CiteULike: weeks's confocal

Dynamic Broadening of the Crystal-Fluid Interface of Colloidal Hard Spheres

Roel Dullens — 2008-07-17T18:53:31-00:00

Physical Review Letters, Vol. 97, No. 22. (2006)

We investigate the structure and dynamics of the crystal-fluid interface of colloidal hard spheres in real space by confocal microscopy. Tuning the buoyancy of the particles allows us to study the interface close to and away from equilibrium. We find that the interface broadens from 8–9 particle diameters close to equilibrium to 15 particle diameters away from equilibrium. Furthermore, the interfacial velocity, i.e., the velocity by which the interface moves upwards, increases significantly. The increasing gravitational drive leads to supersaturation of the fluid above the crystal surface. This dramatically affects crystal nucleation and growth, resulting in the observed dynamic broadening of the crystal-fluid interface.

Quantitative Imaging of Aggregated Emulsions

R Penfold — 2008-06-27T15:07:13-00:00

Langmuir, Vol. 22, No. 5. (28 February 2006), pp. 2005-2015.

Abstract: Noise reduction, restoration, and segmentation methods are developed for the quantitative structural analysis in three dimensions of aggregated oil-in-water emulsion systems imaged by fluorescence confocal laser scanning microscopy. Mindful of typical industrial formulations, the methods are demonstrated for concentrated (30% volume fraction) and polydisperse emulsions. Following a regularized deconvolution step using an analytic optical transfer function and appropriate binary thresholding, novel application of the Euclidean distance map provides effective discrimination of closely clustered emulsion droplets with size variation over at least 1 order of magnitude. The a priori assumption of spherical nonintersecting objects provides crucial information to combat the ill-posed inverse problem presented by locating individual particles. Position coordinates and size estimates are recovered with sufficient precision to permit quantitative study of static geometrical features. In particular, aggregate morphology is characterized by a novel void distribution measure based on the generalized Apollonius problem. This is also compared with conventional Voronoi/Delauney analysis.

Direct observation of a local structural mechanism for dynamic arrest

Patrick — 2008-06-27T14:26:14-00:00

Nat Mater, Vol. 7, No. 7. (July 2008), pp. 556-561.

The mechanism by which a liquid may become arrested, forming a glass or gel, is a long-standing problem of materials science. In particular, long-lived (energetically) locally favoured structures (LFSs), the geometry of which may prevent the system relaxing to its equilibrium state, have long been thought to play a key role in dynamical arrest. Here, we propose a definition of LFSs which we identify with a novel topological method and directly measure with experiments on a colloidal liquid–gel transition. The population of LFSs is a strong function of (effective) temperature in the ergodic liquid phase, rising sharply approaching dynamical arrest, and indeed forms a percolating network that becomes the 'arms' of the gel. Owing to the LFSs, the gel is unable to reach equilibrium, crystal–gas coexistence. Our results provide direct experimental observation of a link between local structure and dynamical arrest, and open a new perspective on a wide range of metastable materials.

Structural Rearrangements That Govern Flow in Colloidal Glasses

Peter Schall — 2008-03-01T21:29:01-00:00

Science, Vol. 318, No. 5858. (21 December 2007), pp. 1895-1899.

Structural rearrangements are an essential property of atomic and molecular glasses; they are critical in controlling resistance to flow and are central to the evolution of many properties of glasses, such as their heat capacity and dielectric constant. Despite their importance, these rearrangements cannot directly be visualized in atomic glasses. We used a colloidal glass to obtain direct three-dimensional images of thermally induced structural rearrangements in the presence of an applied shear. We identified localized irreversible shear transformation zones and determined their formation energy and topology. A transformation favored successive ones in its vicinity. Using continuum models, we elucidated the interplay between applied strain and thermal fluctuations that governs the formation of these zones in both colloidal and molecular glasses. 10.1126/science.1149308

Structure and dynamics of colloidal depletion gels: Coincidence of transitions and heterogeneity

Clare Dibble — 2008-06-19T22:57:38-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 74, No. 4. (2006)

Transitions in structural heterogeneity of colloidal depletion gels formed through short-range attractive interactions are correlated with their dynamical arrest. The system is a density and refractive index matched suspension of 0.20 volume fraction poly(methyl methacyrlate) colloids with the nonadsorbing depletant polystyrene added at a size ratio of depletant to colloid of 0.043. As the strength of the short-range attractive interaction is increased, clusters become increasingly structurally heterogeneous, as characterized by number-density fluctuations, and dynamically immobilized, as characterized by the single-particle mean-squared displacement. The number of free colloids in the suspension also progressively declines. As an immobile cluster to gel transition is traversed, structural heterogeneity abruptly decreases. Simultaneously, the mean single-particle dynamics saturates at a localization length on the order of the short-range attractive potential range. Both immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian distributions of single particle displacements reveal enhanced populations of dynamical trajectories localized on two different length scales. Similar dependencies of number density fluctuations, free particle number, and dynamical length scales on the order of the range of short-range attraction suggests a collective structural origin of dynamic heterogeneity in colloidal gels.

Direct Visual Observation of Thermal Capillary Waves

Dirk Aarts — 2008-05-13T19:12:43-00:00

Science, Vol. 304, No. 5672. (7 May 2004), pp. 847-850.

We studied the free fluid-fluid interface in a phase-separated colloid-polymer dispersion with laser scanning confocal microscopy and directly observed thermally induced capillary waves at the interface in real space. Experimental results for static and dynamic correlation functions validate the capillary wave model down to almost the particle level. The ultralow interfacial tension, the capillary length, and the capillary time are found to be in agreement with independent measurements. Furthermore, we show that capillary waves induce the spontaneous breakup of thin liquid films and thus are of key importance in the process of droplet coalescence. 10.1126/science.1097116

Structure and dynamics of biphasic colloidal mixtures

Ali Mohraz — 2008-06-13T21:12:16-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 6. (2008)

We investigate the structure and dynamics of biphasic colloidal mixtures composed of coexisting attractive and repulsive microspheres by confocal microscopy. Attractive gels formed in the presence of repulsive microspheres are more spatially homogeneous and, on average, are both more locally tenuous and have fewer large voids than their unary counterparts. The repulsive microspheres within these mixtures display heterogeneous dynamics, with some species exhibiting freely diffusive Brownian motion while others are trapped within the gel network during aggregation.

Direct measurement of the free energy by optical microscopy

Roel Dullens — 2008-06-11T18:54:03-00:00

Proceedings of the National Academy of Sciences, Vol. 103, No. 3. (17 January 2006), pp. 529-531.

We report the direct measurement of thermodynamic properties of colloidal hard spheres by optical microscopy. By using confocal microscopy, we obtain three-dimensional snapshots of a colloidal hard-sphere suspension over a wide range of densities. From these snapshots, the available volume to insert an additional sphere and the surface area of that volume are determined, which are directly related to the thermodynamics of the system. This procedure enables us to measure in a direct and noninterfering way, in principle, all thermodynamic properties, here demonstrated for the pressure, the chemical potential, and the free-energy density of a colloidal hard-sphere suspension. The "visual" determination of thermodynamic quantities opens up the possibility to experimentally study the relation between thermodynamics and geometry in real space beyond the hard-sphere potential. 10.1073/pnas.0507052103

Measurement of Forces Inside a Three-Dimensional Pile of Frictionless Droplets

J Zhou — 2008-01-01T22:04:40-00:00

Science, Vol. 312, No. 5780. (16 June 2006), pp. 1631-1633.

We present systematic and detailed measurements of interparticle contact forces inside three-dimensional piles of frictionless liquid droplets. We measured long-range chainlike correlations of the directions and magnitudes of large forces, thereby establishing the presence of force chains in three dimensions. Our correlation definition provides a chain persistence length of 10 mean droplet diameters, decreasing as load is applied to the pile. We also measured the angles between contacts and showed that the chainlike arrangement arises from the balance of forces. Moreover, we found that piles whose height was comparable to the chain persistence length exhibited substantially greater strain hardening than did tall piles, which we attributed to the force chains. Together, the results establish a connection between the microscopic force network and the elastic response of meso- or macroscopic granular piles. The conclusions drawn here should be relevant in jammed systems generally, including concentrated emulsions and piles of sand or other heavy particles. 10.1126/science.1125151

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.

Direct Visualization of Colloidal Rod Assembly by Confocal Microscopy

A Mohraz — 2007-06-25T02:35:18-00:00

Langmuir, Vol. 21, No. 12. (7 June 2005), pp. 5298-5306.

Abstract: The development of model materials and image processing methods to directly visualize and quantify colloidal rod assembly by means of confocal laser scanning microscopy (CLSM) is reported. Monodisperse fluorescent colloidal rods are prepared by the uniaxial extensional deformation of sterically stabilized microspheres at elevated temperatures. The particles are stably dispersed in refractive index matching mixed organic solvents for CLSM. An image processing algorithm is developed to detect rod backbones and extract particle centroids and orientation angles from the CLSM image volumes. By means of these methods we quantify the distribution of rod orientation angles in self-assembled structures of rods formed by sedimentation. We find the observations to be consistent with aspect-ratio-dependent jamming and orientational order/disorder transition in the rod sediments.

Laser tweezer microrheology of a colloidal suspension.

Alexander Meyer — 2007-08-18T23:11:42-00:00

Journal of Rheology, Vol. 50, No. 1. (2006)

The microrheology of a colloidal suspension is measured using laser tweezers. Suspensions of refractive index-matched fluorinated ethylene propylene (FEP) particles are seeded with index-mismatched polystyrene or silica probe particles. Laser trapped probes are then subjected to steady uniform flows, enabling measurements of the suspension microviscosity as a function of FEP volume fraction and flow velocity. The microrheology results agree with bulk rheology, and both exhibit the same volume fraction dependence of the Krieger-Dougherty relationship for hard spheres. As volume fraction increases, the microrheology more closely agrees with the infinite shear bulk viscosity. In this regime, measurements using small probes exhibit additional shear thinning. Using confocal microscopy and fluorescent poly(methylmethacrylate) dispersions, we demonstrate that the nonlinear microrheology is consistent with the development of an anisotropic nonequilibrium pair distribution function between the

Void Structure in Colloidal Dispersions

Kensaku Ito — 2007-09-28T21:09:17-00:00

Science, Vol. 263, No. 5143. (7 January 1994), pp. 66-68.

The time evolution of void structures in highly purified polymer latex dispersions was studied with a confocal laser scanning microscope. In such dispersions, which were initially homogeneous, the voids grew with time when the dispersions were kept standing and formed more quickly in the internal material than in material close to the glass-dispersion interface. Void formation is thus not an artifact arising from the presence of the interface. A similar structural inhomogeneity, in apparently homogeneous systems, is discussed for simple ionic solutions, ionic polymer solutions, and Langmuir-Blodgett films. 10.1126/science.263.5143.66

Localized ordered structure in polymer latex suspensions as studied by a confocal laser scanning microscope

Hiroshi Yoshida — 2007-09-28T21:06:12-00:00

Physical Review B, Vol. 44, No. 1. (1 July 1991), pp. 435-438.

By using a confocal laser scanning microscope and a digital image analyzer; the influence of a glass-suspension interface on colloidal crystals was studied. Particular attention was paid to the interparticle distance (2 D expt ) in the ordered structure and the crystallization process. The 2 D expt was insensitive to the distance from the interface up to about 40 μm. The crystallization process of the ordered structure was characterized by a strong anisotropy originating at the glass-suspension interface.

Fluorescent Monodisperse Silica Ellipsoids for Optical Rotational Diffusion Studies

S Sacanna — 2007-07-01T22:35:33-00:00

Langmuir, Vol. 22, No. 4. (14 February 2006), pp. 1822-1827.

Abstract: We report on the preparation of monodisperse, fluorescent hematite-silica core-shell ellipsoids, with adjustable shapes ranging from spindles to nearly spheres, that are suitable for optical rotational diffusion studies. Hematite cores are grafted with poly(vinylpyrrolidone) which ensures colloidal stability during the silica coating provided by the base-catalyzed hydrolysis and polymerization of tetraethoxysilane. Using tetramethylammonium hydroxide as base instead of the volatile ammonia facilitates continuous seeded growth of silica to colloids with a desired aspect ratio. A convenient feature of the hematite-silica particles is the rapid dissolution of the iron oxide core by acid, producing hollow silica ellipsoids that can be optically matched to near transparency. The control of shape and size of the silica ellipsoids, their optical properties, and the fairly high yield in comparison to other preparation methods for nonspherical model colloids make the ellipsoids very suitable for quantitative studies. As a case in point, we have measured the rotational diffusion coefficient of fluorescent ellipsoids with rotational fluorescence recovery after photobleaching. Dye-labeled ellipsoids can be imaged with confocal microscopy.

Weak Correlations between Local Density and Dynamics near the Glass Transition

JC Conrad — 2006-07-03T19:08:55-00:00

J. Phys. Chem. B, Vol. 109, No. 45. (17 November 2005), pp. 21235-21240.

We perform experiments on two different dense colloidal suspensions with confocal microscopy to probe the relationship between local structure and dynamics near the glass transition. We calculate the Voronoi volume for our particles and show that this quantity is not a universal probe of glassy structure for all colloidal suspensions. We correlate the Voronoi volume to displacement and find that these quantities are only weakly correlated. We observe qualitatively similar results in a simulation of a polymer melt. These results suggest that the Voronoi volume does not predict dynamical behavior in experimental colloidal suspensions; a purely structural approach based on local single particle volume likely cannot describe the colloidal glass transition.

Translational and rotational dynamics of colloidal rods by direct visualization with confocal microscopy

Deshpremy Mukhija — 2007-09-21T19:04:19-00:00

Journal of Colloid and Interface Science, Vol. 314, No. 1. (1 October 2007), pp. 98-106.

We report an experimental method to characterize the dynamics of colloidal rods by measuring their rotation and translation in three dimensions with confocal microscopy. The method relies on solvent viscosification to retard dynamics to time scales that are compatible with 3D confocal optical microscopy. Because the method yields a full three-dimensional characterization of rod displacement and orientation, it is applicable to situations in which complex, anisotropic dynamics emerge. Examples include behavior in liquid crystal phases with both orientational and positional order, suspensions subjected to applied fields such as shear flow or sedimentation, and the emerging area of anisotropic particle dynamics. We demonstrate the performance of the method by quantifying the Brownian motion of fluorescent poly(methyl methacrylate) rods (aspect ratio, L/D=3.1 and 7.0) grafted with poly(dimethylsiloxane) stabilizer. The rods are dispersed at dilute concentration in a solvent mixture of viscosity 2.0 Pa[thin space]s. Rod translational and rotational diffusivities are extracted from the measured translational mean square displacement of the centroid positions and of the rod unit vector u(t), respectively. Rod orientational dynamics are characterized relative to both their azimuthal and polar angles. Probability distributions for the translation and rotation in the frame of rod are computed from the measurements. Experimental values obtained agree well with theory available for the dynamics of isolated rods.

Forced motion of a probe particle near the colloidal glass transition

P Habdas — 2007-09-20T04:40:18-00:00

Europhys. Lett., Vol. 67, No. 3. (2004), pp. 477-483.

We use confocal microscopy to study the motion of a magnetic bead in a dense colloidal suspension, near the colloidal glass transition volume fraction phg. For dense liquid-like samples near phg, below a threshold force the magnetic bead exhibits only localized caged motion. Above this force, the bead is pulled with a fluctuating velocity. The relationship between force and velocity becomes increasingly nonlinear as phg is approached. The threshold force and nonlinear drag force vary strongly with the volume fraction, while the velocity fluctuations do not change near the transition.

Foam drainage on the microscale II. Imaging flow through single Plateau borders

SA Koehler — 2007-09-20T04:35:11-00:00

Journal of Colloid and Interface Science, Vol. 276, No. 2. (15 August 2004), pp. 439-449.

The liquid in foam forms an interconnected network, which is composed of Plateau borders, nodes, and films. One of the dominant pathways for foam drainage is flow through Plateau borders, and we use confocal microscopy to obtain experimental results for the flow fields inside individual Plateau borders. For three types of surfactants detailed comparisons are made with a model based upon the influence of surface viscosity at free boundaries between the gas in the bubbles and the liquid in the Plateau borders. The model describes the flows well, and we find good agreement between the surface viscosity predicted by this model and representative values found in the literature. We also give a qualitative description of the flow in the nodes.

Subdiffusion and the cage effect studied near the colloidal glass transition

Eric Weeks — 2007-09-20T04:31:56-00:00

Chemical Physics, Vol. 284, No. 1-2. (1 November 2002), pp. 361-367.

Real-Space Imaging of Nucleation and Growth in Colloidal Crystallization

U Gasser — 2007-09-20T04:28:54-00:00

Science, Vol. 292, No. 5515. (13 April 2001), pp. 258-262.

Crystallization of concentrated colloidal suspensions was studied in real space using laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of nucleation and growth of crystalline regions, providing the first experimental measure of properties of the nucleating crystallites. By following their evolution, critical nuclei were identified, nucleation rates were determined, and the average surface tension of the crystal-liquid interface was measured. The structure of the nuclei was the same as the bulk solid phase, random-hexagonal-close-packed (rhcp), and their average shape was rather aspherical, with rough rather than faceted surfaces.

Drainage of single Plateau borders: Direct observation of rigid and mobile interfaces

Stephan Koehler — 2007-09-20T04:26:50-00:00

Physical Review E, Vol. 66, No. 4. (October 2002), pp. 040601-040601.

Foam drainage varies with surfactant. We present direct measurements of the flow velocity profiles across single Plateau borders; which make up the interconnected channel-like network for liquid flow. For protein foams the interface is rigid; whereas small-surfactant foams show significant interfacial mobility. The results agree with a model that takes into account the shearing of the liquid-gas interface transverse to the flow direction. A significant consequence is that bubble size and liquid volume fraction in a foam affect the relative importance of surface rheology on the drainage behavior.

Experimental studies of the flow of concentrated hard sphere suspensions into a constriction

L Isa — 2006-06-07T11:46:33-00:00

Journal of Physics: Conference Series, Vol. 40, No. 1. (2006), pp. 124-132.

Interesting flow properties are observed when a concentrated suspension of colloidal particles flows into a geometrical constriction. We present here a description of two different experimental techniques used to study the pressure driven flow of dense suspensions of micron-sized hard spheres into glass capillaries. The first one involves the analysis of the driving pressure during the flow, the other one is based on fast confocal microscopy. Technical details are given, together with a selection of preliminary results.

Confocal microscopy

Denis Semwogerere — 2007-09-20T04:18:38-00:00

(2005)

A confocal microscope creates sharp images of a specimen that would otherwise appear blurred when viewed with a conventional microscope. This is achieved by excluding most of the light from the specimen that is not from the microscope’s focal plane. The image has less haze and better contrast than that of a conventional microscope and represents a thin cross-section of the specimen. Thus, apart from allowing better observation of fine details it is possible to build three-dimensional (3D) reconstructions of a volume of the specimen by assembling a series of thin slices taken along the vertical axis.

Phase Behavior and 3D Structure of Strongly Attractive Microsphere-Nanoparticle Mixtures

JF Gilchrist — 2007-09-20T04:13:37-00:00

Langmuir, Vol. 21, No. 24. (22 November 2005), pp. 11040-11047.

Abstract: We investigate the phase behavior and 3D structure of strongly attractive mixtures of silica microspheres and polystyrene nanoparticles. These binary mixtures are electrostatically tuned to promote a repulsion between like-charged (microsphere-microsphere and nanoparticle-nanoparticle) species and a strong attraction between oppositely charged (microsphere-nanoparticle) species. Using confocal fluorescence scanning microscopy, we directly observe the 3D structure of colloidal phases assembled from these mixtures as a function of varying composition. In the absence of nanoparticle additions, the charged-stabilized microspheres assemble into a polycrystalline array upon sedimentation. With increasing nanoparticle volume fraction, nanoparticle bridges form between microspheres, inducing their flocculation. At even higher nanoparticle volume fractions, the microspheres become well coated with nanoparticles, leading to their charge reversal and subsequent restabilization. We demonstrate how this fluid-gel-fluid transition can be utilized to control the morphology of the colloidal phases formed under gravity-driven sedimentation.

Interparticle Interactions and Direct Imaging of Colloidal Phases Assembled from Microsphere-Nanoparticle Mixtures

CJ Martinez — 2007-09-20T04:12:36-00:00

Langmuir, Vol. 21, No. 22. (25 October 2005), pp. 9978-9989.

Abstract: We investigate the interparticle interactions, phase behavior, and structure of microsphere-nanoparticle mixtures that possess high size and charge asymmetry.1 We employ a novel Monte Carlo simulation scheme2 to calculate the effective microsphere interactions in suspension, yielding new insight into the origin of the experimentally observed behavior.3 The initial settling velocity, final sediment density, and three-dimensional structure of colloidal phases assembled from these binary mixtures via gravitational settling of silica microspheres in water and index-matched solutions exhibit a strong compositional dependence. Confocal laser scanning microscopy is used to directly image and quantify their structural evolution during assembly. Below a lower critical nanoparticle volume fraction (nano < L,C), the intrinsic van der Waals attraction between microspheres leads to the formation of colloidal gels. These gels exhibit enhanced consolidation as nano approaches L,C. When nano exceeds L,C, an effective repulsion arises between microspheres due to the formation of a dynamic nanoparticle halo around the colloids. From this stable fluid phase, the microspheres settle into a crystalline array. Finally, above an upper critical nanoparticle volume fraction (nano > U,C), colloidal gels form whose structure becomes more open with increasing nanoparticle concentration due to the emergence of an effective microsphere attraction,3 whose magnitude exhibits a superlinear dependence on nano.

Invariance of Structure in an Aging Colloidal Glass

Gianguido Cianci — 2007-09-20T04:07:03-00:00

AIP Conference Proceedings, Vol. 832, No. 1. (2006), pp. 21-25.

We study concentrated colloidal suspensions, a model system which has a glass transition. The non-equilibrium nature of the glassy state is most clearly highlighted by aging — the dependence of the system's properties on the time elapsed since vitrification. Fast laser scanning confocal microscopy allows us to image a colloidal glass and track the particles in three dimensions. We analyze the static structure in terms of tetrahedral packing. We find that while the aging of the suspension clearly affects its dynamics, none of the geometrical quantities associated with tetrahedra change with age.

Correlations of structure and dynamics in an aging colloidal glass

Gianguido Cianci — 2007-09-20T04:05:51-00:00

Solid State Communications, Vol. 139, No. 11-12. (September 2006), pp. 599-604.

We study concentrated colloidal suspensions, a model system which has a glass transition. Samples in the glassy state show aging, in that the motion of the colloidal particles slows as the sample ages from an initial state. We study the relationship between the static structure and the slowing dynamics, using confocal microscopy to follow the three-dimensional motion of the particles. The structure is quantified by considering tetrahedra formed by quadruplets of neighboring particles. We find that while the sample clearly slows down during aging, the static properties as measured by tetrahedral quantities do not vary. However, a weak correlation between tetrahedron shape and mobility is observed, suggesting that the structure facilitates the motion responsible for the sample aging.

Structure of dense colloidal liquids in tight spaces

Eric Weeks — 2007-09-20T04:00:14-00:00

Vol. 19 (2007), pp. 45-49.

We use three-dimensional confocal microscopy to study the structure of a dense colloidal liquid confined between two parallel glass plates. The colloidal sample is at a volume fraction of 50\% and is a binary mixture of 2~$μ$m and 3~$μ$m diameter particles to prevent crystallization. The plate separation ranges from 50 small particle diameters to 3 small particle diameters. While particles form layers immediately adjacent to the confining walls, we otherwise see little influence of the confinement on structure.

Microscopic Structure and Elasticity of Weakly Aggregated Colloidal Gels

AD Dinsmore — 2007-09-19T16:41:52-00:00

Physical Review Letters, Vol. 96, No. 18. (2006)

We directly probe the microscopic structure, connectivity, and elasticity of colloidal gels using confocal microscopy. We show that the gel is a random network of one-dimensional chains of particles. By measuring thermal fluctuations, we determine the effective spring constant between pairs of particles as a function of separation; this is in agreement with the theory for fractal chains. Long-range attractions between particles lead to freely rotating bonds, and the gel is stabilized by multiple connections among the chains. By contrast, short-range attractions lead to bonds that resist bending, with dramatically suppressed formation of loops of particles.

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

D Semwogerere — 2007-09-19T16:15:32-00:00

J. Fluid Mech., Vol. 581 (2007), pp. 437-451.

Particle migration in pressure-driven flow of a Brownian suspension

M Frank — 2007-09-19T16:15:32-00:00

J. Fluid Mech, Vol. 493 (2003), pp. 363-378.

Confocal microscopy of colloids

V Prasad — 2007-09-19T15:10:32-00:00

Journal of Physics: Condensed Matter, Vol. 19 (2007), pp. 113102-113102.

Colloids have increasingly been used to characterize or mimic many aspects of atomic and molecular systems. With confocal microscopy these colloidal particles can be tracked spatially in three dimensions with great precision over large time scales. This review discusses equilibrium phases such as crystals and liquids, and non-equilibrium phases such as glasses and gels. The phases that form depend strongly on the type of particle interaction that dominates. Hard-sphere-like colloids are the simplest, and interactions such as the attractive depletion force and electrostatic repulsion result in more nontrivial phases which can better model molecular materials. Furthermore, shearing or otherwise externally forcing these colloids while under microscopic observation helps connect the microscopic particle dynamics to the macroscopic flow behavior. Finally, directions of future research in this field are discussed.

Direct visualization of ageing in colloidal glasses

Rachel Courtland — 2005-08-20T23:20:53-00:00

J. Phys.: Condens. Matter, Vol. 15 (2003), pp. S359-S365.

We use confocal microscopy to directly visualize the dynamics of ageing colloidal glasses. We prepare a colloidal suspension at high density, a simple model system that shares many properties with other glasses, and initiate experiments by stirring the sample. We follow the motion of several thousand colloidal particles after the stirring and observe that their motion significantly slows as the sample ages. The ageing is both spatially and temporally heterogeneous. Furthermore, while the characteristic relaxation timescale grows with the age of the sample, nontrivial particle motions continue to occur on all timescales.

Real-Space Structure of Colloidal Hard-Sphere Glasses

A van Blaaderen — 2007-05-17T16:29:59-00:00

Science, Vol. 270 (November 1995), pp. 1177-1179.

The real-space structure of hard-sphere glasses quenched from colloidal liquids in thermodynamic equilibrium has been determined. Particle coordinates were obtained by combining the optical sectioning capability of confocal fluorescence microscopy with the structure of specially prepared fluorescent silica colloids. Both the average structure and the local structure of glasses, with volume fractions ranging from 0.60 to 0.64, were in good agreement with glasses and random close packings generated by computer simulations. No evidence of a divergent correlation length was found. The method used to obtain the three-dimensional particle coordinates is directly applicable to other colloidal structures, such as crystals, gels, and flocs.<p>

Direct Observation of Dynamical Heterogeneities in Colloidal Hard-Sphere Suspensions

Willem Kegel — 2007-05-18T06:58:01-00:00

Science, Vol. 287, No. 5451. (14 January 2000), pp. 290-293.

The real-space dynamics in a model system of colloidal hard spheres was studied by means of time-resolved ßuorescence confocal scanning microscopy. Direct experimental evidence for the presence of dynamical heterogeneities in a dense liquid was obtained from an analysis of particle trajectories in two dimensional slices of the bulk sample. These heterogeneities manifest themselves as a non-Gaussian probability distribution of particle displacements and also affect the onset of long-time diffusive behavior.

Three-Dimensional Confocal Microscopy of Colloids

AD Dinsmore — 2007-06-11T21:35:48-00:00

Applied Optics, 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.

Properties of Cage Rearrangements Observed near the Colloidal Glass Transition

Eric Weeks — 2007-04-02T22:33:06-00:00

Physical Review Letters, Vol. 89, No. 9. (2002), pp. 095704-095704.

We use confocal microscopy to study particle motion in colloidal systems. Near the glass transition; motion is inhibited; as particles spend time trapped in transient “cages” formed by neighboring particles. We measure the cage sizes and lifetimes; which; respectively; shrink and grow as the glass transition approaches. Cage rearrangements are more prevalent in regions with lower concentrations and higher disorder. Neighboring rearranging particles typically move in parallel directions; although a nontrivial fraction moves in antiparallel directions; usually from particle pairs with initial separations corresponding to local maxima and minima of the pair correlation function g ( r ); respectively.

Contribution of Slow Clusters to the Bulk Elasticity Near the Colloidal Glass Transition

Jacinta Conrad — 2007-08-15T16:26:46-00:00

Physical Review Letters, Vol. 97, No. 26. (2006)

We use confocal microscopy to visualize individual particles near the colloidal glass transition. We identify the most slowly-relaxing particles and show that they form spatially correlated clusters that percolate across the sample. In supercooled fluids, the largest cluster spans the system on short time scales but breaks up on longer time scales. In contrast, in glasses, a percolating cluster exists on all accessible time scales. Using molecular dynamics simulation, we show that these clusters make the dominant contribution to the bulk elasticity of the sample.

Short- and long-range correlated motion observed in colloidal glasses and liquids

Eric Weeks — 2007-08-29T17:41:41-00:00

Journal of Physics: Condensed Matter, Vol. 19, No. 20. (2007), pp. 205131-205131.

We use a confocal microscope to examine the motion of individual particles in a dense colloidal suspension. Close to the glass transition, particle motion is strongly spatially correlated. The correlations decay exponentially with particle separation, yielding a dynamic length scale of O(2-3s) (in terms of particle diameter s). This length scale grows modestly as the glass transition is approached. Further, the correlated motion exhibits a strong spatial dependence on the pair correlation function g(r). Motion within glassy samples is weakly correlated, but with a larger spatial scale for this correlation.

Colloidal Glass Transition Observed in Confinement

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

Physical Review Letters, Vol. 99, No. 2. (2007), pp. 025702-025702.

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.