CiteULike: dchen's colloids

Colloidal Aggregation in a Nematic Liquid Crystal: Topological Arrest of Particles by a Single-Stroke Disclination Line

Takeaki Araki — 2008-05-13T00:24:02-00:00

Physical Review Letters, Vol. 97, No. 12. (2006)

We numerically study many-body interactions among colloidal particles suspended in a nematic liquid crystal, using a fluid particle dynamics method, which properly incorporates dynamical coupling among particles, nematic orientation, and flow field. Based on simulation results, we propose a new type of interparticle interaction in addition to well-known quadrupolar interaction for particles accompanying Saturn-ring defects. This interaction is mediated by the defect of the nematic phase: upon nematic ordering, a closed disclination loop binds more than two particles to form a sheetlike dynamically arrested structure. The interaction depends upon the topology of a disclination loop binding particles, which is determined by aggregation history.

Measuring the Kinetics of Biomolecular Recognition with Magnetic Colloids

Cohen Tannoudji — 2008-05-07T23:01:32-00:00

Physical Review Letters, Vol. 100, No. 10. (2008)

We introduce a general methodology based on magnetic colloids to study the recognition kinetics of tethered biomolecules. Access to the full kinetics of the reaction is provided by an explicit measure of the time evolution of the reactant densities. Binding between a single ligand and its complementary receptor is here limited by the colloidal rotational diffusion. It occurs within a binding distance that can be extracted by a reaction-diffusion theory that properly accounts for the rotational Brownian dynamics. Our reaction geometry allows us to probe a large diversity of bioadhesive molecules and tethers, thus providing a quantitative guidance for designing more efficient reactive biomimetic surfaces, as required for diagnostic, therapeutic, and tissue engineering techniques.

Polyelectrolyte-Compression Forces between Spherical DNA Brushes

Kati Kegler — 2008-05-07T21:14:58-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Optical tweezers are employed to measure the forces of interaction within a single pair of DNA-grafted colloids, dependent on the molecular weight of the DNA chains, and the concentration and valence of the surrounding ionic medium. The resulting forces are short range and set in as the surface-to-surface distance between the colloidal cores reaches the value of the brush height. The measured force-distance relation is analyzed by means of a theoretical treatment that quantitatively describes the effects of compression of the chains on the surface of the opposite-lying colloid. Quantitative agreement with the experiment is obtained for all parameter combinations.

Colloidal Assembly on Magnetically Vibrated Stripes

Pietro Tierno — 2008-05-07T15:36:01-00:00

Physical Review Letters, Vol. 100, No. 14. (2008)

We investigate the collective organization of paramagnetic colloidal particles externally driven above the periodic stripes of a uniaxial ferrimagnetic garnet film. An external field modulation induces vibration of the stripe walls and produces random motion of the particles. Defects in the stripe pattern break the symmetry of the potential and favor particle nucleation into large clusters above a critical density. Mismatch between particle size and pattern wavelength generates assemblies with different morphological order. At even higher field strengths, repulsive dipolar interactions between the particles induce cluster melting. We propose a novel approach to generate and externally control a variety of colloidal assemblies.

Density Dependent Interactions and Structure of Charged Colloidal Dispersions in the Weak Screening Regime

Rojas Ochoa — 2008-05-05T20:12:24-00:00

Physical Review Letters, Vol. 100, No. 17. (2008)

We determine the structure of charge-stabilized colloidal suspensions at low ionic strength over an extended range of particle volume fractions using a combination of light and small angle neutron scattering experiments. The variation of the structure factor with concentration is analyzed within a one-component model of a colloidal suspension. We show that the observed structural behavior corresponds to a nonmonotonic density dependence of the colloid effective charge and the mean interparticle interaction energy. Our findings are corroborated by similar observations from primitive model computer simulations of salt-free colloidal suspensions.

Dynamics of the Solid and Liquid Phases in Dilute Sheared Brownian Suspensions: Irreversibility and Particle Migration

Jennifer Brown — 2008-05-04T18:06:26-00:00

Physical Review Letters, Vol. 99, No. 24. (2007)

Magnetic resonance measurements of migration and irreversible dynamics in the capillary shear flow of a Brownian suspension are presented. The results demonstrate the presence of phenomena typically associated with concentrated noncolloidal systems and indicate the role of many body hydrodynamics in dilute Brownian suspension transport. The application of concepts from chaos theory and nonequilibrium statistical mechanics is demonstrated.

New developments in colloid science

DA Weitz — 2008-05-02T17:46:28-00:00

(2004)

Entropically driven self-assembly and interaction in suspension

2008-04-29T23:03:20-00:00

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 359, No. 1782. (15 May 2001), pp. 921-937.

In this paper we present fundamental studies elucidating the role of entropy in particle suspensions. We focus on systems composed of large colloidal particles along with a second, usually smaller species such as a particle or polymer. We describe direct measurements of these interactions in suspension, and we systematically show how these forces can be used to control the self-assembly of colloidal particles. The paper provides a unified review of the experiments from our laboratory, and in a few cases touches on very recent results.

Direct imaging of three-dimensional structure and topology of colloidal gels

AD Dinsmore — 2008-04-29T22:57:53-00:00

Journal of Physics: Condensed Matter, Vol. 14, No. 33. (2002), pp. 7581-7597.

We present novel measurements of the structure of colloidal gels. Using confocal microscopy, we obtain the precise three-dimensional positions of a large number of particles. We develop quantitative descriptions of the topology of the gel, including the number of bonds per particle, the chemical or bond fractal dimension, the number of flexible pivot points and other topological parameters that describe the chainlike structure. We investigate the dependence of these parameters on the particle volume fraction and the strength of the attraction that holds the particles together. While all samples have approximately the same fractal and chemical dimensions, we find that gels formed with stronger attraction or larger volume fraction have fewer bonds per particle, more filamentous chains and a greater number of flexible pivot points. Finally, we discuss the topological results in the context of the gel's elasticity. Measurements of the elastic constants of individual chainlike segments are explained with a simple model. The distribution of elastic constants, however, has a general form that is not understood.

Colloids: A useful boundary

Anthony Dinsmore — 2008-04-29T20:15:42-00:00

Nat Mater, Vol. 6, No. 12. (December 2007), pp. 921-922.

Bicontinuous emulsions stabilized solely by colloidal particles.

EM Herzig — 2008-04-29T20:39:54-00:00

Nature materials, Vol. 6, No. 12. (December 2007), pp. 966-971.

Recent large-scale computer simulations suggest that it may be possible to create a new class of soft solids, called 'bijels', by stabilizing and arresting the bicontinuous interface in a binary liquid demixing via spinodal decomposition using particles that are neutrally wetted by both liquids. The interfacial layer of particles is expected to be semi-permeable; hence, if realized, these new materials would have many potential applications, for example, as micro-reaction media. However, the creation of bijels in the laboratory faces serious obstacles. In general, fast quench rates are necessary to bypass nucleation, so that only samples with limited thickness can be produced, which destroys the three-dimensionality of the putative bicontinuous network. Moreover, even a small degree of unequal wettability of the particles by the two liquids can lead to ill-characterized, 'lumpy' interfacial layers and therefore irreproducible material properties. Here, we report a reproducible protocol for creating three-dimensional samples of bijel in which the interfaces are stabilized by essentially a single layer of particles. We demonstrate how to tune the mean interfacial separation in these bijels, and show that mechanically, they indeed behave as soft solids. These characteristics and their tunability will be of great value for microfluidic applications.

Structure of dense colloidal liquids in tight spaces

Eric — 2008-04-29T18:44:50-00:00

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 µm and 3 µm 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.

Phase behaviour of concentrated suspensions of nearly hard colloidal spheres

PN Pusey — 2005-08-23T18:57:37-00:00

Nature, Vol. 320, No. 6060. (27 March 1986), pp. 340-342.

Confocal microscopy of colloids

Semwogerere — 2007-07-27T10:53:20-00:00

Journal of Physics: Condensed Matter, Vol. 19, No. 11. (2007)

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 non-trivial 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 behaviour. Finally, directions of future research in this field are discussed.

Real-Space Structure of Colloidal Hard-Sphere Glasses

Alfons van Blaaderen — 2008-04-28T23:56:24-00:00

Science, Vol. 270, No. 5239. (17 November 1995), pp. 1177-1179.

10.1126/science.270.5239.1177

A comparative study on the phase behaviour of highly charged colloidal spheres in a confining wedge geometry

Ana Fontecha — 2007-09-27T16:53:09-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 31. (2005), pp. S2779-S2786.

We studied the structures formed in aqueous dispersions of charged colloidal spheres under a constant low salt concentration of c = 6 × 10[?]6 mol l[?]1. Particles of diameter 2a = 1000 nm were confined to a low angle wedge geometry with plate separation 0<S<50 µm and observed with video microscopy. Irrespective of the initial particle density n we reproducibly observe the particles to migrate to the narrow wedge side on the timescale of a few days. Thereby an interface between a crystalline structure and a near particle free region is formed, which propagates slowly until the dilute region is exhausted of particles. While the origin of this separation is still unclear, the final extension of the crystalline region is stable on the timescale of months. Within the crystal phase we observe a characteristic sequence of structures with increasing plate separation similar to that seen in previous experiments on hard sphere-like systems but here with non-touching particles. Moreover, we find that mechanical equilibrium is a necessary prerequisite for observing the full richness of different phases. A detailed comparison to recent theoretical calculations for bilayers was performed and semi-quantitative agreement with the predictions observed.

Nature of the divergence in low shear viscosity of colloidal hard-sphere dispersions

Zhengdong Cheng — 2007-07-11T20:42:14-00:00

Physical Review E, Vol. 65, No. 4. (April 2002), 041405.

Measurements of the low-shear viscosity η o with a Zimm-Crothers viscometer for dispersions of colloidal hard spheres are reported as a function of volume fraction φ up to 0.56. Nonequilibrium theories based on solutions to the two-particle Smoluchoski equation or ideal mode coupling approximations do not capture the divergence. However; the nonhydrodynamic contribution to the relative viscosity Δη o is correlated over a wide range of volume fractions by the Doolittle and Adam-Gibbs equations; indicating an exponential divergence at φ m =0.625±0.015. The data extend the previously proposed master curve; providing a test for improved theories for the many-body thermodynamic and hydrodynamic interactions that determine the viscosity of hard-sphere dispersions.

Microrheology from Rotational Diffusion of Colloidal Particles

Efren Reyes — 2007-05-18T11:12:10-00:00

Physical Review Letters, Vol. 94, No. 10. (2005)

The microrheology of viscoelastic fluids is obtained from rotational diffusion of optically anisotropic spherical colloidal probes, measured by depolarized dynamic light scattering. The storage and loss moduli obtained from the rotational mean squared displacement is in excellent agreement with those obtained from translational diffusion and by mechanical measurements. We also show that this method is applicable to samples with strong light scattering components. This extends the capabilities of the microrheological methods based on the diffusional motion of colloidal probes.

Laser tweezer microrheology of a colloidal suspension

Alexander Meyer — 2008-04-28T19:22:56-00:00

Journal of Rheology, Vol. 50, No. 1. (2006), pp. 77-92.

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 probe and bath particles, with a denser region at the leading surface of the probe and a wake trailing it. The nonlinear response and underlying microstructure are in qualitative agreement with recent theory [T. M. Squires and J. F. Brady, Phys. Fluids 17, 073101 (2005)]. ©2006 The Society of Rheology

Forced motion of a probe particle near the colloidal glass transition

Habdas — 2008-04-28T19:13:52-00:00

Europhys. Lett., 67 (3), p. 477 (2004)

We use confocal microscopy to study the motion of a magnetic bead in a dense colloidal suspension, near the colloidal glass transition volume fraction $φ_ g$. For dense liquid-like samples near $φ_ g$, 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 $φ_ g$ 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.

Effect of particle size distribution on crystallisation and the glass transition of hard sphere colloids

SI Henderson — 2007-09-27T16:45:30-00:00

Physica A: Statistical and Theoretical Physics, Vol. 233, No. 1-2. (15 November 1996), pp. 102-116.

Phase behaviour, crystallisation kinetics and particle dynamics are compared for two colloidal suspensions of hard spherical particles with different particle size distributions; one is narrow and roughly symmetrical and the other is broader and skewed towards smaller particles. Both suspensions exhibit the equilibrium phase behaviour expected for a system of identical hard spheres and they show a glass transition, indicated by the cessation of homogeneous nucleation and the partial arrest of concentration fluctuations, at approximately the same volume fraction, [Phi]g [approximate] 0.58. Interestingly, compared to the suspension with the narrower size distribution, crystallisation rates are significantly slower in the more polydisperse suspension. In its colloidal glass state no crystal growth occurs on secondary nuclei, such as the shear-aligned crystals that can be induced in both suspensions by regular rocking.

Dense Packing and Symmetry in Small Clusters of Microspheres

Vinothan Manoharan — 2007-04-04T07:42:56-00:00

Science, Vol. 301, No. 5632. (25 July 2003), pp. 483-487.

When small numbers of colloidal microspheres are attached to the surfaces of liquid emulsion droplets, removing fluid from the droplets leads to packings of spheres that minimize the second moment of the mass distribution. The structures of the packings range from sphere doublets, triangles, and tetrahedra to exotic polyhedra not found in infinite lattice packings, molecules, or minimum-potential energy clusters. The emulsion system presents a route to produce newcolloidal structures and a means to study howdifferent physical constraints affect symmetry in small parcels of matter. 10.1126/science.1086189

CHEMISTRY: Colloidal Molecules and Beyond

Alfons van Blaaderen — 2007-04-04T08:25:28-00:00

Science, Vol. 301, No. 5632. (25 July 2003), pp. 470-471.

10.1126/science.1087140

Yielding and flow of sheared colloidal glasses

G Petekidis — 2008-04-26T00:17:53-00:00

Journal of Physics: Condensed Matter, Vol. 16, No. 38. (2004), pp. S3955-S3963.

We have studied some of the rheological properties of suspensions of hard-sphere colloids with particular reference to behaviour near the concentration of the glass transition. First we monitored the strain on the samples during and after a transient step stress. We find that, at all values of applied step stress, colloidal glasses show a rapid, apparently elastic, recovery of strain after the stress is removed. This recovery is found even in samples which have flowed significantly during stressing. We attribute this behaviour to 'cage elasticity', the recovery of the stress-induced distorted environment of any particle to a more isotropic state when the stress is removed. Second, we monitored the stress as the strain rate γ of flowing samples was slowly decreased. Suspensions which are glassy at rest show a stress which becomes independent of γ as γ \to 0 . This limiting stress can be interpreted as the yield stress of the glass and agrees well both with the yield stress deduced from the step stress and recovery measurements and that predicted by a recent mode coupling theory of sheared suspensions. Thus, the behaviours under steady shearing and transient step stress both support the idea that colloidal glasses have a finite yield stress. We note however that the samples do exhibit a slow accumulation of strain due to creep at stresses below the yield stress.

Self-Diffusion of Interacting Colloids Far from Equilibrium

Xia Qiu — 2008-04-26T00:12:25-00:00

Physical Review Letters, Vol. 61, No. 22. (28 November 1988), 2554.

Direct visualization of ageing in colloidal glasses

Rachel Courtland — 2008-04-26T00:03:39-00:00

Journal of Physics: Condensed Matter, Vol. 15, No. 1. (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.

Laponite: Aging and Shear Rejuvenation of a Colloidal Glass

Daniel Bonn — 2008-04-25T23:08:28-00:00

Physical Review Letters, Vol. 89, No. 1. (13 June 2002), 015701.

Electric-field-induced capillary attraction between like-charged particles at liquid interfaces

MG Nikolaides — 2008-04-24T19:25:39-00:00

Nature, Vol. 420, No. 6913. (21 November 2002), pp. 299-301.

Universal features of the ﬂuid to solid transition for attractive colloidal particles

Prasad — 2008-04-24T18:37:13-00:00

Faraday Discuss., 2003

Attractive colloidal particles can exhibit a fluid to solid phase transition if the magnitude of the attractive interaction is sufficiently large, if the volume fraction is sufficiently high, and if the applied stress is sufficiently small. The nature of this fluid to solid transition is similar for many different colloid systems, and for many different forms of interaction. The jamming phase transition captures the common features of these fluid to solid translations, by unifying the behavior as a function of the particle volume fraction, the energy of interparticle attractions, and the applied stress. This paper describes the applicability of the jamming state diagram, and highlights those regions where the fluid to solid transition is still poorly understood. It also presents new data for gelation of colloidal particles with an attractive depletion interaction, providing more insight into the origin of the fluid to solid transition.

Yielding and crystallization of colloidal gels under oscillatory shear

PA Smith — 2008-04-24T17:45:46-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

We have studied the behavior of a colloidal gel under oscillatory shear. The quiescent gel was an arrested structure formed by a 40% volume fraction hard-sphere suspension in which a “depletion” interparticle attraction was induced by adding nonadsorbing polymer. We applied progressively larger amplitude oscillatory shear to the sample, and observed its behavior using conventional and confocal microscopy as well as dynamic light scattering echo spectroscopy. We find that, to within experimental uncertainties, the point at which irreversible particle rearrangements (or yielding) occur coincides with the observation of crystallization. We summarize our findings in a “shear state diagram.” The strain amplitude required for yielding/crystallization increases with decreasing oscillation frequency. We can quantitatively account for our observations by estimating the effect of shear on the probability for a particle to escape from the attractive potential of its neighbor using a Kramers approach.

Limits to Gelation in Colloidal Aggregation

S Manley — 2008-04-23T23:55:21-00:00

Physical Review Letters, Vol. 93, No. 10. (2004), 108302.

Onset of Buckling in Drying Droplets of Colloidal Suspensions

N Tsapis — 2008-04-23T21:45:07-00:00

Physical Review Letters, Vol. 94, No. 1. (2005)

Minute concentrations of suspended particles can dramatically alter the behavior of a drying droplet. After a period of isotropic shrinkage, similar to droplets of a pure liquid, these droplets suddenly buckle like an elastic shell. While linear elasticity is able to describe the morphology of the buckled droplets, it fails to predict the onset of buckling. Instead, we find that buckling is coincident with a stress-induced fluid to solid transition in a shell of particles at a droplet's surface, occurring when attractive capillary forces overcome stabilizing electrostatic forces between particles.

Optically Anisotropic Colloids of Controllable Shape

A Fernández-Nieves — 2008-04-23T21:34:08-00:00

Advanced Materials, Vol. 17, No. 6. (2005), pp. 680-684.

No abstract.

Self-assembled Shells Composed of Colloidal Particles: Fabrication and Characterization

MF Hsu — 2008-04-23T21:31:14-00:00

Langmuir, Vol. 21, No. 7. (29 March 2005), pp. 2963-2970.

Abstract: We construct shells with tunable morphology and mechanical response with colloidal particles that self-assemble at the interface of emulsion droplets. Particles self-assemble to minimize the total interfacial energy, spontaneously forming a particle layer that encapsulates the droplets. We stabilize these layers to form solid shells at the droplet interface by aggregating the particles, connecting the particles with adsorbed polymer, or fusing the particles. These techniques reproducibly yield shells with controllable properties such as elastic moduli and breaking forces. To enable diffusive exchange through the particle shells, we transfer them into solvents that are miscible with the encapsulant. We characterize the mechanical properties of the shells by measuring the response to deformation by calibrated microcantilevers.

Gravitational Collapse of Colloidal Gels

S Manley — 2008-04-23T21:18:48-00:00

Physical Review Letters, Vol. 94, No. 21. (2005)

We present a unified framework for understanding the compaction of colloidal gels under their own weight. The dynamics of the collapse are determined by the value of the gravitational stress g, as compared to the yield stress Y of the network. For g<Y, gels collapse poroelastically, and their rate of compression decays exponentially in time. For g>Y, the network eventually yields, leading to rapid settling. In both cases, the rate of collapse is backflow limited, while its overall magnitude is determined by a balance between gravitational stress and network elastic stress.

Time-Dependent Strength of Colloidal Gels

S Manley — 2008-04-23T21:08:32-00:00

Physical Review Letters, Vol. 95, No. 4. (2005)

Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure.

Fluids of Clusters in Attractive Colloids

Peter Lu — 2008-04-23T19:32:58-00:00

Physical Review Letters, Vol. 96, No. 2. (2006)

We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact.

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.

Buckling and Crumpling of Drying Droplets of Colloid-Polymer Suspensions

Y Sugiyama — 2008-04-23T17:51:21-00:00

Langmuir, Vol. 22, No. 14. (4 July 2006), pp. 6024-6030.

Abstract: Spray drying of complex liquids to form solid powders is important in many industrial applications. One of the challenges associated with spray drying is controlling the morphologies of the powders produced; this requires an understanding of how drying mechanics depend on the ingredients and conditions. We demonstrate that the morphology of powders produced by spray drying colloidal polystyrene (PS) suspensions can be significantly altered by changing the molecular weight of dissolved poly(ethylene oxide) (PEO). Samples containing high-molecular-weight PEO produce powders with more crumpled morphologies than those containing low-molecular-weight PEO. Observations of drying droplets suspended by a thin film of vapor suggest that this occurs because the samples with high-molecular-weight PEO buckle earlier in the drying process when the droplets are larger. Earlier buckling times are likely caused by the decreased stability, demonstrated by bulk rheology experiments, of PS particles in the presence of high-molecular-weight PEO at elevated temperatures. We present a consistent picture in which decreased particle stability hastens droplet buckling and leads to more crumpled powder morphologies; this underscores the importance of interparticle forces in determining the buckling of particle-laden droplets.

Direct imaging of repulsive and attractive colloidal glasses

Laura Kaufman — 2007-06-25T01:35:08-00:00

The Journal of Chemical Physics, Vol. 125, No. 7. (2006)

Coherent anti-Stokes Raman scattering microscopy is performed on glassy systems of poly(methylmethacrylate) colloidal particles in density- and refractive-index-matched solvents. Samples are prepared with varying amounts of linear polystyrene, which induces a depletion driven attraction between the nearly hard-sphere particles. Images collected over several hours confirm the existence of a reentrant glass transition. The images also reveal that the dynamics of repulsive and attractive glasses are qualitatively different. Colloidal particles in repulsive glasses exhibit cage rattling and escape, while those in attractive glasses are nearly static while caged but exhibit large displacements upon (infrequent) cage escape. ©2006 American Institute of Physics

Microfluidic Assembly of Homogeneous and Janus Colloid-Filled Hydrogel Granules

RF Shepherd — 2008-04-23T17:33:28-00:00

Langmuir, Vol. 22, No. 21. (10 October 2006), pp. 8618-8622.

Abstract: The microfluidic assembly of colloid-filled hydrogel granules of varying shape and composition is described. First, drops are formed by shearing a concentrated colloidal microsphere-acrylamide suspension in a continuous oil phase using a sheath-flow device. Both homogeneous and Janus (hemispherically distinct) spheres and disks are produced by confining the assembled drops in microchannels of varying geometry. Next, photopolymerization is carried out shortly after drop breakup to preserve their morphology. Representative wet and dried granules are characterized using fluorescence and scanning electron microscopy, respectively. Our approach offers a facile route for assembling colloid-filled hydrogel granules with controlled shape and composition.

Synthesis of Nonspherical Colloidal Particles with Anisotropic Properties

JW Kim — 2008-04-23T17:26:53-00:00

J. Am. Chem. Soc., Vol. 128, No. 44. (8 November 2006), pp. 14374-14377.

Abstract: We describe a promising and flexible technique for fabricating uniform nonspherical particles with anisotropic phase and surface properties. Our approach is based on the seeded polymerization technique in which monomer-swollen particles are polymerized. The polymerization causes a phase separation to occur, giving rise to two-phase nonspherical particles. We show that the elastic contraction of the swollen polymer particles induced by elevated polymerization temperatures plays an important role in the phase separation. Moreover, chemical anisotropy of nonspherical particles can be obtained by using immiscible polymer pairs and by employing surface treatments. Furthermore, we are able to produce amphiphilic dumbbell particles consisting of two different bulbs: hydrophilic poly (ethylene imine)-coated polystyrene and hydrophobic polystyrene. Controlled geometries of these amphiphilic nonspherical particles will allow a wide range of potential applications, such as engineered colloid surfactants.

Colloidal Crystals

Pawel Pieranski — 2008-04-23T17:22:31-00:00

Contemporary Physics; Jan/Feb83, Vol. 24 Issue 1, p25, 49p, 54 diagrams, 5 graphs, 2c, 7bw (1983)

Uniform Nonspherical Colloidal Particles with Tunable Shapes

JW Kim — 2008-04-17T20:35:27-00:00

Advanced Materials, Vol. 19, No. 15. (2007), pp. 2005-2009.

No abstract.

Fluids of clusters in attractive colloids.

PJ Lu — 2008-04-17T00:53:44-00:00

Physical review letters, Vol. 96, No. 2. (20 January 2006)

We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact.

Preparation of monodisperse PMMA microspheres in nonpolar solvents by dispersion polymerization with a macromonomeric stabilizer

Sascham Klein — 2007-05-27T21:14:34-00:00

Colloid & Polymer Science, Vol. 282, No. 1. (1 December 2003), pp. 7-13.

We discuss a dispersion polymerization procedure for preparing monodisperse and micron-sized poly(methyl methacrylate) (PMMA) particles in hexanes with methacryloxypropyl-terminated polydimethylsiloxane stabilizers. We investigate the effects of the stabilizer molecular weight, stabilizer concentration, and monomer concentration on the particle size and polydispersity. We find that a minimum molecular weight of 10 000 g/mol is necessary to synthesize colloidally stable PMMA dispersions. The particle polydispersity is minimal (=5%) for stabilizer to monomer weight ratios of 0.02 to 0.1, while PMMA particles prepared under conditions outside this range are polydisperse. The particle diameter can be varied from 0.4 to 1.5 µm by appropriate choices of stabilizer and monomer concentrations. Stable PMMA suspensions can be prepared at up to 26.3% solids. The dispersions are stable in most liquid aliphatics, and are monodisperse enough to form ordered domains at high concentration. This single-stage synthesis, requiring only commercially available materials, may be of interest to those seeking a simple way to prepare highly monodisperse non-aqueous dispersions in the micron size range.

Length-Scale-Dependent Relaxation in Colloidal Gels

Emanuela Del Gado — 2008-03-19T17:19:20-00:00

Physical Review Letters, Vol. 98, No. 2. (2007)

We use molecular dynamics computer simulations to investigate the relaxation dynamics of a simple model for a colloidal gel at a low volume fraction. We find that due to the presence of the open spanning network this dynamics shows at low temperature a nontrivial dependence on the wave vector which is very different from the one observed in dense glass-forming liquids. At high wave vectors the relaxation is due to the fast cooperative motion of the branches of the gel network, whereas at low wave vectors the overall rearrangements of the heterogeneous structure produce the relaxation process.

Viscoelasticity of Dynamically Self-Assembled Paramagnetic Colloidal Clusters

Pietro Tierno — 2008-03-19T17:16:50-00:00

Physical Review Letters, Vol. 98, No. 2. (2007)

Paramagnetic particles in a liquid above a solid dynamically self-assemble into two-dimensional (2D) viscoelastic clusters in a precessing magnetic field if the precession angle exceeds the magic angle. Hexagonal clusters rotate with a frequency proportional to the precession frequency of the magnetic field. The rotation is explained by viscoelastic shear waves excited in the clusters that can be visualized slightly above the magic angle. The cluster rotation and the visualization of viscoelastic modes are independent techniques to probe the rheological properties of the cluster. We find agreement between both techniques when determining the 2D cluster viscosity c10-11 N s/m.

Noncentral Forces in Crystals of Charged Colloids

D Reinke — 2008-03-19T17:11:08-00:00

Physical Review Letters, Vol. 98, No. 3. (2007)

The elastic properties of fcc crystals consisting of charge stabilized colloidal particles are determined from real space imaging experiments using confocal microscopy. The normal modes and the force constants of the crystal are obtained from the fluctuations of the particles around their lattice sites using the equipartition theorem. We show that the Cauchy relation is not fulfilled and that only noncentral many-body forces can account for the elastic properties.

Forces of Interaction between DNA-Grafted Colloids: An Optical Tweezer Measurement

K Kegler — 2008-03-19T16:59:43-00:00

Physical Review Letters, Vol. 98, No. 5. (2007)

Optical tweezers are employed to measure the forces of interaction between single DNA-grafted colloids. Parameters to be varied are the length of the DNA, the grafting density, and the ion concentration of the surrounding medium. From the measured force-separation dependence an interaction length at a given force is deduced. It shows in the mushroom regime a scaling with the grafting density which levels off for brushes. For the latter the transition from an osmotic to a salted brush can be traced in detail by varying the ion concentration in accordance with mean field theories.