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PubMed2000

Simulating formation of voids in charged colloids by brownian dynamics

Using Brownian dynamics simulations and Sogami's effective pair potential with a long-range attractive term for the colloidal particles, we report that the mechanism of making voids in charged colloids is the long-range attractive interaction between particles. The present results are in good agreement with the experimental observations of stable voids in equilibrium with ordered structures, which may not be explained by repulsive potentials without a long-range attractive interaction such as the Derjaguin-Landau-Vervey-Overbeck potential. Our results are also in a good agreement with Monte Carlo simulation results. The voids have been reported with ordered structure in charged colloids by Brownian dynamics simulations.

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PubMed2000

Slow dynamics of equilibrium density fluctuations in suspensions of colloidal hard spheres near the glass transition

A mean-field theory for the dynamics of equilibrium suspensions of colloidal hard spheres near the glass transition is presented based on the standpoint recently proposed by the present author. It is shown that although the relative magnitude of the density fluctuations to the mean equilibrium density is small even near the glass transition, they are described by a nonlinear stochastic equation which originates from the long-range hydrodynamic interactions between particles. A nonlinear mean-field equation for the particle mean-square displacement is then derived. This equation is used to analyze the recent experimental data for equilibrium colloidal suspensions. Analyses show that no divergence of the alpha- and beta-relaxation times is found in the experimental data, although the dynamic properties of the colloidal liquid exhibit a drastic slowing down in the so-called supercooled region.

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PubMed2001

Surface ordering in a concentrated suspension of colloidal particles investigated by x-ray scattering methods.

The spatial arrangement of colloids near the free surface of a concentrated suspension of colloidal silica in water is investigated by means of x-ray scattering. The weakly charged particles are found to organize in layers along the surface normal direction. The degree of layering decreases with increasing distance from the surface and three layers are identified from the scattering profile. In the lateral direction, the scattering profile indicates a random spatial arrangement of particles at the surface. Based on the findings, a simple structural model for the near surface arrangement of colloidal particles in this system is proposed.

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PubMed2001

Orientational order in gravity dispersed clay colloids: a synchrotron x-ray scattering study of Na fluorohectorite suspensions.

Colloidal suspensions of clay particles in aqueous salt solutions make ideal model systems for the study of interactions between plate-shaped particles, due to the ease in tuning their electrostatic repulsion with the concentration of the salt. Numerous gel and sol structures are possible, including nematic liquid crystalline order, although only qualitative identification of the latter in clay colloids has been available so far. We present synchrotron x-ray diffraction from gravity dispersed solutions of Na fluorohectorite, a synthetic swelling clay, over a large NaCl concentration range. Our use of liquid scattering techniques allows us to identify regions in which particles reorient from horizontal to vertical alignments in strata coexisting at different heights within the sample. We identify two distinct gel regions characterized by differences in orientational anisotropy and domain size. Our results provide direct evidence for nematic order, as well as unique structural information regarding particle morphology and alignment within each of the colloid phases.

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PubMed2002

Adsorption of colloidal particles on a charged surface: cluster Monte Carlo simulations.

We investigate the adsorption of colloidal particles on a charged surface by the cluster Monte Carlo simulation. Under the primitive model of asymmetric electrolytes, the density profiles of colloidal particles at a surface are analyzed as a function of the electrostatic coupling parameter. In the strong coupling regime numerical results show the like-charged adsorption between colloidal particles and a charged surface, which cannot be explained by the Derjaguin-Landau-Verwey-Overbeek theory.

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PubMed2002

Thermodiffusion in magnetic colloids evidenced and studied by forced Rayleigh scattering experiments.

This paper shows how forced Rayleigh scattering can be used as an experimental tool for studying thermodiffusion (Soret effect). The systems investigated are magnetic colloids of different types. A framework including thermodiffusion and dielectrophoresis is described in which the evolutions of temperature and of colloid concentration are clearly distinguished. The framework is then shown to account for experiments on steady-state concentration gratings coupled with transient temperature ones, and the parameters are determined therefrom. Dielectrophoretic forces are found to be negligible. Studying different types of magnetic colloids with various dilution rates shows that the sign of the Soret effect is controlled by the nature of the particle coating made up of electrostatic charges or of surfactant, and that its mechanism is located at the nanoparticle core-solvent interface.

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PubMed2002

Electro-optic response and switchable Bragg diffraction for liquid crystals in colloid-templated materials.

We report optical switching studies on nematic liquid crystal incorporated into structures based on self-assembled colloids. We compare the electro-optic responses of liquid crystal imbibed into colloid-templated polymers, liquid crystal imbibed in the interstitial space of colloid crystals, and conventional polymer-dispersed liquid crystals. We characterize the Bragg diffraction of our templated liquid-crystal/polymer composites as a function of electric field and measure switching times. The response of liquid crystal in connected networks differs qualitatively from that of liquid crystal in isolated cavities.

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PubMed2002

Structural aging of crystals of hard-sphere colloids.

We report a detailed experimental study of the aging of the (initial) random hexagonal close-packed (rhcp) crystals formed in suspensions of hard-sphere colloids near the melting point. By suspending the same colloidal particles in two different mixtures of solvents we are able to tune the strength of the gravitational forces acting on the particles. The crystal structure is deduced from diffraction patterns measured by the light scattering equivalent of powder x-ray crystallography. A spontaneous aging of the structure is observed over long periods of time, consisting of a fraction of pure face-centered cubic (fcc) crystals growing at the expense of the randomly stacked crystallites. The rate of growth of the new crystals is small and consistent with the predictions by Pronk and Frenkel [13]. Gravity is also revealed to affect the crystals and favor fcc order but through a slow gradual rearrangement of the random stacking. An important new observation is that small mechanical perturbations can strongly affect the structure of the colloidal crystals, promoting fcc growth and interfering with the spontaneous aging process. Previous experimental results are also discussed in the light of these new findings.

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PubMed2003

Phase behavior and crystallization kinetics of poly-12-hydroxystearic-coated polymethylmethacrylate colloids.

Polymethylmethacrylate (PMMA) colloids sterically stabilized by a layer of chemically grafted poly-12-hydroxystearic (PHSA) are widely used in experiments as model hard spheres. However, due to the coating, the interaction between particles is slightly soft. Here we report a numerical study of the effect of the PHSA coating on the phase behavior and crystallization kinetics of PMMA colloids based on parameters determined from surface-force measurements on PHSA-PMMA-coated mica surfaces [B. A. de L. Costello and P. F. Luckham, J. Colloid Interface Sci. 156, 72 (1993); B. A. de L. Costello et al., Langmuir 8, 464 (1992)]. We find that the core volume fraction of particles at freezing measured by Pusey and van Megen [Nature 320, 340 (1986)] can only be reproduced by using a thickness of the PHSA layer that is considerably larger than literature values. This may indicate that the particles are in fact slightly charged. Compared to perfect hard spheres, the crystallization rate in these slightly soft particles was found to be increased by about two orders of magnitudes.

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PubMed2003

Is the binary hard-sphere mixture a good reference system for sterically stabilized colloids?

The relevance of the hard-sphere mixture model as a starting point for the study of sterically stabilized colloids is discussed. Two physical situations are distinguished: true molecular solvent-colloid mixtures, and pseudobinary mixtures of two supramolecular objects. For the former, the limitation of the hard-sphere mixture model are recalled. Its potential use as a reference system for perturbation treatments is then analyzed. The accuracy of the latter is tested numerically. This study shows that the hard-sphere mixture is, in general, not a good reference system for sterically stabilized colloids in molecular solvents. For pseudobinary mixtures, the potential of mean force between the bigger solutes induced by the smaller ones is considered. The influence of a very short-range heteroattraction is discussed.

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PubMed2004

Three-body interactions in colloidal systems.

We present a direct measurement of three-body interactions in a colloidal system comprised of three charged colloidal particles. Two of the particles have been confined by means of a scanned laser tweezers to a line-shaped optical trap where they diffused due to thermal fluctuations. Upon the approach of a third particle, attractive three-body interactions have been observed. The results are in qualitative agreement with additionally performed nonlinear Poisson-Boltzmann calculations, which also allow us to investigate the microionic density distributions in the neighborhood of the interacting colloidal particles.

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PubMed2004

Renormalized jellium model for charge-stabilized colloidal suspensions.

We introduce a renormalized jellium model to calculate the equation of state for charged colloidal suspensions. An almost perfect agreement with Monte Carlo simulations is found. Our self-consistent approach naturally allows to define the effective charge of particles at finite colloidal density. Although this quantity may differ significantly from its counterpart obtained from the standard Poisson-Boltzmann cell approach, the osmotic pressures for both models are in good agreement. We argue that by construction, the effective charge obtained using the jellium approximation is more appropriate to the study of colloidal interactions. We also discuss a possibility of a fluid-fluid critical point and show how our equation of state can be used to shed light on the surprising results found in recent sedimentation experiments.

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PubMed2005

Effect of disorder on the optical properties of colloidal crystals.

Colloidal crystals offer a promising route for the formation of three-dimensional photonic crystals. The primary constraint in working with these materials is the disorder present in these self-assembled materials. Sphere vacancies, line dislocations, and random position errors all lead to a degradation in the optical properties. It is important to characterize these effects so as to guide further developments in colloidal crystal optics. Here, we report a systematic and quantitative study of disorder in colloidal crystals with visible diffractive properties. Using optical spectroscopy and digital imaging we have correlated several measures of structural disorder with variations in the transmissive and reflective optical properties. We observe a critical size distribution above which rapid deterioration of the lowest stop band is observed. Below this critical size distribution, we observe excellent optical quality, nearly independent of the size distribution. Single sphere vacancies also increase in crystals formed from more polydisperse spheres. The primary effect of this type of defect is to increase the broadband diffuse scattering.

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PubMed2005

Soft colloids driven and sheared by traveling wave fields.

We study the dynamics of soft colloids interacting via a Gaussian pair potential in an external moving potential which is periodic in the spatial coordinate of the direction of motion. Both dynamical density functional theory and Brownian dynamics computer simulations are used to predict the steady-state density profiles. Two different situations are investigated: the first corresponds to a light wave that travels with a constant velocity v through the quiescent solvent containing the colloidal suspension. The second setup consists of two parallel repulsive walls with a periodic topographical substructure. One of the walls is at rest relative to the solvent while the other is in motion, inducing a shearing of the suspension. In the first case, we find that the amplitude of the steady-state density behaves nonmonotonically with the traveling speed v of the wave if the shape of the wave contains an edge: for increasing v , it first grows and then decreases. In the second setup we show that a strongly confined suspension induces a shear resistance which is a nonmonotonic function of the wall velocity. These effects are verifiable in real-space experiments on colloidal suspensions exposed to external laser-optical fields. In both situations, the dynamical density functional theory is in good agreement with the Brownian dynamics simulation data.

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PubMed2005

Change in relaxation scenario at the order-disorder transition of a colloidal fluid of hard spheres seen from the Gaussian limit of the self-intermediate scattering function.

Self-intermediate scattering functions (ISFs) are measured by dynamic light scattering for the colloidal fluid of hard spheres for both equilibrium and nonequilibrium (undercooled) conditions, i.e., for volume fractions below and above the known freezing transition of the hard-sphere system. The delay time tau(m) where the mean-squared displacement, or the low wave-vector limit of the ISF, exhibits its maximum stretching is identified as a characteristic of the non-Markovian process(es) and is used to separate the ISF into fast (tau < tau(m)) and slow (tau > tau(m)) contributions. Each of these contributions exposes qualitative differences in the dynamics of the particles between the equilibrium and nonequilibrium colloidal fluids. These changes in the relaxation scenario signal the colloidal fluid's awareness of its traversal of the freezing volume fraction.

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PubMed2005

Stability of the binary colloidal crystals AB2 and AB13.

Suspensions of binary mixtures of hard-sphere poly-methylmethacrylate colloidal particles were studied at six different size ratios alpha. The main aim was to determine the range of size ratios over which the binary colloidal crystals AB2 and AB13 are stable. Combining these results with those of earlier work, we found stability of AB2 for 0.60 approximately > alpha approximately > 0.425, in good agreement with theoretical predictions by computer simulation and cell model methods. AB13 was observed for 0.62 approximately > alpha approximately > 0.485, the lower limit being significantly smaller than the theoretical prediction of about 0.525. Rough measurements of crystallization rates showed that AB2 tended to crystallize fastest at small size ratios, whereas the opposite was true for AB13. These findings should provide a guide to the optimum conditions for materials applications of these binary colloidal crystals.

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PubMed2005

Steric interaction between spherical colloidal particles.

The reliability of the Derjaguin approximation for the calculation of the mixing term between sterically stabilized colloidal particles is studied. For this purpose, the steric potential obtained from the experiment of Doroszkowski and Lambourne [J. Polym. Sci., Part C: Polym. Symp. 34, 253 (1971)] is regarded as an exact result. Several analytical expressions corresponding to the mixing term of the steric potential are tested. Vincent et al. [Colloids Surf. 18, 261 (1986)] obtained four of them using the Derjaguin approximation along with different profiles for the volume fraction of segments in grafted polymer layers. As will be shown, the exact calculation of the volume of interaction between two spheres with adsorbed polymer layers already leads to a considerable improvement of the theoretical prediction for the simplest case of constant spatial distribution of polymer monomers. This equation is also better than the four additional expressions that result from using Bagchi's formalism [J. Colloid Interface Sci. 47, 86 (1974)] with similar segment profiles. The deviations of Bagchi's formalism can be substantially minimized using Flory-Krigbaum theory instead of the Flory-Huggins formalism for the calculation of the free energy of mixing. The equations derived here for the steric potentials were derived for particles of distinct radii.

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PubMed2006

Volume terms for charged colloids: a grand-canonical treatment.

We present a study of thermodynamic properties of suspensions of charged colloids on the basis of linear Poisson-Boltzmann theory. We calculate the effective Hamiltonian of the colloids by integrating out the ionic degrees of freedom grand canonically. This procedure not only yields the well-known pairwise screened-Coulomb interaction between the colloids, but also additional volume terms that affect the phase behavior and the thermodynamic properties, such as the osmotic pressure. These calculations are greatly facilitated by the grand-canonical character of our treatment of the ions and allow for relatively fast computations compared to earlier studies in the canonical ensemble. Moreover, the present derivation of the volume terms are relatively simple, make a direct connection with Donnan equilibrium, yield an explicit expression for the effective screening constant, and allow for extensions to include, for instance, nonlinear effects.

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