Anisotropic colloidal particles in critical fluids

We consider anisotropic colloidal particles with dumbbell or lens shapes that are immersed in a critical binary fluid mixture. The orientation-dependent long-ranged universal interactions mediated by the critical solvent between a particle and a wall or between two particles are investigated for mesoscopic particle sizes small compared to the correlation length and interparticle distances. Exact results are obtained using a "small particle operator expansion." The amplitudes of the isotropic and anisotropic operators in the expansion depend on the size and aspect ratio of the dumbbell or lens and are determined by density profiles in the Ising model at the critical point in a wedge geometry with symmetry-breaking fixed-spin boundary conditions. Dumbbells and ellipsoids with a symmetry preserving surface are also considered.     

Anisotropic colloidal particles interacting with polymers in a good solvent

We study the consequences of chain self-avoidance for the interaction between nonadsorbing polymers and colloidal particles of anisotropic shape, such as ellipsoids, lenses, and dumbbells. In the framework of a field theoretic operator expansion for small mesoscopic particles, we obtain exact results for self-avoiding polymers in d=2 spatial dimensions and we compare ideal and self-avoiding polymers in 2相似文献   

Anisotropic and hindered diffusion of colloidal particles in a closed cylinder

Video microscopy and particle tracking were used to measure the spatial dependence of the diffusion coefficient (D(α)) of colloidal particles in a closed cylindrical cavity. Both the height and radius of the cylinder were equal to 9.0 particle diameters. The number of trapped particles was varied between 1 and 16, which produced similar results. In the center of the cavity, D(α) turned out to be 0.75D(0) measured in bulk liquid. On approaching the cylindrical wall, a transition region of about 3 particle diameters wide was found in which the radial and azimuthal components of D(α) decrease to respective values of 0.1D(0) and 0.4D(0), indicating asymmetrical diffusion. Hydrodynamic simulations of local drag coefficients for hard spheres produced very good agreement with experimental results. These findings indicate that the hydrodynamic particle-wall interactions are dominant and that the complete 3D geometry of the confinement needs to be taken into account to predict the spatial dependence of diffusion accurately.     

Kinetic features of plasma etching of polycarbonate in oxygen plasma

Changes in surface properties and composition of the surface layer of polycarbonate of the Lexan LS2 and Lexan 8010 brands after treatment in the positive column of glow-discharge oxygen plasma have been studied. The amount of the polymer loaded for processing has been shown to affect the kinetics of etching and modifying the polycarbonate surface. It has been found that an increase in the amount of the polymer in the plasma reduces its etching rate and also affects the electrical parameters of plasma, which in turn can cause a change in the flux of active species onto the polymer surface and, consequently, a change in the rate of plasma-initiated heterogeneous processes.     

Novel fabrication of Janus particles from the surfaces of electrospun polymer fibers

A novel synthetic approach for the efficient fabrication of Janus silica particles was demonstrated by embedment of zero-dimensional colloids on one-dimensional polymer fiber surfaces, followed by the surface modification on the exposed silica hemispheres. Electrospinning of poly(methyl methacrylate) and poly(4-vinyl pyridine) blends produced polymer fibers with high specific surface area and desired surface hydrophilicities. Fiber compositions determined the colloid adsorption density and uniformity. The colloid embedding resulted from the polymer softening was manipulated by the isothermal heat treatment. Subsequent silianization completed the amino functionalities on hemispherical surfaces of embedded silica colloids. Janus particles with uniform asymmetric chemical features were further labeled with gold nanoparticles before their recovery from fiber substrates. Fabrication of Janus particles, including colloid adsorption, temperature-driven embedding, and hemispherical surface modification, were investigated and are discussed.     

Dry etching of colloidal crystal films

Two types of non-close-packed colloidal crystal films were prepared by etching the films made of polystyrene nanospheres using a hyperthermal neutral beam of oxygen gas. Etching without sintering above glass transition temperature of the polymer particles resulted in the non-close-packed structure of the nanospheres, in which polystyrene nanospheres in different lattice planes touched each other due to the reduction in the size of the nanospheres that occurred during the etching process. In contrast, a different non-close-packed structure with inter-connecting networks between etched nanospheres was generated by annealing of the colloidal crystal and a subsequent etching process. The photonic bandgap could be tuned during this dry etching of colloidal photonic crystals. This connected open structure could be used as a template for a silica inverse opal by chemical vapor deposition. An alternative dry etching process, reactive ion etching, mainly affected the morphology of particles near the top surface, and only a slight change in the stop band position of the colloidal crystal film was observed.     

Fabrication of one-dimensional colloidal assemblies from electrospun nanofibers

Electrospun nanofibers were used as confining geometries for fabricating 1-D colloidal assemblies. Silica particles dispersed in several different polymer solutions were cast into nanofibers by an electrospinning process. The silica particle configurations were examined in terms of the size ratio of silica particles to nanofibers and the properties of the dispersing medium. As the electrospun fiber was extended highly, the silica particles dispersed in the polymer solution began to assemble spontaneously into a pearl-necklace structure. We also demonstrated the alignment of 1-D silica assemblies using a designed configuration of collector electrodes.     

Analysis of colloidal particles V. size-exclusion chromatography of colloidal semiconductor particles

An HPLC technique for the size determination of colloidal cadmium sulphide and zinc sulphide in a diameter range from 20 down to 2 nm using silica with pore sizes from 30 to 100 nm is described. The growth of the particles during the run was suppressed by the addition of stabilizers to the eluent and by the use of reversed-phase silica as the stationary phase for inorganic stabilizers. The calibration of the column sets by electron microscopy resulted in a linear relationship between the logarithm of the particle diameter and the elution time. The analysis was carried out within 4–10 min. The lateral resolution lay between 1.3% for larger particles and 1.9% for smaller particles. Below a diameter of 13 nm these values were better than those found from electron microscopy. From the comparison of the calibration lines for various colloidal materials, the differences in their electrical double layers could be estimated. The limitations of the method are discussed and the size-exclusion chromatographic and electron microscopic methods are compared.     

Interaction of active particles of oxygen plasma with polypropylene

Changes in the chemical composition of surface were studied and the rates of formation of gaseous degradation products during polypropylene treatment in oxygen plasma and its flowing afterglow were determined by means of attenuated total internal reflectance IR spectroscopy and mass spectrometry. It was found that surface oxidation at minimal degradation rates is reached upon the joint action of O₂(a ¹Δ _g ) molecules and ground-state oxygen molecules, whereas O(³ P) atoms participate in both oxidation processes and the processes of degradation of oxygen-containing groups leading to the formation of gaseous products. It was shown that only the action of plasma leads to the formation of vinyl and vinylidene double bonds, which disappear in reactions with O₂(a ¹Δ _g ) molecules and O(³ P) atoms.     

Release of antibiotics from electrospun bicomponent fibers

Biocompatible nanofibers that are capable of adapting to the physiological conditions of the human body have become increasingly important for clinical applications in recent years. Electrospun fiber mats offer particular advantages due to their large surface area and their sorption/release properties. If loaded with drugs, delivery properties can be tailored to a specific release rate. This research work focuses on poly(L-lactic acid) (PLA) and poly(ε-caprolactone) (PCL) incorporating three different model antibiotics as well as bicomponent fibers made from PLA and PCL containing the same model drugs. Tetracycline and chlorotetracycline hydrochloride, and amphotericin B were selected as model drugs and their release properties and antimicrobial effectiveness studied. The surface morphology and the average diameter of the fibers strongly depended on the individual spinning system which in turn influenced the release of the therapeutic compounds from the fibers. Tetracycline was discharged from PCL at the highest rate while amphotericin B was slowest. PCL almost completely liberated any of the drugs over time while PLA only released about 10% total. By forming bicomponent PCL–PLA fibers surface and release characteristics could be modified to fit a sensible drug delivery.     

Dynamics of colloidal particles in ice

We use x-ray photon correlation spectroscopy (XPCS) to probe the dynamics of colloidal particles in polycrystalline ice. During freezing, the dendritic ice morphology and rejection of particles from the ice created regions of high particle density, where some of the colloids were forced into contact and formed disordered aggregates. The particles in these high density regions underwent ballistic motion, with a characteristic velocity that increased with temperature. This ballistic motion is coupled with both stretched and compressed exponential decays of the intensity autocorrelation function. We suggest that this behavior could result from ice grain boundary migration.     

Characterization of cellulose fibers electrospun using ionic liquid

Nonwoven fibers of cellulose were obtained by electrospinning of cellulose in an ionic liquid, 1-butyl-3-methylinmidazolium chloride (BMIMCl), which is known to be one of the non-volatile solvents. The electrospinning setup was modified in such a way that the syringe was contained in a constant-temperature chamber because of the high melting point of BMIMCl, and the electrospun fibers were collected on the water, which immediately removed the remnant solvent from the electrospun cellulose fibers. The effect of the viscosity of the cellulose solution in BMIMCl on the size and the structure of the fibers was investigated. The crystalline structure of cellulose was examined by X-ray diffraction. Also, the effect of dimethyl sulfoxide, which was expected to induce swelling of cellulose, was studied. The minimum diameter of the continuous electrospun cellulose fibers obtained in this work ranged between 500 and 800 nm.     

Screening of charged spheroidal colloidal particles

We study the effective screened electrostatic potential created by a spheroidal colloidal particle immersed in an electrolyte, within the mean field approximation, using Poisson-Boltzmann equation in its linear and nonlinear forms, and also beyond the mean field by means of Monte Carlo computer simulation. The anisotropic shape of the particle has a strong effect on the screened potential, even at large distances (compared to the Debye length) from it. To quantify this anisotropy effect, we focus our study on the dependence of the potential on the position of the observation point with respect with the orientation of the spheroidal particle. For several different boundary conditions (constant potential, or constant surface charge) we find that, at large distance, the potential is higher in the direction of the large axis of the spheroidal particle.     

Janus balance of amphiphilic colloidal particles

We introduce the notion of "Janus balance" (J), defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle (a "Janus particle") from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. The J value can be calculated simply from the interfacial contact angle and the geometry of Janus particles, without the need to know the interfacial energy. It is demonstrated that Janus particles of the same chemical composition but different geometries will have the highest adsorption energy when J = 1. Even for particles of homogeneous chemical makeup, the Janus balance concept can be applied when considering the contact angle hysteresis in desorbing the particle from equilibrium into the water or oil phase. The Janus balance concept may enable predictions of how a Janus particle behaves with respect to efficiency and function as a solid surfactant, as the Janus balance of solid surfactants is the analog of the classical hydrophile-lipophile balance of small surfactant molecules.     

Surface modification of colloidal silica particles

Colloidal silica particles in organic solvents were grafted, using several reagents, in order to make them hydrophobic. The hydrophobicity of the beads could be easily varied. Quasielastic light scattering and transmission electron microscopy experiments showed that no aggregation occurs during the reaction when monofunctional agents are used. Elemental analysis and solid-state NMR measurements gave us the rate of surface modification of the silanol groups. We also studied the rheological behaviour of the grafted particles as a function of the volume fraction in solvents of various hydrophobicity. Hexamethyldisilazane-grafted particles display hard-sphere behaviour in polar solvents such as 2-propanol, but not in alkanes, whereas dimethyldodecylchlorosilane-grafted particles flocculated in polar solvents but could easily be dispersed in apolar solvents. Received: 15 March 2000 Accepted: 11 July 2000     

Solvent-free synthesis of Janus colloidal particles

Taking advantage of the quick and efficient access of vapor to surfaces, a simple, solvent-free method is demonstrated to synthesize Janus colloidal particles in large quantity and with high efficiency. First, at the liquid-liquid interface of emulsified molten wax and water, untreated silica particles adsorb and are frozen in place when the wax solidifies. The exposed surfaces of the immobilized particles are modified chemically by exposure to silane vapor and, in principle, subsequent dissolution of the wax opens up the inner particle surface for further chemical modification. Applying this scheme, this paper describes the production of amphiphilic Janus particles (hydrophobic on one side, hydrophilic on the other) and dipolar Janus particles (positively charged on one side, negatively charged on the other). Janus geometry is confirmed by fluorescence microscopy and flow cytometry. Amphiphilic Janus particles are found to adsorb strongly to the water-oil interface, whereas dipolar particles assemble into chains in the aqueous phase.     

Anisotropic diffusion in confined colloidal dispersions: the evanescent diffusivity

We employ an analogy to traditional dynamic light scattering to describe the inhomogeneous and anisotropic diffusion of colloid particles near a solid boundary measured via evanescent wave dynamic light scattering. Following this approach, we generate new expressions for the short-time self- and collective diffusivities of colloidal dispersions with arbitrary volume fraction. We use these expressions in combination with accelerated Stokesian dynamics simulations to calculate the diffusivities in the limit of large and small scattering wave numbers for evanescent penetration depths ranging from four particle radii to one-fifth of a particle radius and volume fractions from 10% to 40%. We show that at high volume fractions, and larger penetration depths, the boundaries have little effect on the dynamics of the suspension parallel to the wall since, to a first approximation, the boundary acts hydrodynamically much as another nearby particle. However, near and normal to the wall, the diffusivity shows a strong dependence on penetration depth for all volume fractions. This is due to the lubrication interactions between the particles and the boundary as the particle moves relative to the wall. These results are novel and comprehensive with respect to the range of penetration depth and volume fraction and provide a complete determination of the effect of hydrodynamic interactions on colloidal diffusion adjacent to a rigid boundary.