CiteULike: ghunter's colloid

Phase diagram of mixtures of hard colloidal spheres and discs: A free-volume scaled-particle approach

SM Oversteegen — 2008-08-08T12:30:57-00:00

The Journal of Chemical Physics, Vol. 120, No. 5. (2004), pp. 2470-2474.

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Entropically Driven Colloidal Crystallization on Patterned Surfaces

K Lin — 2008-08-04T22:38:31-00:00

Physical Review Letters, Vol. 85, No. 8. (2000)

We investigate the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases are induced entropically by the presence of dissolved, nonadsorbing polymers. A rich variety of two-dimensional fluidlike and solidlike phases was observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single fcc crystal more than 30 layers thick. The general approach provides a new route for directed self-assembly of novel mesoscopic structures.

Colloidal suspensions in confined geometries

Clemens Bechinger — 2008-07-28T14:49:45-00:00

Current Opinion in Colloid & Interface Science, Vol. 7, No. 3-4. (August 2002), pp. 204-209.

In the presence of geometric confinements such as topographically patterned surfaces, strong light fields, or channels the phase behavior and the dynamical properties of colloidal suspensions are strongly modified in comparison to the bulk properties. Because such geometric confinements play an important role in many physical, biological and chemical processes, investigations of the properties of colloidal suspensions in such situations may help to obtain a better understanding of, e.g. transport of particles through cell membranes or catalytic reactions in zeolitic materials.

Hydrodynamic interaction of particles with grafted polymer brushes and applications to rheology of colloidal dispersions

AA Potanin — 2008-05-27T14:55:09-00:00

Physical Review E, Vol. 52, No. 1. (1 July 1995), 730.

Low Reynolds Number Interactions between Colloidal Particles near the Entrance to a Cylindrical Pore

Venkatachalam Ramachandran — 2008-05-15T15:54:27-00:00

Journal of Colloid and Interface Science, Vol. 229, No. 2. (15 September 2000), pp. 311-322.

The interaction between stable colloidal particles arriving at a pore entrance was studied using a numerical method for the case where the particle size is smaller than but of the same order as the pore size. The numerical method was adapted from a front-tracking technique developed for studying incompressible, multifluid flow by S. O. Unverdi and G. Tryggvason (J. Comp. Phys.100, 25, 1992). The method is based on the finite difference solution of Navier-Stokes equation on a stationary, structured, Cartesian grid and the explicit representation of the particle-liquid interface using an unstructured grid that moves through the stationary grid. The simulations are in two dimensions, considering both deformable and nondeformable particles, and include interparticle colloidal interactions. The interparticle and particle-pore hydrodynamic interactions, which are very difficult to determine using existing analytical and semi-numerical, semi-analytical techniques in microhydrodynamics, are naturally accounted for in our numerical method and need not be explicity determined. Two- and three-particle motion toward a pore has been considered in our simulations. The simulations demonstrate how the competition between hydrodynamic forces and colloidal forces acting on particles dictate their flow behavior near the pore entrance. The predicted dependence of the particle flow behavior on the flow velocity and the ratio of pore size to particle size are qualitatively consistent with the experimental observations of V. Ramachandran and H. S. Fogler (J. Fluid Mech.385, 129, 1999).

Plugging by hydrodynamic bridging during flow of stable colloidal particles within cylindrical pores

Venkatachalam Ramachandran — 2008-05-15T15:50:14-00:00

Journal of Fluid Mechanics, Vol. 385, No. -1. (2000), pp. 129-156.

Development of particle migration in pressure-driven flow of a Brownian suspension

Denis Semwogerere — 2008-05-14T11:41:14-00:00

Journal of Fluid Mechanics, Vol. 581, No. -1. (2007), pp. 437-451.

An experimental investigation into the influence of Brownian motion on shear-induced particle migration of monodisperse suspensions of micrometre-sized colloidal particles is presented. The suspension is pumped through a 50 μm × 500 μm rectangular cross-section glass channel. The experiments are characterized chiefly by the sample volume fraction (φ = 0.1 − 0.4), and the flow rate expressed as the Péclet number (Pe = 10 − 400). For each experiment we measure the entrance length, which is the distance from the inlet of the channel required for the concentration profile to develop to its non-uniform steady state. The entrance length increases strongly with increasing Pe for Pe ≪ 100, in marked contrast to non-Brownian flows for which the entrance length is flow-rate independent. For larger Pe, the entrance length reaches a constant value which depends on the other experimental parameters. Additionally, the entrance length decreases with increasing φ; this effect is strongest for low φ. Modelling of the migration based on spatial variation of the normal stresses due to the particles captures the primary features observed in the axial evolution over a range of Pe and φ.

Shear Zones and Wall Slip in the Capillary Flow of Concentrated Colloidal Suspensions

Lucio Isa — 2008-03-18T22:57:29-00:00

Physical Review Letters, Vol. 98, No. 19. (2007)

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a “paste”) in a square capillary using confocal microscopy. The flow consists of a “plug” in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.

Three-Dimensional Confocal Microscopy of Colloids

AD Dinsmore — 2008-04-28T15:37:19-00:00

Appl. Opt., Vol. 40, No. 24. (2001), pp. 4152-4159.

Confocal microscopy is used in the study of colloidal gels, glasses, and binary fluids. We measure the three-dimensional positions of colloidal particles with a precision of approximately 50 nm (a small fraction of each particle’s radius) and with a time resolution sufficient for tracking the thermal motions of several thousand particles at once. This information allows us to characterize the structure and the dynamics of these materials in qualitatively new ways, for example, by quantifying the topology of chains and clusters of particles as well as by measuring the spatial correlations between particles with high mobilities. We describe our experimental technique and describe measurements that complement the results of light scattering.

Methods of Digital Video Microscopy for Colloidal Studies

John Crocker — 2007-05-18T07:19:49-00:00

Journal of Colloid and Interface Science, Vol. 179, No. 1. (15 April 1996), pp. 298-310.

We describe a set of image processing algorithms for extracting quantitative data from digitized video microscope images of colloidal suspensions. In a typical application, these direct imaging techniques can locate submicrometer spheres to within 10 nm in the focal plane and 150 nm in depth. Combining information from a sequence of video images into single-particle trajectories makes possible measurements of quantities of fundamental and practical interest such as diffusion coefficients and pair-wise interaction potentials. The measurements we describe in detail combine the outstanding resolution of digital imaging with video-synchronized optical trapping to obtain highly accurate and reproducible results very rapidly.

Deposition of Colloidal Asphaltene in Capillary Flow: Experiments and Mesoscopic Simulation

Edo Boek — 2008-04-28T11:22:35-00:00

Energy Fuels, Vol. 22, No. 2. (19 March 2008), pp. 805-813.

Abstract: The aggregation and deposition of colloidal asphaltene in reservoir rock is a significant problem in the oil industry. To obtain a fundamental understanding of this phenomenon, we have studied the deposition and aggregation of colloidal asphaltene in capillary flow by experiment and simulation. For the simulation, we have used the stochastic rotation dynamics (SRD) method, in which the solvent hydrodynamic emerges from the collisions between the solvent particles, while the Brownian motion emerges naturally from the interactions between the colloidal asphaltene particles and the solvent. The asphaltene colloids interact through a screened Coulomb potential. We vary the well depth µcc and the flow rate v to obtain Peflow >> 1 (hydrodynamic interactions dominate) and Re << 1 (Stokes flow). In the simulations, we impose a pressure drop over the capillary length and measure the corresponding solvent flow rate. We observe that the transient solvent flow rate decreases when the asphaltene particles become more sticky. For a well depth µcc = 2kBT, a monolayer deposits on the capillary wall. With an increasing well depth, the capillary becomes totally blocked. The clogging is transient for µcc = 5kBT, but appears to be permanent for µcc = 1020kBT. We compare our simulation results with flow experiments in glass capillaries, where we use extracted asphaltenes in toluene, reprecipitated with n-heptane. In the experiments, the dynamics of asphaltene precipitation and deposition were monitored in a slot capillary using optical microscopy under flow conditions similar to those used in the simulation. Maintaining a constant flow rate of 5 µL min1, we found that the pressure drop across the capillary first increased slowly, followed by a sharp increase, corresponding to a complete local blockage of the capillary. Doubling the flow rate to 10 µL min1, we observe that the initial deposition occurs faster but the deposits are subsequently entrained by the flow. We calculate the change in the dimensionless permeability as a function of time for both experiment and simulation. By matching the experimental and simulation results, we obtain information about (1) the interaction potential well depth for the particular asphaltenes used in the experiments and (2) the flow conditions associated with the asphaltene deposition process.

Funneling of Flow into Grain-to-grain Contacts Drives Colloid−Colloid Aggregation in the Presence of an Energy Barrier

Meiping Tong — 2008-04-28T10:59:04-00:00

Environ. Sci. Technol., Vol. 42, No. 8. (15 April 2008), pp. 2826-2832.

Abstract: Deposition behaviors of carboxylate-modified polystyrene latex microspheres (five sizes ranging from 0.1 to 2.0 µm) were examined in packed porous media, impinging jet, and porous media-packed flow chamber systems under a variety of environmentally relevant ionic strength and flow conditions in the presence of an energy barrier to deposition. Temporally constant deposition rate coefficients were observed for all microsphere sizes under baseline conditions, whereas temporal increases in colloid deposition rate coefficients (ripening) occurred for all microsphere sizes in response to slight increases in solution ionic strength and slight decreases in fluid velocity. This transition from clean bed deposition to ripening was triggered by relatively subtle changes in solution chemistry and fluid velocity. Direct observation of colloid deposition in a flow chamber packed with porous media revealed that colloidal aggregates formed at grain-to-grain contacts in the porous media. The absence of ripening in an impinging jet system (unbounded flat surface) examined under equivalent conditions to the packed porous media further indicated that colloid aggregation was driven by the funneling of fluid into the grain-to-grain contacts. Comparison of colloid breakthrough in porous media comprised of smooth-spherical versus angular grains demonstrated that the propensity to trigger ripening increased with the number and length of grain-to-grain contacts.

Confocal microscopy of colloidal particles: Towards reliable, optimum coordinates

MC Jenkins — 2008-02-19T17:20:12-00:00

Advances in Colloid and Interface Science, Vol. 136, No. 1-2. (15 January 2008), pp. 65-92.

Over the last decade, the light microscope has become increasingly useful as a quantitative tool for studying colloidal systems. The ability to obtain particle coordinates in bulk samples from micrographs is particularly appealing. In this paper we review and extend methods for optimal image formation of colloidal samples, which is vital for particle coordinates of the highest accuracy, and for extracting the most reliable coordinates from these images. We discuss in depth the accuracy of the coordinates, which is sensitive to the details of the colloidal system and the imaging system. Moreover, this accuracy can vary between particles, particularly in dense systems. We introduce a previously unreported error estimate and use it to develop an iterative method for finding particle coordinates. This individual-particle accuracy assessment also allows comparison between particle locations obtained from different experiments. Though aimed primarily at confocal microscopy studies of colloidal systems, the methods outlined here should transfer readily to many other feature extraction problems, especially where features may overlap one another.

Colloid-stabilized emulsions: behaviour as the interfacial tension is reduced

PS Clegg — 2008-04-20T18:26:19-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 45. (2005), pp. S3433-S3438.

We present confocal microscopy studies of novel particle-stabilized emulsions. The novelty arises because the immiscible fluids have an accessible upper critical solution temperature. The emulsions have been created by beginning with particles dispersed in the single-fluid phase. On cooling, regions of the minority phase nucleate. While coarsening, these nuclei become coated with particles due to the associated reduction in interfacial energy. The resulting emulsion is arrested, and the particle-coated interfaces have intriguing properties. Having made use of the binary-fluid phase diagram to create the emulsion we then make use of it to study the properties of the interfaces. As the emulsion is re-heated toward the single-fluid phase the interfacial tension falls and the volume of the dispersed phase drops. Crumpling, fracture or coalescence can follow. The results show that the elasticity of the interfaces has a controlling influence over the emulsion behaviour.

The colloid structural forces as a tool for particle characterization and control of dispersion stability

E Basheva — 2008-04-20T18:25:12-00:00

Phys. Chem. Chem. Phys., Vol. 9 (2007), pp. 5183-5198.

Film Stratification in the Presence of Colloidal Particles

GN Sethumadhavan — 2008-04-20T18:17:43-00:00

Langmuir, Vol. 17, No. 7. (3 April 2001), pp. 2059-2062.

Abstract: This is probably the first study about the drainage of curved liquid films in the presence of colloidal particles. The systems did not contain any surfactant. In the presence of monodispersed colloidal particles, thinning occurs in a stepwise manner (stratification). It has been shown that the size of the film is an important parameter in the stepwise thinning process. This investigation found a critical film size below which at least one layer of particles stays in the film at equilibrium; a "spot" (a thinner section of the film), even if formed, does not expand and is in equilibrium with the film. The area of the spot expands linearly with time. The rate of spot-area expansion increases linearly with the film perimeter and can be increased or decreased merely by changing the film size. The stepwise film thinning and the effects of film size and particle concentration on film stability are discussed on the basis of the diffusive-osmotic mechanism.

Nonequilibrium Sedimentation of Colloids on the Particle Scale

Patrick Royall — 2008-03-18T23:05:10-00:00

Physical Review Letters, Vol. 98, No. 18. (2007)

We investigate sedimentation of model hard-sphere-like colloidal dispersions confined in horizontal capillaries using laser scanning confocal microscopy, dynamical density functional theory, and Brownian dynamics computer simulations. For homogenized initial states we obtain quantitative agreement of the results from the respective approaches for the time evolution of the one-body density distribution and the osmotic pressure on the walls. We demonstrate that single-particle information can be obtained experimentally in systems that were initialized further out of equilibrium such that complex lateral patterns form.

Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?

Marjolein Dijkstra — 2008-04-01T10:19:42-00:00

Journal of Physics: Condensed Matter, Vol. 18, No. 3. (2006), pp. 825-836.

We study the sedimentation equilibrium of low salt suspensions of binary mixtures of charged colloids, both by Monte Carlo simulations of an effective colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture. We show that the theoretically predicted lifting and layering effect, which involves the entropy of the screening ions and a spontaneous macroscopic electric field (Zwanikken and van Roij 2005 Europhys. Lett. 71 480), can also be understood on the basis of an effective colloid-only system with pairwise screened-Coulomb interactions. We consider, by theory and by simulation, both repelling like-charged colloids and attracting oppositely charged colloids, and we find a re-entrant lifting and layering phenomenon when the charge ratio of the colloids varies from large positive through zero to large negative values.

Lane formation in colloidal mixtures driven by an external field

J Dzubiella — 2008-04-01T09:58:17-00:00

Physical Review E, Vol. 65, No. 2. (11 January 2002), 021402.

The influence of an external field on a binary colloidal mixture performing Brownian dynamics in a solvent is investigated by nonequilibrium computer simulations and simple theory. In our model; one half of the particles are pushed into the field direction while the other half of them are pulled into the opposite direction. For increasing field strength; we show that the system undergoes a nonequilibrium phase transition from a disordered state to a state characterized by lane formation parallel to the field direction. The lanes are formed by the same kind of particles moving collectively with the field. Lane formation accelerates particle transport parallel to the field direction but suppresses massively transport perpendicular to the field. We further show that lane formation also occurs in a time-dependent oscillatory field. If the frequency of the external field exceeds a critical value; however; the system exhibits a transition back to the disordered state. Our results can be experimentally verified in binary colloidal suspensions exposed to external fields under nonequilibrium conditions.

Shear thickening in a model colloidal suspension

Jerome Delhommelle — 2008-03-31T14:54:25-00:00

The Journal of Chemical Physics, Vol. 123, No. 7. (2005)

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Stability of Liquid Films Containing Monodisperse Colloidal Particles

Gopi Sethumadhavan — 2008-03-30T02:34:43-00:00

Journal of Colloid and Interface Science, Vol. 240, No. 1. (1 August 2001), pp. 105-112.

Thin liquid films containing colloidal particles are considered to be the key structural elements of three-phase foams containing liquid, gas, and colloidal particles. This study is aimed at understanding the stability of such films in the absence of any surfactants. The particles form a layered structure in the film and produce a stepwise thinning in the thin liquid films. We report here for the first time the effects of particle concentration and size on film thickness transition of curved liquid films containing monodispersed colloidal particles. The rate of stepwise film thinning was observed to be high when particle concentration was low and both particle size and film size were large. The phenomenon of stepwise film thinning (i.e., stratification) is rationalized on the basis of diffusion of colloidal particles from the film to the meniscus, i.e., the diffusive osmotic mechanism. There exists a critical film size below which at least one layer of particles always stays in the film (i.e., black spot expansion does not occur). This critical size is dependent upon both particle size and concentration. Also, Monte Carlo simulations of the film show that, at a high particle concentration, better particle in-layer structure develops that increases the energy barrier, inhibiting particle diffusion from the film to the bulk meniscus.

Nonequilibrium pattern formation in strongly interacting driven colloids

H Löwen — 2008-03-30T02:32:28-00:00

Faraday Discuss., Vol. 123 (2003), pp. 99-105.

Should “lane formation” occur systematically in driven liquids and colloids?

Jerome Delhommelle — 2008-03-30T02:28:57-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 71, No. 1. (2005)

We report on nonequilibrium molecular dynamics simulations of binary mixtures of particles in a color field. Both nonequilibrium molecular dynamics and Brownian dynamics generally assume that the mechanical noise is of thermal origin only and that, at a given temperature, its amplitude remains constant however strong the applied field is. We show that this postulate systematically results in the strong ordering of particles into lanes. By applying a nonequilibrium molecular dynamics method which does not exert any constraint on the noise amplitude, we show that releasing this constraint prevents the systematic "lane formation" from occurring. We observe the onset of density inhomogeneities and jamming instead. This behavior is reminiscent of the shear-thickening regime observed experimentally on colloidal suspensions and in simulations taking into account hydrodynamic interactions.

Lane formation in oppositely charged colloids driven by an electric field: Chaining and two-dimensional crystallization

M Rex — 2008-03-30T02:27:11-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 5. (2007)

A binary mixture of oppositely charged colloids which is driven by an external electric field is studied by extensive Brownian dynamics computer simulations, ignoring hydrodynamic interactions. The particle interaction is modeled via a screened Coulomb potential together with a steric repulsion. A strong electric field leads to lane formation of oppositely driven lanes. Each lane comprises particles of the same charge. A nonequilibrium “phase diagram” classifying different steady states is obtained as a function of the colloidal volume fraction and the Coulomb coupling. Different steady states are characterized by structural correlations perpendicular and parallel to the applied field. We find a variety of different phases involving lane chains at small volume fraction and low screening, and lanes with two-dimensional crystalline order perpendicular to the field at high volume fraction. The lateral crystalline order can be a square, triangular, or rhombic lattice. In between there is a lateral network structure. These predictions can be verified in real-space experiments on oppositely charged colloids.

Ionic colloidal crystals of oppositely charged particles

Mirjam Leunissen — 2005-09-07T17:30:20-00:00

Nature, Vol. 437, No. 7056., pp. 235-240.

Instability of a fluid–fluid interface in driven colloidal mixtures

A Wysocki — 2008-03-30T02:23:00-00:00

Journal of Physics: Condensed Matter, Vol. 16, No. 41. (2004), pp. 7209-7224.

A Rayleigh-Taylor-like interface instability is studied in a compressible Brownian Yukawa fluid mixture on the 'molecular' scales of length and time of the individual particles. As a model, a two-dimensional phase-separated symmetric binary mixture of colloidal particles of type A and B with a fluid-fluid interface separating an A-rich phase from a B-rich phase is investigated, by means of Brownian computer simulations, when brought into non-equilibrium via a constant external driving field which acts differently on the different particles and perpendicular to the interface. Two different scenarios are observed which occur either for high or for low interfacial free energies as compared to the driving force. In the first scenario for high interfacial tension, the critical wavelength lc of the unstable interface modes is in good agreement with the classical Rayleigh-Taylor formula provided that dynamically rescaled values for the interfacial tension are used. The wavelength lc increases with time, representing an effect of self-healing of the interface due to a local density increase near the interface. The Rayleigh-Taylor formula is confirmed even if lc is of the order of a molecular correlation length. In the second scenario for very large driving forces as compared to the interfacial line tensions, on the other hand, the particles penetrate the interface easily due to the driving field and form microscopic lanes with a width different from the predictions of the classical Rayleigh-Taylor formula. The results are of relevance for phase-separating colloidal mixtures in a gravitational or electric field.

Dynamical instability in driven colloids

J Chakrabarti — 2008-03-30T02:21:21-00:00

EPL (Europhysics Letters), Vol. 61, No. 3. (2003), pp. 415-421.

Brownian-dynamics computer simulations show a dynamical crossover in a strongly interacting colloidal suspension consisting of oppositely driven particles, wherein a uniform state transforms, with increasing driving force, into a locally demixed state characterized by strongly anisotropic stripe-like domains which are homogeneous in the direction parallel to the drive but have finite spatial extent of a double-correlation length in the transverse direction. A phenomenological dynamic density-functional theory has been proposed which accounts for such a strongly anisotropic state as arising from an instability of a homogeneous state.

The sediment of mixtures of charged colloids: Segregation and inhomogeneous electric fields

J Zwanikken — 2008-03-27T23:56:29-00:00

EPL (Europhysics Letters), Vol. 71, No. 3. (2005), pp. 480-486.

We theoretically study sedimentation-diffusion equilibrium of dilute binary, ternary, and polydisperse mixtures of colloidal particles with different buoyant masses and/or charges. We focus on the low-salt regime, where the entropy of the screening ions drives spontaneous charge separation and the formation of an inhomogeneous macroscopic electric field. The resulting electric force lifts the colloids against gravity, yielding highly nonbarometric and even nonmonotonic colloidal density profiles. The most profound effect is the phenomenon of segregation into layers of colloids with equal mass-per-charge, including the possibility that heavy colloidal species float onto lighter ones.

Measuring colloidal interactions with confocal microscopy

Patrick Royall — 2008-03-27T21:32:28-00:00

The Journal of Chemical Physics, Vol. 127, No. 4. (2007)

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Enumeration of distinct mechanically stable disk packings in small systems

GJ Gao — 2008-03-14T14:55:57-00:00

PHILOSOPHICAL MAGAZINE, Vol. 87, No. 3-5. (January 2007), pp. 425-431.

Publisher's Note: Random close packing revisited: Ways to pack frictionless disks [Phys. Rev. E [bold 71], 061306 (2005)]

Ning Xu — 2008-03-14T14:51:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 1. (2005)

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Random close packing revisited: Ways to pack frictionless disks

Ning Xu — 2008-01-01T22:16:32-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 71, No. 6. (2005)

We create collectively jammed (CJ) packings of 50-50 bidisperse mixtures of smooth disks in two dimensions (2D) using an algorithm in which we successively compress or expand soft particles and minimize the total energy at each step until the particles are just at contact. We focus on small systems in 2D and thus are able to find nearly all of the collectively jammed states at each system size. We decompose the probability P() for obtaining a collectively jammed state at a particular packing fraction into two composite functions: (1) the density of CJ packing fractions (), which only depends on geometry, and (2) the frequency distribution (), which depends on the particular algorithm used to create them. We find that the function () is sharply peaked and that () depends exponentially on . We predict that in the infinite-system-size limit the behavior of P() in these systems is controlled by the density of CJ packing fractions—not the frequency distribution. These results suggest that the location of the peak in P() when N can be used as a protocol-independent definition of random close packing.

Phase equilibria in stratified thin liquid films stabilized by colloidal particles

J Blawzdziewicz — 2008-03-14T14:44:32-00:00

EPL (Europhysics Letters), Vol. 71, No. 2. (2005), pp. 269-275.

Phase equilibria between regions of different thickness in thin liquid films stabilized by colloidal particles are investigated using a quasi-two-dimensional thermodynamic formalism. Appropriate equilibrium conditions for the film tension, normal pressure, and chemical potential of the particles in the film are formulated, and it is shown that the relaxation of these parameters occurs consecutively on three distinct time scales. Film stratification is described quantitatively for a hard-sphere suspension using a Monte Carlo method to evaluate thermodynamic equations of state. Coexisting phases are determined for systems in constrained- and full-equilibrium states that correspond to different stages of film relaxation.

Simultaneous investigation of sedimentation and diffusion of a single colloidal particle near an interface.

RJ Oetama — 2008-02-01T12:27:50-00:00

J Chem Phys, Vol. 124, No. 16. (28 April 2006)

We describe here a new procedure for the simultaneous investigation of sedimentation and diffusion of a colloidal particle in close proximity to a solid, planar wall. The measurements were made using the optical technique of total internal reflection microscopy, coupled with optical radiation pressure, for dimensionless separation distances (gap width/radius of particle) ranging from 0.01 to 0.05. In this region, the hydrodynamic mobility and diffusion coefficient are substantially reduced below bulk values. The procedure involved measuring the mean and the variance of vertical displacements of a Brownian particle settling under gravity toward the plate. The spatially varying diffusion coefficient was calculated from the displacements at small times (where diffusive motion was dominant). The mobility relationship for motion normal to a flat plate was tested by measuring the average distance of travel versus time as the particle settled under the constant force of gravity. For the simple Newtonian fluid used here (aqueous salt solution), the magnitude of the diffusion coefficient and mobility, plus their dependence on separation distance, showed excellent agreement with predictions. This new technique could be of great value in measuring the mobility and diffusion coefficient for near-contact motion in more complex fluids for which the hydrodynamic correction factors are not known a priori, such as shear-thinning fluids.

Effective interaction of a charged colloidal particle with an air-water interface

EC Mbamala — 2008-02-01T12:25:23-00:00

Journal of Physics: Condensed Matter, Vol. 14, No. 19. (2002), pp. 4881-4900.

Within the framework of linear and non-linear Poisson-Boltzmann theory, we study the effective interaction of a single charged colloidal sphere in an aqueous electrolytic solution with an air-water interface. The effects of varying the salt concentration and the colloidal surface charge density on the effective interaction are being investigated, with a view to understanding some physical phenomena, which include electrostatic adsorption and trapping at the air-water interface. Results show an electrostatic double-layer barrier to the colloid's approach to the interface which can be lowered considerably by increasing the salt concentration. For enough added salt, the charged colloid should be able to suddenly pop up at the air-water surface, an effect which has actually been observed in recent experiments. We discuss the relevance of our results to other experimental observations, and emphasize the close analogy between the problem considered here and the classical problem of the interaction of two colloids in a bulk suspension.

Colloidal Glass Transition Observed in Confinement

Carolyn Nugent — 2007-07-13T19:38:05-00:00

Physical Review Letters, Vol. 99, No. 2. (2007)

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.

Computer simulations of phase transitions of bulk and confined colloid-polymer systems

CY Chou — 2008-01-28T15:20:11-00:00

Physica A: Statistical Mechanics and its Applications, Vol. 369, No. 2. (15 September 2006), pp. 275-290.

Grand canonical Monte Carlo, histogram reweighting and finite-size scaling methods are used to determine the phase transitions of bulk (three-dimensional) and confined (quasi-two-dimensional) neutral colloid-polymer systems. The colloids are modeled as hard spheres and the polymer molecules as hard chains, and the only attractive forces are effective ones induced by depletion effects. In contrast to the predictions of mean field and other approximate theories, the nature of the coexistence phases is found to not depend solely on the polymer-to-colloid size ratio, q, but on the colloid diameter, the polymer radius of gyration, and the polymer monomer size. The threshold values of q for the onset of liquid-liquid phase separation differ significantly from earlier predictions, and depend strongly on the dimensionality of space. Extrapolation to the "protein limit" of very small colloid and very long polymer indicates that immiscibility persists at this limit in three dimensions, while it does not always do so for confined systems.

Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls

Andrea Fortini — 2008-01-28T15:17:41-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 73, No. 5. (2006)

Using Gibbs ensemble Monte Carlo simulations and density functional theory we investigate the fluid-fluid demixing transition in inhomogeneous colloid-polymer mixtures confined between two parallel plates with separation distances between one and ten colloid diameters covering the complete range from quasi-two-dimensional to bulklike behavior. We use the Asakura-Oosawa-Vrij model in which colloid-colloid and colloid-polymer interactions are hard-sphere like, while the pair potential between polymers vanishes. Two different types of confinement induced by a pair of parallel walls are considered—namely, either through two hard walls or through two semipermeable walls that repel colloids but allow polymers to freely penetrate. For hard (semipermeable) walls we find that the capillary binodal is shifted towards higher (lower) polymer fugacities and lower (higher) colloid fugacities as compared to the bulk binodal; this implies capillary condensation (evaporation) of the colloidal liquid phase in the slit. A macroscopic treatment is provided by a symmetric Kelvin equation for general binary mixtures based on the proximity in chemical potentials of statepoints at capillary coexistence and the reference bulk coexistence. Results for capillary binodals compare well with those obtained from the classic version of the Kelvin equation due to [Evans and Marini Bettolo Marconi, J. Chem. Phys. 86, 7138 (1987)] and are quantitatively accurate away from the fluid-fluid critical point, even at small wall separations. However, the significant shift of the critical polymer fugacity towards higher values upon increasing confinement, as found in simulations, is not reproduced. For hard walls the density profiles of polymers and colloids inside the slit display oscillations due to packing effects for all statepoints. For semipermeable walls either similar structuring or flat profiles are found, depending on the statepoint considered.

Confined dynamics, forms and transitions in colloidal systems: from clay to DNA

Pierre Levitz — 2008-01-28T15:15:56-00:00

Magnetic Resonance Imaging, Vol. 23, No. 2. (February 2005), pp. 147-152.

Colloidal suspensions are a classic example of confining systems developing large specific surfaces, presenting a rich variety of shapes and exhibiting complex organization on a length scale ranging from 1 nm to several micrometers. Two distinct confined dynamics are generally considered in such systems: (1) the embedded fluid dynamics entrapped in the pore network with two main contributions, surface interaction and long-range connectivity, and (2) the dynamics of the host matrix, associated with a time evolution of the interfacial geometry. This last contribution is particularly important during dynamic and structural transitions of colloidal suspensions such as jamming, glass transition, phase separations and flocculation. It is generally believed that the characteristic time scale needed to describe colloidal movement and interfacial geometrical reorganization is much slower than the dynamics of the embedded fluid (except in the trivial situation where the fluid molecule is irreversibly adsorbed to a colloidal surface). Thus, few connections are made between these two distinct dynamics. In this presentation, we show how the slow and confined water dynamics at proximity of a colloidal surface provides an original way to probe colloidal shape and colloidal orientation dynamics. Two topics are presented. First of all, water field-cycling NMR relaxometry is used to probe the glass transition and the strong rotational slowing down of a colloidal system made of plate-like particles, a synthetic clay (laponite). Second, we analyze the case of long colloidal thin rods (either mineral or biologic such as DNA cylinders) dispersed in very diluted suspensions. At large distance and/or long time, these particles appear as a portion of a line. We discuss how the embedded fluid dynamics can be sensitive to this morphological crossover and may provide information about the particle shape. Some comparisons with recent experiments are presented.