SEARCH · Search PubMed

Results for “Colloids”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,333 records · Page 74Linked to original sources

PubMed2004

Synthesis of amorphous silicon colloids by trisilane thermolysis in high temperature supercritical solvents.

Colloidal submicrometer-diameter amorphous silicon (a-Si) particles are synthesized with >90% yield by thermal decomposition of trisilane (Si3H8) in supercritical hexane at temperatures ranging from 400 to 500 degrees C and pressures up to 345 bar. A range of synthetic conditions was explored to optimize the quality of the product. Under the appropriate synthetic conditions, the colloids are spherical and unagglomerated. The colloids can be produced with average diameters ranging from 50 to 500 nm by manipulating the precursor concentration, temperature, and pressure. Relatively narrow particle size distributions, as measured by transmission electron microscopy (TEM), with standard deviations about the mean as low as approximately +/-10% could be obtained in some cases. We explored the thermal annealing of the amorphous silicon particles after isolation from the reactor and found that crystallization to diamond structure silicon occurred at temperatures as low as 650 degrees C. The amorphous and crystalline materials were characterized by X-ray diffraction and high resolution scanning and transmission electron microscopy.

Journal Article↗

PubMed2004

Colloidal structures from bulk demixing in liquid crystals.

This experimental paper deals with phase separations of binary mixtures composed of a continuous liquid crystal phase and an isotropic dispersed phase. In contrast to isotropic binary mixtures, the investigated mixtures do not lead to a full phase separation but to a self-ordering of colloidal particles, as reported earlier (Loudet, J. C. et al. Nature 2000, 407, 611). We present here further aspects of such phase separations which include the kinetics of the phase separation, the origin of the formation of dislocation-like patterns, the influence of surfactants, chiral additives, and temperature on the formed colloidal structures. The present results show that (i) the dislocations in chain arrays can be seen as kinetically frozen defects, (ii) temperature can be used to control the size of the domains formed upon demixing, (iii) a slight change in surface chemistry, via the addition of surfactants, profoundly alters the formed colloidal structures, and (iv) chiral additives allow the formation of unique helical pearl chains which reflect the symmetry of the liquid crystal phase they are embedded in.

Journal Article↗

PubMed2004

Are thermoresponsive microgels model systems for concentrated colloidal suspensions? A rheology and small-angle neutron scattering study.

The structure of concentrated temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel suspensions has been investigated employing rheology and small-angle neutron scattering (SANS). A previously described model expression for the particle form factor P(inho)(q) is extended by a model hard sphere structure factor S(q), and the average radial density profiles phi(r) are calculated from the amplitude of the form factor A(q) and the structure factor S(q). By this procedure, a direct real space description of the spatial ordering in the neighborhood of a single particle is obtained. The overall particle size and the correlation length xi of the concentration fluctuations of the internal polymer network decrease with concentration, revealing the increasing compression of the spheres. Thus, the particle form factor P(inho)(q) of the swollen PNiPAM microgels depends on concentration. The particle-particle interaction potential does not change significantly between 25 and 32 degrees C. Even approximately 1 K below the lower critical solution temperature (LCST), the experimental scattering intensity distributions I(q)/c are described very well by the hard sphere structure factor when an equivalent hard sphere particle size R(HS) and volume fraction eta(HS) are used. Microgels with different degrees of cross-linking and particle size resemble true hard sphere behavior up to effective volume fractions of phi(eff) < 0.35. At higher effective volume fractions phi(eff) > 0.35 strong deviations from true hard spheres are observed. Interpenetration of the outer, less cross-linked regions of the soft spheres as well as particle compression occurred at higher concentrations. In agreement with this, the equilibrium colloidal phase behavior and rheology also has some features of soft sphere systems. At temperatures well above the LCST, the interaction potential becomes strongly attractive and the collapsed microgel spheres form aggregates consisting of flocculated particles without significant long-range order. Hence, an attractive interaction potential in concentrated suspensions of PNiPAM microgels leads to distinctively different structures as compared to attractive hard sphere colloids. When the peculiar structural properties of the PNiPAM microgels are considered, they can be used as model systems in colloidal science.

Journal Article↗

PubMed2005

Stimuli-responsive surface localized ionic cluster (SLICs) formation from nonspherical colloidal particles.

Structural features of phospholipids provide a unique opportunity for utilizing these amphiphilic species to stabilize the synthesis of colloidal dispersion particles by controlling concentration levels relative to dispersion synthesis components. 1,2-Bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DCPC) phospholipid was utilized as cosurfactant in the synthesis of sodium dioctyl sulfosuccinate (SDOSS) stabilized methyl methacrylate/n-butyl acrylate (MMA/nBA) colloidal dispersions. Aqueous dispersions containing various concentration levels of DCPC result in the formation of cocklebur particle morphologies, and when prepared in the presence of Ca2+ and annealed at various temperatures, stimuli-responsive behaviors of coalesced films were elucidated. The formation of surface localized ionic clusters (SLICs) at the film-air (F-A) and film-substrate (F-S) interfaces is shown to be responsive to concentration levels of DCPC, Ca2+/DCPC ratios, and temperature. These studies show that it is possible to control stratification and mobility to the F-A and F-S interfaces during and after coalescence. Using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and internal reflection infrared imaging (IRIRI) spectroscopies, molecular entities responsible for SLIC formation were determined. These studies also show that stimuli-responsive behaviors during film formation can be controlled by colloidal solution morphologies and synergistic interactions of individual components.

Journal Article↗

PubMed2005

Measurement and interpretation of particle-particle and particle-wall interactions in levitated colloidal ensembles.

This paper reports measurements of particle-wall and particle-particle interactions in levitated colloidal ensembles using integrated total internal reflection microscopy (TIRM) and video microscopy (VM) techniques. In levitated colloidal ensembles with area fractions of phiA = 0.03-0.25, ensemble TIRM measured height distribution functions are used to interpret particle-wall interactions, and VM measured pair distribution functions are used to interpret particle-particle interactions using inverse Ornstein-Zernike (OZ) and three-dimensional inverse Monte Carlo (MC) analyses. An inconsistent finding is the observation of an anomalous long-range particle-particle attraction and recovery of the expected Derjaguin-Landau-Verwey-Overbeek (DLVO) particle-wall interactions for all concentrations examined. Because particle-wall and particle-particle potentials are expected to be consistent in several respects, the analytical and experimental methods employed in this investigation are examined for possible sources of error. Comparison of inverse OZ and three-dimensional inverse MC analyses are used to address uncertainties related to dimensionality, effects of particle concentration, and assumptions of the OZ theory and closure relations. The possible influence of charge heterogeneity and particle size polydispersity on measured distribution functions is discussed with regard to inconsistent particle-wall and particle-particle potentials. Ultimately, achieving a consistent understanding of particle-wall and particle-particle interactions in interfacial and confined colloidal systems is essential to numerous complex fluid and advanced material technologies.

Journal Article↗

PubMed2005

Self-assembly of colloidal pyramids in magnetic fields.

We study routes toward the construction of 2D colloidal pyramids. We find that magnetic beads may self-assemble into pyramids near a nonmagnetic 1D boundary as long as the number of beads in the pyramid does not exceed 10. We have also found that a strong magnetic field gradient could act as a boundary, thus assisting the self-assembly of magnetic colloids in water, and have observed the formation of stable microscopic pyramids within a certain magnetic field range. Our results indicate that colloidal pyramids can be formed in a number of ways by utilizing external fields.

Journal Article↗

PubMed2005

Theoretical study of cooperativity in multivalent polymers for colloidal stabilization.

Multivalent polymers, i.e., copolymers with multiple binding sites, have been proposed recently for stabilization of fusogentic liposomes and other liposomal colloids useful for drug delivery. The performance of such polymers critically depends on their molecular architecture, in particular the strength and frequency of surface anchoring sites along the backbone of a highly soluble polymer. In this work, we investigate the adsorption and surface forces due to multivalent polymers based on coarse-grained polymer models. We find that for W-type polymers that form dangling tails when all anchoring segments are attached to a surface, increasing the chain length at fixed polymer composition leads to a stronger repulsive barrier in the polymer-mediated surface forces thereby increasing the ability of the polymer to stabilize colloidal particles. This prediction conforms to an earlier experiment indicating that increasing the number of hydrophobic anchors along poly(ethylene glycol) polymers results in the cooperative behavior for both surface adsorption and steric stabilization. For M-type multivalent polymers that have weakly anchoring sites placed at the ends, however, addition of binding sites at fixed polymer composition could lead to negative cooperativity, i.e., the more binding sites, the less the amount of adsorption or the weaker the ability of surface protection. The theory also predicts that polymers with two anchoring sites (e.g., telechelic copolymers) are most efficient for colloidal stabilization.

Journal Article↗

PubMed2006

Photophysical effects between spirobenzopyran-methyl methacrylate-functionalized colloidal particles.

Colloidal particles were derivatized with end-grafted polymethylmethacrylate polymer brushes containing varying concentrations of spirobenzopyran photochromic molecules. The polymers were grown from initiator-functionalized silica particles by an atom-transfer radical polymerization (ATRP). These core-shell colloids formed stable suspensions in toluene with the spirobenzopyran in its closed, nonpolar form. However, UV-induced photoswitching of the photochrome to its open, polar merocyanine isomer caused rapid aggregation. The nature of this colloidal stability transition was examined with respect to the spirobenzopyran content in the polymeric brush and solvent polarity. Turbidimetry, wettability studies, UV-vis spectroscopy, suspension rheology, SEM, and visual inspection were utilized to characterize the system photoswitchability. It was found that the system exhibiting the greatest transition in toluene was the copolymer brush composed of 20% spirobenzopyran and 80% methyl methacrylate.

Journal Article↗

PubMed2006

Doped colloidal photonic crystal structure with refractive index chirping to the [111] crystallographic axis.

A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the face-centered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.

Journal Article↗

PubMed2006

Size-dependent self-organization of colloidal particles on chemically patterned surfaces.

A study of the self-organization of colloidal particles during the evaporation of particle solutions on chemically patterned surfaces is presented. On a surface with hydrophilic and hydrophobic regions, colloidal particles form compact structures on the hydrophilic sites. When a colloidal solution containing a mixture of particles with a variation in size is used, the number density of each type of particle deposited on the hydrophilic islands after evaporation decreases with increasing particle size. This makes it possible to produce a concentration gradient of the particles on islands of different sizes. It is shown that this technique could allow for particle separation.

Journal Article↗

PubMed2006

Formation and drying of colloidal crystals using nanosized silica particles.

Much interest has been generated in the fabrication of colloidal crystals from suspensions because of the promise of photonic band gap applications. However, since the case of small, nonsedimenting colloidal particles indeed remains rather rarely treated, spherical silica particles with diameters varying from 75 down to 20 nm have been used in the present work to fabricate colloidal crystals by drying the suspending liquid. Typical events that take place during the drying process of a particulate film, such as cracking, compaction and penetration of air into a porous network, have been evaluated using existing theories, and the maximum stress in the drying film could be approximated. Investigation on the dry film structure by scanning electron microscopy showed the arrangement of particles in a close-packed system. To interpret the formation of such crystals, the amplitudes of the interparticle and capillary forces have been estimated from existing models. The repulsive interparticle forces allow the particles to remain stable and thus rearrange up to fairly high particle concentration. These modeling results showed the dominance of the capillary contribution at the end of the drying process. Nitrogen adsorption/desorption measurements gave very coherent results regarding both pore volume and pore size of the dry particulate films when compared to the expected ordered packing arrangements.

Journal Article↗

PubMed2006

Evolution of interparticle capillary forces during drying of colloidal crystals.

Photonic crystals are periodic structures that have the capability to manipulate the photons in the same way as semiconductors do for electrons. The self-assembly strategy that utilizes colloidal crystals as a template to form photonic crystals has received a great deal of recent research interest because it is simple and cost-effective. Experimental studies and theoretical analysis have speculated that capillary forces play a pivotal role in forming the colloidal crystals during the crystal growth process and that particularly during the drying stage the changing of the magnitude of capillary forces is critical to the resultant microstructure. This paper presents a computational analysis of the changing capillary forces, which may throw light on a refined strategy for controlling colloidal crystal growth.

Journal Article↗

PubMed2006

Fuzzy ternary particle systems by surface-initiated atom transfer radical polymerization from layer-by-layer colloidal core-shell macroinitiator particles.

We report the synthesis of ternary polymer particle material systems composed of (a) a spherical colloidal particle core, coated with (b) a polyelectrolyte intermediate shell, and followed by (c) a grafted polymer brush prepared by surface-initiated polymerization as the outer shell. The layer-by-layer (LbL) deposition process was utilized to create a functional intermediate shell of poly(diallyl-dimethylammonium chloride)/poly(acrylic acid) multilayers on the colloid template with the final layer containing an atom transfer radical polymerization (ATRP) macroinitiator polyelectrolyte. The intermediate core-shell architecture was analyzed with FT-IR, electrophoretic mobililty (zeta-potential) measurements, atomic force microscopy, and transmission electron microscopy (TEM) techniques. The particles were then utilized as macroinitiators for the surface-initiated ATRP grafting process for poly(methyl methacrylate) polymer brush. The polymer grafting was confirmed with thermo gravimetric analysis, FT-IR, and TEM. The polymer brush formed the outermost shell for a ternary colloidal particle system. By combining the LbL and surface-initiated ATRP methods to produce controllable multidomain core-shell architectures, interesting functional properties should be obtainable based on independent polyelectrolyte and polymer brush behavior.

Journal Article↗

PubMed2006

Colloidal woodpile structure: three-dimensional photonic crystal with a dual periodicity.

With planar photolithography and self-assembly techniques, multilayer colloidal crystals with a woodpile structure were fabricated. They represent a new kind of photonic crystals, that is, three-dimensional (3D) photonic crystals with a dual periodicity; one comes from the face-centered cubic (fcc) structure within the colloidal crystal strips and the other one results from the periodic arrangement of the colloidal crystal strips.

Journal Article↗

PubMed2006

Rotation in suspension of a rod-shaped colloid.

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.

Journal Article↗

PubMed2006

Non-affine structural evolution of soft colloidal crystalline latex films under stretching as observed via synchrotron X-ray scattering.

A polymer dispersion consisting of soft latex spheres with a diameter of 135 nm was used to produce a crystalline film with face-centered cubic (fcc) packing of the spheres. Different from conventional small-molecule and hard-sphere colloidal crystals, the crystalline latex film in the present case is soft (i.e., easily deformable). The structural evolution of this soft colloidal latex film under stretching was investigated by in-situ synchrotron ultra-small-angle X-ray scattering. The film exhibits polycrystalline scattering behavior corresponding to fcc structure. Stretching results not only in a large deformation of the crystallographic structure but also in considerable nonaffine deformation at high draw ratios. The unexpected nonaffine deformation was attributed to slippage between rows of particles and crystalline grain boundaries. The crystalline structure remains intact even at high deformation, suggesting that directional anisotropic colloidal crystallites can be easily produced.

Journal Article↗

PubMed2006

Patterned assembly of colloidal particles by confined dewetting lithography.

We report the assembly of colloidal particles into confined arrangements and patterns on various cleaned and chemically modified solid substrates using a method which we term "confined dewetting lithography" or CDL for short. The experimental setup for CDL is a simple deposition cell where an aqueous suspension of colloidal particles (e.g., polystyrene spheres) is placed between a floating deposition template (i.e., metal microgrid) and the solid substrate. The voids of the deposition template serve as an array of micrometer-sized reservoirs where several hydrodynamic processes are confined. These processes include water evaporation, meniscus formation, convective flow, rupturing, dewetting, and capillary-bridge formation. We discuss the optimal conditions where the CDL has a high efficiency to deposit intricate patterns of colloidal particles using polystyrene spheres (PS; 4.5, 2.0, 1.7, 0.11, 0.064 microm diameter) and square and hexagonal deposition templates as model systems. We find that the optimization conditions of the CDL method, when using submicrometer, sulfate-functionalized PS particles, are primarily dependent on minimizing attractive particle-substrate interactions. The CDL methodology described herein presents a relatively simple and rapid method to assemble virtually any geometric pattern, including more complex patterns assembled using PS particles with different diameters, from aqueous suspensions by choosing suitable conditions and materials.

Journal Article↗

PubMed2001

Suppression of crystal nucleation in polydisperse colloids due to increase of the surface free energy.

The formation of small crystallites is governed by two competing factors: the free energy gained upon transferring constituent atoms, molecules or colloidal particles from the metastable liquid to the more stable solid, and the free energy needed to create the surface area of the crystallite. Because the ratio of surface area to bulk is large for small particles, small crystallites dissolve spontaneously under conditions where larger crystallites are stable and macroscopic crystal growth occurs only if spontaneously formed crystallites exceed a critical minimum size. On theoretical grounds, the probability of forming such critical crystal nuclei is expected to increase rapidly with supersaturation. However, experiments show that the rate of crystal nucleation in many systems goes through a maximum as the supersaturation is increased. It is commonly assumed that the nucleation rate peaks because, even though the probability of forming critical nuclei increases with increasing concentration, the rate of growth of such nuclei decreases. Here we report simulations of crystal nucleation in suspensions of colloidal spheres with varying size distributions that show that the probability that critical nuclei will form itself goes through a maximum as the supersaturation is increased. We find that this effect, which is strongest for systems with the broadest particle size distribution, results from an increase with supersaturation of the solid-liquid interfacial free energy. The magnitude of this effect suggests that vitrification at high supersaturations should yield colloidal glasses that are truly amorphous, rather than nano-crystalline.

Journal Article↗