for the colloidal glass transition.

A theoretical approach is developed to derive a hierarchy of mode-coupling equations for the dynamics of concentrated colloidal suspensions, which improves the prediction of the colloidal glass transition. Our derivation is based on a matrix formalism for stochastic dynamics and the resulting recursive expressions for irreducible memory functions. The 1st order truncation of the generalized mode-coupling closure recovers mode-coupling theory, whereas its 2nd and 3rd order truncations provide corrections. The predictions of the transition volume fraction and Debye-Waller parameter for the hard-sphere colloidal system improve with the increasing mode-coupling order and compare favorably with experimental measurements.

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PubMed2006

Shape-induced frustration of hexagonal order in polyhedral colloids.

The effect of a nonspherical particle shape and shape polydispersity on the structure of densely packed hard colloidal particles was studied in real space by confocal microscopy. We show that the first layer at the wall of concentrated size-monodisperse but shape-polydisperse polyhedral colloids exhibits significant deviations from a hexagonal lattice. These deviations are identified as bond-orientational fluctuations which lead to percolating "mismatch lines." While the shape-induced geometrical frustration of the hexagonal symmetry suppresses translational order, bond-orientational order is clearly retained, indicating a hexaticlike structure of the polyhedral colloids.

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PubMed2006

Aggregation kinetics in a model colloidal suspension.

We present molecular dynamics simulations of aggregation kinetics in a colloidal suspension modeled as a highly asymmetric binary mixture. Starting from a configuration with largely uncorrelated colloidal particles the system relaxes by coagulation-fragmentation dynamics to a structured state of low-dimensionality clusters with an exponential size distribution. The results show that short-range repulsive interactions alone can give rise to so-called cluster phases. For the present model and probably other, more common colloids, the observed clusters appear to be equilibrium phase fluctuations induced by the entropic intercolloidal attractions.

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