CiteULike: Tag colloidal

Tangential Forces between Nontouching Colloidal Particles

D Velegol — 2008-02-12T00:33:46-00:00

Physical Review Letters, Vol. 83, No. 6. (1999), 1243.

By measuring the orientation of colloidal doublets suspended in water under the opposing alignment forces of gravity and electrophoresis; we observed tangential forces between nontouching particles. We found a new phenomenon of time-dependent switching between “slipping” (no tangential force) and “sticking” (tangential forces producing rigid-body behavior of the doublet) for heterodoublets of silica/polystyrene. Tangential forces between particle surfaces across a fluid gap and their transient nature are important to the dynamics of colloidal suspensions and the evolution of floc morphology.

Novel fluorescent colloids as a DNA fluorescence probe

L Jinshui — 2007-03-30T14:17:48-00:00

Anal. Bioanal. Chem., Vol. 377, No. 2. (2003), pp. 346-349.

Fluorescent perylene colloids in the 80-90 nm size range have been prepared by the reprecipitation method. These nanoparticles were modified by cetyltrimethylammonium bromide (CTAB) which inhibited their growth. The nanoparticles also readily interacted with DNA. The fluorescence emission was measured at ? ex/? em=400/565 nm. The fluorescence decrease of colloid-CTAB in aqueous solution was measured in the presence of nucleic acids. Under the optimum conditions, the ratio of fluorescence intensity in the absence and presence of nucleic acids was proportional to the concentration of nucleic acids over the range 0.02-5.1 ?g mL-1 for FS (fish sperm) DNA or CT (calf thymus) DNA. The detection limits were 0.01 ?g mL-1 for FS DNA and 0.012 ?g mL -1 for CT DNA, respectively. Based on this approach, a new quantitative method for DNA assay is presented in this paper. © Springer-Verlag 2003.

Shape-controlled synthesis of colloidal platinum nanoparticles

TS Ahmadi — 2007-03-20T01:50:28-00:00

SCIENCE, Vol. 272, No. 5270. (1996), pp. 1924-1926.

The shapes and sizes of platinum nanoparticles were controlled by changes in the ratio of the concentration of the capping polymer material to the concentration of the platinum cations used in the reductive synthesis of colloidal particles in solution at room temperature. Tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral particle shapes were observed, whose distribution was dependent on the concentration ratio of the capping polymer material to the platinum cation. Controlling the shape of platinum nanoparticles is potentially important in the field of catalysis.

Capped colloids as light-mills in optical traps

FS Merkt — 2006-09-28T08:54:22-00:00

New J. Phys., Vol. 8, No. 9. (September 2006), 216.

Rotation in Suspension of a Rod-Shaped Colloid

L Hong — 2007-05-16T17:25:39-00:00

Langmuir, Vol. 22, No. 17. (15 August 2006), pp. 7128-7131.

Abstract: The simple strategy of coating a closely packed colloid monolayer with a nanometer-thick metal film to connect colloidal spheres and then gently sonicating produces a series of colloid metastructures, including rods and planar sheets. These structures can be fluorescently labeled, which can serve as a probe to monitor their dynamics in complex environments. The metal coating modulates fluorescence emission as these structures rotate in suspension. By analyzing the time sequence of fluorescence images using single-particle tracking techniques, here we measure the rotational dynamics of a rodlike tetramer, quantifying rotation along the long axis.

Surfactant-mediated two-dimensional crystallization of colloidal crystals

L Ramos — 2007-08-06T03:07:58-00:00

Science, Vol. 286, No. 5448. (17 December 1999), pp. 2325-2328.

Colloidal particles can form unexpected two-dimensional ordered colloidal crystals when they interact with surfactants of the opposite charge. Coulomb interactions lead to self-limited adsorption of the particles on the surface of vesicles formed by the surfactants. The adsorbed particles form ordered but fluid rafts on the vesicle surfaces, and these ultimately form robust two-dimensional crystals. This use of attractive Coulomb interaction between colloidal particles and surfactant structures offers a potential new route to self-assembly of ordered colloidal structures.

Rapid Fabrication of Binary Colloidal Crystals by Stepwise Spin-Coating

D Wang — 2007-08-06T03:05:42-00:00

Advanced Materials, Vol. 16, No. 3. (2004), pp. 244-247.

No abstract.

Rapid Fabrication of Two- and Three-Dimensional Colloidal Crystal Films via Confined Convective Assembly

M H Kim — 2007-08-06T03:03:29-00:00

Advanced Functional Materials, Vol. 15, No. 8. (2005), pp. 1329-1335.

We have developed a self-assembly method for fabricating well-ordered two-dimensional (2D) and three-dimensional (3D) colloidal crystal films. With a minute amount of a polystyrene colloidal suspension and without any special equipment, the proposed method can be used to rapidly deposit high-quality colloidal crystal films over a large surface area. By controlling the lift-up rate of the substrate, we modulate the meniscus thinning rate, which determines whether the colloidal particles are assembled into two or three dimensions. The proposed method can be used to fabricate not only monolayered colloidal crystals with colloidal particles of various sizes, but also multilayered colloidal crystals. In addition, the method enables us to fabricate binary colloidal crystals by consecutively depositing large and small particles.

Preparation and absorption properties of polystyrene/Ag/TiO2 multiple coated colloids

JH Zhang — 2007-07-29T20:05:58-00:00

J Mater Res, Vol. 20, No. 4. (2005), pp. 965-970.

We demonstrated a facile route based on the use of acetone and polyvinylpyrrolidone (PVP) to prepare polystyrene (PS)/Ag/TiO2 multilayered colloids with controllable shell thickness. In this route, PVP absorbed directly onto PS colloid surface, and the Ag seed shell composed of Ag nanoparticles was synthesized directly under the PVP shell by swelling the surface layer of the PS core. Because the PVP shell increased the affinity of the Ag shell to TiO2, the hydrolyzed titania particles could deposit directly onto the core to form the outer TiO2 shell. A seed growth technique and the controllable hydrolysis reaction of tetra-n-butyl titanate were developed to grow the shell thickness of Ag and TiO2, respectively. Studies of the absorption properties indicate that the optical properties of these multilayered composite colloids can be modified by changing the coating species and shell thickness. © 2005 Materials Research Society.

Monodispersed Colloidal Spheres: Old Materials with New Applications

Y Xia — 2007-08-06T03:01:11-00:00

Advanced Materials, Vol. 12, No. 10. (2000), pp. 693-713.

This article presents an overview of current research activities that center on monodispersed colloidal spheres whose diameter falls anywhere in the range of 10 nm to 1 &mgr;m. It is organized into three parts: The first part briefly discusses several useful methods that have been developed for producing monodispersed colloidal spheres with tightly controlled sizes and well-defined properties (both surface and bulk). The second part surveys some techniques that have been demonstrated for organizing these colloidal spheres into two- and three-dimensionally ordered lattices. The third part highlights a number of unique applications of these crystalline assemblies, such as their uses as photonic bandgap (PBG) crystals; as removable templates to fabricate macroporous materials with highly ordered and three-dimensionally interconnected porous structures; as physical masks in lithographic patterning; and as diffractive elements to fabricate new types of optical sensors. Finally, we conclude with some personal perspectives on the directions towards which future research in this area might be directed.

Influence of Particle Volume Fraction on Packing in Responsive Hydrogel Colloidal Crystals

SB Debord — 2007-08-15T16:03:08-00:00

J. Phys. Chem. B, Vol. 107, No. 13. (3 April 2003), pp. 2927-2932.

Abstract: We describe the influence of particle concentration on the formation of crystals from soft colloidal particles. Specifically, laser scanning confocal and differential interference contrast microscopies are used to directly observe the packing in entropic crystals formed from submicron-sized particles composed of the thermoresponsive polymer poly-N-isopropylacrylamide (pNIPAm) and the thermoresponsive/pH-responsive copolymer poly-N-isopropylacrylamide-co-acrylic acid (pNIPAm-AAc). Close-packed crystals are observed when either the pNIPAm and pNIPAm-AAc particles are concentrated by centrifugation. At that point the particle-particle spacing is smaller than the hydrodynamic diameter as determined by photon correlation spectroscopy, reflecting compression of the soft particles. Upon dilution of the pNIPAm crystals, the melting point of the assembly is reached when the repulsive particle interactions are no longer sufficient to dictate long-range order in the assembly. However, excellent crystallization is observed even after a 9-fold dilution of the acrylic acid-containing particles, whereupon the particle size and hence the center-to-center particle spacing is observed to be more than twice that of the original sample. These results are not explainable using a simple hard-sphere packing model, which would predict crystal melting upon dilution, and are therefore interpreted in terms of particle compressibility (for higher concentration samples) as well as an attractive pair- or multibody-potential (for low-concentration samples).

Fluctuation-Dissipation Theorem in an Aging Colloidal Glass

Sara Farouji — 2007-11-26T20:20:36-00:00

Physical Review Letters, Vol. 98, No. 10. (2007)

We provide a direct experimental test of the fluctuation-dissipation theorem (FDT) in an aging colloidal glass. The use of combined active and passive microrheology allows us to independently measure both the correlation and response functions in this nonequilibrium situation. Contrary to previous reports, we find no deviations from the FDT over several decades in frequency (1 Hz–10 kHz) and for all aging times. In addition, we find two distinct viscoelastic contributions in the aging glass, including a nearly elastic response at low frequencies that grows during aging.

Formation of dried colloidal monolayers and multilayers under the influence of electric fields

HJ Schope — 2007-08-06T02:56:17-00:00

(2003)

In this letter I investigate the crystallization of colloidal monolayers and multilayers under convective self-assembly using inclined deposition and under the influence of electric fields. Applying a periodic external field during the crystallization process parallel to the substrate and perpendicular to the growth direction of the layer, the particles order along the field lines and attach as strings to the crystalline interface. The crystalline interface itself melts and freezes periodically due to the applied shear; grain boundaries can be annealed and the formation of large monocrystalline arrays on a very short timescale can be observed. Application of electric fields may thus yield a significantly improved quality of the resulting synthetic opals.

Investigation of the optical properties of hollow aluminium 'nano-caps'

J Liu — 2005-11-05T04:42:21-00:00

Nanotechnology, Vol. 16, No. 12. (December 2005), pp. 3023-3028.

Design of a scanning laser optical trap for multiparticle manipulation

C Mio — 2007-03-21T20:29:25-00:00

Review of Scientific Instruments, Vol. 71, No. 5. (2000), pp. 2196-2200.

In recent years, single-beam optical traps have been used to manipulate individual colloids and biological objects such as cells. We have implemented a rapidly scanning laser optical trap with rates as high as 1200 Hz where a single laser beam is used to trap multiple colloids simultaneously. The optics are optimized to achieve a small laser focus size and a large scanning pattern in the sample. This approach provides great pattern flexibility and, because of the use of piezoelectrics, small particles (1 µm in diameter) in low-viscosity solvents, such as water, can be readily manipulated. ©2000 American Institute of Physics.

Translational and rotational diffusion of model nanocolloidal dispersions studied by molecular dynamics simulations

DM Heyes — 2007-03-06T22:13:19-00:00

Journal of Physics: Condensed Matter, Vol. 10, No. 45. (1998), pp. 10159-10178.

Molecular dynamics (MD) simulations have been used to study the translational and rotational relaxation of model spherical nanocolloidal particles in solution at infinite dilution. The solvent was modelled at the molecular level. Simulations were carried out with two types of model nanocolloidal particle, one that was smooth (`structureless') and the other built from a cluster of atoms (`rough'). Both types had variable diameter, IMG, compared to that of the solvent molecule, IMG. The Weeks-Chandler-Andersen (WCA) interaction between the colloid and the WCA solvent molecules was used. Nanocolloidal particles that were up to an order of magnitude larger than those of the solvent molecules were simulated. The effects of the relative solvent and colloidal particle mass density, and colloid size on the translational and rotational self-diffusion coefficients were investigated. At liquid-like number densities IMG the translational, D , and rotational, IMG, self-diffusion coefficients for the nanocolloids of all sizes were statistically independent of the ratio of colloidal to solvent particle mass density for the the values, up to IMG, explored. As solvent number density decreased, the translational self-diffusion coefficients of the colloidal particles showed more evidence than the rotational self-diffusion coefficients of a decrease with increasing colloid particle density. Both D and IMG decreased with increasing size of the colloidal particle in close agreement with the classical solutions, the Stokes-Einstein and Stokes-Einstein-Debye relationships respectively. Differences in the translational diffusion coefficients of smooth and rough colloidal particles were not statistically significant at IMG, but at IMG the D were lower for the rough particles. Reorientational motion occurred by small-step diffusion.

Two-Dimensional Interfacial Colloidal Crystals

Pawel Pieranski — 2007-08-02T21:24:35-00:00

Physical Review Letters, Vol. 45, No. 7. (1980), 569.

Polystyrene spheres (2450 Å in diameter) are trapped in a surface energy well at water/air interface. Because of asymmetry of charge distribution; electrical dipoles are associated with each interfacial particle. The dipole-dipole repulsive interactions organize the polystyrene spheres into a two-dimensional triangular lattice. The direct microscopic observations of such an interfacial colloidal crystal are reported for the first time.

Light Absorption by the Clusters of Colloidal Gold and Silver Particles Formed During Slow and Fast Aggregation

NG Khlebtsov — 2007-03-25T20:43:48-00:00

Colloid Journal, Vol. 62, No. 6. (1 November 2000), pp. 765-779.

Controlled assembly of two-dimensional colloidal crystals

QH Wei — 2007-08-02T21:22:17-00:00

Applied Physics Letters, Vol. 77 (September 2000), 1641.

Under the influence of capillary forces, colloidal particles embedded in a soap film self-organize to form polycrystalline monolayers when the film is withdrawn from a concentrated suspension. Here, we show that mechanically generated capillary waves on the free surface of the bulk colloidal suspension can cause migration of grain boundaries, and under certain conditions, completely eliminate them. This dramatic effect, we call “mechanic annealing,” provides a robust means of growing two-dimensional single crystals with size that has never been achieved before. The method is expected to be applicable to a variety of self-assembling systems.

Tailored Surfaces Using Optically Manipulated Colloidal Particles

C Mio — 2007-03-21T20:14:00-00:00

Langmuir, Vol. 15, No. 25. (7 December 1999), pp. 8565-8568.

Abstract: Optical trapping techniques have been used extensively to manipulate biological objects and micrometer-sized colloids in a wide variety of investigations. We have used an extension of this technique, scanning laser optical trapping, to simultaneously trap multiple colloids in a designed pattern and have locked-in this artificially created structure through photopolymerization of the monomer-containing solvent. This technique can be used as a means of constructing templates for lithography or as a starting point for creation of larger three-dimensional colloidal structures.

The microrheology of colloidal dispersions : II. Brownian diffusion of doublets of spheres

EB Vadas — 2007-08-11T00:44:03-00:00

Journal of Colloid and Interface Science, Vol. 57, No. 2. (November 1976), pp. 308-326.

The rotary Brownian diffusion of isolated doublets of uniform spheres of 1- and 2-[mu]m diam was measured in water at rest and in shear (Poiscuille) flow through narrow tubes. The translational Brownian diffusion of single spheres and doublets was also measured in suspensions at rest; under the conditions of the Poiseuille flow experiments, translational diffusion was too small to be measured. Expressions for the orientation distribution function of a set of noninteracting doublets, initially randomly oriented were derived. From these, mean values of the change in particle orientation with time were computed and compared with experimental values from a large number of measurements on single, isolated doublets. In the stationary suspensions, the experimental and theoretical mean values were in good agreement over time intervals of up to 2 sec. In Poiseuille flow, the Brownian contribution to the change in particle orientation was calculated from the measured angular displacements by subtracting the theoretically predicted displacement due to shear and the mean values found to be in agreement with theory, i.e., Brownian motion is independent of shear over short time, t << T. Over longer times this agreement is no longer expected, as found experimentally.

Colloidal quantum dots. From scaling laws to biological applications

P Alivisatos — 2007-11-08T01:49:32-00:00

Pure Appl. Chem., Vol. 72, No. 1-2. (2000), pp. 3-9.

Over a twenty-year period, condensed matter physicists and physical chemists have elucidated a series of scaling laws which successfully describe the size dependence of solid state properties [1,2]. Often the experiments were performed under somewhat exotic conditions, for instance on mass-selected clusters isolated in molecular beams or on quantum dots grown by molecular beam epitaxy and interrogated at low temperatures and in high magnetic fields. As a result, we now have an understanding of how thermodynamic, optical, electrical, and magnetic properties evolve from the atomic to the solid state limit. This area of research is presently undergoing a remarkable transformation. The scaling laws, previously the direct subject of research, now provide a tool for the design of advanced new materials. In the case of colloidal quantum dots, or semiconductor nanocrystals, these new insights are poised to have impact in disciplines remote from solid state physics [3].

Metal-Enhanced Fluorescence (MEF) Due to Silver Colloids on a Planar Surface: Potential Applications of Indocyanine Green to in Vivo Imaging

CD Geddes — 2007-07-26T00:23:09-00:00

J. Phys. Chem. A, Vol. 107, No. 18. (8 May 2003), pp. 3443-3449.

Abstract: We examined the effects of metallic silver colloids on the fluorescence spectral properties of indocyanine green (ICG), which is a dye widely used for in vivo medical testing. Silver colloids from a suspension bind spontaneously to amine-coated surfaces. These colloid-coated surfaces were found to cause a 30-fold increase in the intensity of ICG, which was held close to the metal surface by adsorbed albumin. The increased intensities of ICG were also associated with decreased lifetimes and increased photostability, which are indicative of modifying the fluorophores radiative decay rate. These results suggest the use of metal colloid-enhanced ICG for applications to retinal angiography and vascular imaging and as a contrast agent for optical tomography.

Fluorescence properties of labeled proteins near silver colloid surfaces.

BP Maliwal — 2007-10-28T17:22:56-00:00

Biopolymers, Vol. 70, No. 4. (December 2003), pp. 585-594.

The fluorescence properties of a monolayer of labeled avidin molecules were studied near silver island films. We first adsorbed a monolayer of biotinylated-BSA as a base that was used to capture labeled avidin molecules. For labeled avidin on silver island films, we observed an increase of the fluorescence intensity of between 18 and 80 with one-photon excitation and up to several hundredfold or larger with two-photon excitation. The probes were moderately more photostable in the presence of silver islands. There was also a dramatic decrease in the lifetimes with the amplitude-weighted values decreasing from 7- to 35-fold. The data suggest that these spectral changes are due to both increased rates of excitation near the metallic particles and increases in the rates of radiative decay. Because these silver island surfaces are very heterogeneous, we are hopeful that larger increases in intensity and photostability can be obtained for probes situated at an optimal distance from the ideal island surfaces.

In Situ, Nonlinear Optical Probe of Surfactant Adsorption on the Surface of Microparticles in Colloids

H Wang — 2007-08-15T15:56:42-00:00

Langmuir, Vol. 16, No. 6. (21 March 2000), pp. 2475-2481.

Abstract: The demonstration of a nonlinear optical technique for directly monitoring adsorption of surfactants on the surface of microparticles in colloids is reported. In this approach, dye molecules with strong hyperpolarizability are first adsorbed on the particle surface to give detectable second-harmonic generation. The surfactant is then added to the colloidal solution in competition with dye for adsorption on the surface. The displacement of the dye molecules on the surface results in a decrease of the second-harmonic signal, indicating the adsorption of the surfactant molecules. A continuous flow/titration system in combination with a high-repetition-rate femtosecond laser allows the adsorption to be monitored in real time. This approach was first demonstrated on a methacrylate polymeric surfactant on latex and talc particles in an aqueous solution. The adsorption free energy and surface density of this surfactant on these particles have been determined.

Surface-enhanced luminescence from Eu3+ complex nearby Ag colloids

H Nabika — 2007-10-28T17:21:38-00:00

Eur. Phys. J. D, Vol. 24, No. 1-3. (2003), pp. 369-372.

The effect of colloidal dispersion of silver on the luminescent properties of Eu (III) complex with pyridine-3,5-dicarboxylic acid (PyDC) was investigated. The luminescence from Fo?rster type europium complex was enhanced several times with the presence of Ag colloid and the enhancement factor showed Ag concentration dependence. The observed enhancement effect was discussed in view of surface-enhancement effect and optical absorption due to surface plasmon resonance, both arising from excitation of surface plasmon polariton. The coordination structure around Eu (III) ion was also affected with the presence of Ag colloid, which induced the change in the intensity ratio between two emission bands of 5D0 ? 7F1 and 5D0 ? 7F2 transitions of Eu (III) ion.

Multilayers of colloidal particles

RK Iler — 2008-01-29T05:57:45-00:00

Journal of Colloid and Interface Science, Vol. 21, No. 6. (June 1966), pp. 569-594.

A new technique has been developed by which alternate layers of positively and negatively charged colloidal particles, such as silica and alumina, can be deposited from sols onto a smooth surface such as glass. By this means films of controlled, uniform thickness can be built up, showing interference colors. By using uniform colloidal particles such as silica or polystyrene latex about 100 m[mu] in diameter as a visible indicator film, the adsorption of invisibly small particles, polyvalent ions, surfactants, and water-soluble polymers can be simply observed and studied.

Capillary forces and structuring in layers of colloid particles

PA Kralchevsky — 2007-08-06T02:39:25-00:00

pp. 383-401.

'Capillary forces' are interactions between particles mediated by fluid interfaces. Recent advances in this field have been achieved by experiments and theory on lateral capillary forces, which are due to the overlap of menisci formed around separate particles attached to an interface. In particular, we should mention the cases of 'finite menisci' and 'capillary multipoles'. The capillary-bridge forces were investigated in relation to capillary condensation and cavitation, surface-force measurements and antifoaming by oily drops. The studies on colloidal self-assembly mediated by capillary forces developed in several promising directions. The obtained structures of particles have found numerous applications.

Optical Trapping of Titania/Silica Core-Shell Colloidal Particles

P Viravathana — 2008-01-13T05:59:30-00:00

Journal of Colloid and Interface Science, Vol. 221, No. 2. (15 January 2000), pp. 301-307.

Manipulation of colloidal systems via optical trapping techniques requires a refractive index mismatch between particles and solvent which leads to strong interparticle van der Waals interactions. Investigation of the behavior of systems without such strong attractive interactions, however, requires the uncoupling of particle refractive index and particle-particle interactions. To accomplish this, the synthesis of core-shell titania/silica particles has been performed. By index matching a silica shell on a titania core using a mixture of toluene and propanol, the van der Waals interactions between particles can be minimized. Due to the mismatch of the refractive index between the solvent and titania core, however, a strong trapping force can be generated, making optical manipulation feasible. In order to confirm that the silica shell was indeed matched, pure silica particles were synthesized by the method of Stober (1968) and added to the core-shell system. In these mixed systems of core-shell and pure silica particles in silica-index-matching solvents, only the core-shell particles were trappable.

A DNA-based method for rationally assembling nanoparticles into macroscopic materials

Chad Mirkin — 2007-03-14T19:31:29-00:00

Nature, Vol. 382, No. 6592. (1996), pp. 607-609.

Low-frequency complex magnetic susceptibility of magnetic composite microspheres in colloidal dispersion

Ben Erne — 2007-12-17T20:04:43-00:00

Journal of Magnetism and Magnetic Materials, Vol. 311, No. 1. (April 2007), pp. 145-149.

A procedure is presented to determine the permanent magnetic dipole moment of composite microspheres containing magnetic nanoparticles with a blocked magnetic dipole moment. The composite particles are dispersed in a solvent, and the complex magnetic susceptibility is measured from 0.1 to 1000 Hz using a highly sensitive new setup. Composite particles with a permanent magnetic dipole moment are revealed by a characteristic frequency that corresponds to the Brownian rotation of the microspheres. From measured susceptibility spectra, we calculate the permanent magnetic dipole moment of recently developed cobalt ferrite-doped silica and latex microspheres.

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.

Microscopic artificial swimmers

Rémi Dreyfus — 2005-10-05T22:35:21-00:00

Nature, Vol. 437, No. 7060., pp. 862-865.

Self-Motile Colloidal Particles: From Directed Propulsion to Random Walk

JR Howse — 2007-10-26T18:01:29-00:00

Physical Review Letters, Vol. 99, No. 4. (July 2007), 048102.

The motion of an artificial microscale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substantially enhanced diffusion coefficient. Our results suggest strategies for designing artificial chemotactic systems.

Fluorescence spectra characters of silver-coated gold colloidal nanoshells

Z Jian — 2007-10-28T16:59:08-00:00

Colloids Surf. A Physicochem. Eng. Asp., Vol. 232, No. 2-3. (2004), pp. 155-161.

Aqueous solutions containing a high yield of suspended pure silver, pure gold and silver-coated gold nanoshells have been synthesized via an electrochemical method. UV-vis absorption spectra and fluorescence emission spectra have been measured at room temperature. The absorption spectra show two peaks at 445 and 485nm, which due to the plasmon resonance of silver-coated gold nanoparticles. Fluorescence emission peaks have been observed in the wavelength region of 550-700nm. The emission peak corresponding to solid silver particles is more intensive than the peaks corresponding to silver-coated gold nanoshells and solid gold particles. The relationship between relative thickness of silver shell and fluorescence intensity has been illuminated from the quasi-static and local field theory. With decreasing the thickness of silver shell, the maximum of local field enhancement factor decrease and red shifts nonlinearly. © 2003 Elsevier B.V. All rights reserved.

Rotational diffusion of colloidal particles near confining walls

RB Jones — 2007-07-26T03:23:32-00:00

The Journal of Chemical Physics, Vol. 123, No. 16. (2005)

We study the rotational diffusion of a spherical colloid confined in a narrow channel between parallel plane hard walls. The walls damp translational diffusion much more than rotational diffusion so that there is expected to be little translation-rotation coupling. Using a recent calculation of the nonisotropic rotational mobilities arising from the hydrodynamic interactions with the walls, we set up the rotational Smoluchowski equation for either a particle with a permanent dipole moment or a polarizable particle with axisymmetric polarizabilities subject to an external electric field. Using the Smoluchowski equation dynamics we calculate the time-correlation functions of orientation that are measured in depolarized light scattering for the cases of no external field, external field normal to the walls, and external field parallel to the walls. The decay of correlations is shown to be given by a weighted sum of decaying exponentials and can be characterized by an initial and a mean characteristic decay time. The weights and decay rates of each component and the characteristic decay times are studied numerically for a range of field strengths. The nonisotropic rotational mobilities make these decay times highly sensitive to the distance of the particle from the confining walls. This position dependence can be used as a method of measuring the rotational mobilities or, conversely, the rate of decay of correlations can be used as a probe of particle position between the confining walls. ©2005 American Institute of Physics

Direct Observation of Hydrodynamic Rotation-Translation Coupling between Two Colloidal Spheres

S Martin — 2007-07-03T19:15:21-00:00

Physical Review Letters, Vol. 97, No. 24. (2006)

By combining optical tweezers with polarization microscopy, the hydrodynamic coupling between position and orientation fluctuations in a pair of colloidal spheres has been measured. Imaging of birefringent particles under crossed polarizers allows for the simultaneous determination of the positions and orientations of both particles. The temporal cross-correlation function between random displacements of one particle and orientation fluctuations of its neighbor allows for the quantification of the hydrodynamic rotation-translation coupling between the spheres. Our results are in good agreement with predictions for the hydrodynamic mobility tensors calculated in the creeping-flow limit of the Navier-Stokes equation.

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.

Kinetically Locked-In Colloidal Transport in an Array of Optical Tweezers

Pamela Korda — 2008-01-13T01:01:42-00:00

Physical Review Letters, Vol. 89, No. 12. (2002), 128301.

We describe measurements of colloidal transport through arrays of micrometer-scale potential wells created with holographic optical tweezers. Varying the orientation of the trap array relative to the external driving force results in a hierarchy of lock-in transitions analogous to symmetry-selecting processes in a wide variety of systems. Focusing on colloid as a model system provides the first opportunity to observe the microscopic mechanisms of kinetic lock-in transitions and reveals a new class of statistically locked-in states. This particular realization also has immediate applications for continuously fractionating particles; biological cells; and macromolecules.

Measurement of the hydrodynamic corrections to the Brownian motion of two colloidal spheres

JC Crocker — 2007-08-13T20:05:06-00:00

J. Chem. Phys., Vol. 106 (February 1997), pp. 2837-2840.

Not Available

Elasticity-Mediated Self-Organization and Colloidal Interactions of Solid Spheres with Tangential Anchoring in a Nematic Liquid Crystal

II Smalyukh — 2007-07-29T22:15:27-00:00

Physical Review Letters, Vol. 95, No. 15. (2005)

Using laser tweezers, we study colloidal interactions of solid microspheres in the nematic bulk caused by elastic distortions around the particles with tangential surface anchoring. The interactions overcome the Brownian motion when the interparticle separation rp is less than 3 particle diameters. The particles attract when the angle between rp and the uniform far-field director n^0 is between 0° and 70° and repel when 75°90°. The particles aggregate in chains directed at 30° to n^0 and, at higher concentrations, form complex kinetically trapped structures.

Optical trapping of colloidal particles and measurement of the defect line tension and colloidal forces in a thermotropic nematic liquid crystal

II Smalyukh — 2007-07-29T22:13:23-00:00

Applied Physics Letters, Vol. 86 (January 2005), 1913.

We demonstrate optical trapping and manipulation of transparent microparticles suspended in a thermotropic nematic liquid crystal with low birefringence. We employ the particle manipulation to measure line tension of a topologically stable disclination line and to determine colloidal interaction of particles with perpendicular surface anchoring of the director. The three-dimensional director fields and positions of the particles manipulated by laser tweezers are visualized by fluorescence confocal polarizing microscopy.

Reflection-mode scattering-type scanning near-field optical microscope using a laser trapped gold colloidal particle as a scattering probe

T Konada — 2007-08-29T03:37:40-00:00

Journal of Applied Physics, Vol. 89 (June 2001), pp. 6481-6483.

We have developed a reflection-mode scattering-type scanning near-field optical microscope using a laser trapped gold colloidal particle as a scattering probe and succeeded in observing the reflectance change of an opaque semiconductor sample with the alternating layers of GaAs and Al0.55Ga0.45As. The spatial resolution became as high as 200 nm when using a 200 nm gold colloidal particle. The results indicated that the resolution obtained in the experiment is in good agreement with the trapped particle size and overcame the diffraction limit (420 nm) of the lens system.

The microrheology of colloidal dispersions : IV. Pairs of interacting spheres in shear flow

TGM van de Ven — 2007-12-22T20:50:47-00:00

Journal of Colloid and Interface Science, Vol. 57, No. 3. (December 1976), pp. 505-516.

The effect of interaction forces on the trajectories of pairs of equal-sized spheres in shear flow is calculated. Depending on the functional form of the interaction potential there are three modes of behavior: (i) the spheres can collide and form a touching doublet, (ii) they can be captured in an energy minimum, orbiting around each other after capture, or (iii) the interactions are transitory and the spheres separate. For interactions determined jointly by van der Waals attraction and electrostatic repulsion, we have calculated the conditions under which (i) touching doublets from (i.e., in the so-called primary energy minimum) and the system coagulates, (ii) nontouching doublets form (in a secondary minimum) and the system flocculates, or (iii) no permanent doublets form and the system is stable. The results indicate that when Brownian motion can be neglected, dilute spherical sols can be unstable at low and high shear rates, but stable in between.

Measurement of the effective refractive index of a turbid colloidal suspension using light refraction

A Reyes-Coronado — 2005-04-06T19:51:30-00:00

New Journal of Physics, Vol. 7, No. 1. (April 2005), 89.

A sphere buoyant in a dilute polymer solution

S Grisafi — 2006-07-07T14:28:21-00:00

Journal of Applied Polymer Science, Vol. 40, No. 5-6. (1990), pp. 781-788.

The density and pressure fields of a dilute polymer solution are nonuniform at the surface of a small sphere even when there is no fluid motion. The polymer solution is modeled by an ensemble of Hookean dumbbells. A hard core potential is assumed as the only interaction between the sphere and the macromolecules. The pressure field about the sphere is transversely isotropic. The perpendicular component of the pressure at the interface is greater than the isotropic pressure in the bulk of the fluid. The parallel component of the pressure at the interface is less than the bulk isotropic pressure. The surface adsorption (cf., surface pressure) and the film thickness vary with a parameter that is the ratio of the sphere radius, a, to the root mean square extension of the dumbbells, R0. The spherical results coincide with the plane wall results for a/R0 greater than approximately 100.

Suspensions of Rodlike Particles: Literature Review and Data Correlations

E Ganani — 2007-12-28T22:44:35-00:00

Journal of Composite Materials, Vol. 19, No. 3. (1 January 1985), pp. 194-215.

Recent experimental work on the macroscopic rheological properties of suspensions of rodlike particles in Newtonian and non-Newtonian fluids is reviewed. Correlations are proposed for the shear viscosity-shear rate relationship for semi-concentrated and concentrated suspensions. In the former case, shear thinning behavior is found with the slope of the power law region depending upon fiber aspect ratio. For the concen trated systems, the data are substantially independent of volume fraction and aspect ratio and show slight shear thinning. 10.1177/002199838501900301

Clusters of Amphiphilic Colloidal Spheres

L Hong — 2008-02-29T20:20:35-00:00

Langmuir, Vol. 24, No. 3. (5 February 2008), pp. 621-625.

Abstract: Orientation-dependent interactions can drive unusual self-assembly of colloidal particles. This study, based on combined epifluorescence microscopy and Monte Carlo simulations, shows that amphiphilic colloidal spheres, hydrophobic on one hemisphere and charged on the other, assemble in water into extended structures not formed by spheres of uniform surface chemical makeup. Small, compact clusters each comprised of less than 10 of these Janus spheres link up, as increasing salt concentration enhances electrostatic screening, into wormlike strings.

Equilibrium Glassy Phase in a Polydisperse Hard-Sphere System

Pinaki Chaudhuri — 2008-05-05T20:00:41-00:00

Physical Review Letters, Vol. 95, No. 24. (2005)

The phase diagram of a polydisperse hard-sphere system is examined by numerical minimization of a discretized form of the Ramakrishnan-Yussouff free-energy functional. Crystalline and glassy local minima of the free energy are located and the phase diagram in the density-polydispersity plane is mapped out by comparing the free energies of different local minima. The crystalline phase disappears and the glass becomes the equilibrium phase beyond a "terminal" value of the polydispersity. A crystal-to-glass transition is also observed as the density is increased at high polydispersity. The phase diagram obtained in our study is qualitatively similar to that of hard spheres in a quenched random potential.

Imaging the Sublimation Dynamics of Colloidal Crystallites

JR Savage — 2007-04-02T22:50:47-00:00

Science, Vol. 314, No. 5800. (3 November 2006), pp. 795-798.

We studied the kinetics of sublimating crystals with single-particle resolution by experiments with colloidal spheres and by computer simulations. A short-range attraction between spheres led to crystallites one to three layers thick. The spheres were tracked with optical microscopy while the attraction was reduced and the crystals sublimated. Large crystallites sublimated by escape of particles from the perimeter. The rate of shrinkage was greatly enhanced, however, when the size decreased to less than 20 to 50 particles, depending on the location in the phase diagram. At this size, the crystallites transformed into a dense amorphous structure, which rapidly vaporized. The enhancement of kinetics by metastable or unstable phases may play a major role in the melting, freezing, and annealing of crystals. 10.1126/science.1128649