Showing papers on "Particle published in 1991"
TL;DR: In this article, the authors used the atomic force microscope to measure the forces between a planar surface and an individual colloid particle, a silica sphere of radius 3.5 µm, attached to the force sensor in the microscope and measured in solutions of sodium chloride.
Abstract: THE forces between colloidal particles dominate the behaviour of a great variety of materials, including paints, paper, soil, clays and (in some circumstances) cells. Here we describe the use of the atomic force microscope to measure directly the force between a planar surface and an individual colloid particle. The particle, a silica sphere of radius 3.5 µm, was attached to the force sensor in the microscope and the force between the particle and the surface was measured in solutions of sodium chloride. The measurements are consistent with the double-layer theory1,2 of colloidal forces, although at very short distances there are deviations that may be attributed to hydration forces3–6 or surface roughness, and with previous studies on macroscopic systems4–6. Similar measurements should be possible for a wide range of the particulate and fibrous materials that are often encountered in industrial contexts, provided that they can be attached to the microscope probe.
1,900 citations
TL;DR: In this article, the ultraviolet-visible absorption spectra of colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory, are given.
Abstract: The ultraviolet–visible absorption spectra are given for 10 nm diameter colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory. For most of the elements the spectra cover the range 200–900 nm. Well resolved absorption bands are observed for colloidal Sc, Ti, V, Y, Cd, Eu, Yb, Hg and Th as well as for colloids of Cu, Ag, Au and the s-block metals. However, for the majority of the colloidal metallic elements in this size range there is only a continuous absorption in the visible range, rising to broad and poorly resolved absorption bands in the ultraviolet near 200 nm. The difference in the way that the spectra of colloidal particles of different metals change when the particle shape is varied from spherical to spheroidal is investigated systematically in the dipole approximation. This is achieved by means of contour plots of the absorbance cross-section for the particles vs. the real and imaginary parts of the dielectric function for the metals, and this method of investigation is extended also to hollow spherical particles. The results suggest that Ca, Sr, Ba, Eu, Yb, Th and possibly Sc, Ti, V and Y may merit experimental investigation as new metals for exhibiting surface-enhanced Raman scattering.
1,501 citations
Book•
01 Jan 1991
TL;DR: In this paper, the wave dynamics of burst and detonation of gas particle suspensions and powders are studied in a channel theory of inertia-free and percolation flows of heterogeneous media.
Abstract: Part 1 Volume 1: equations of mechanics of heterogeneous continuous media mechanics of processes near dispersed particles drops and bubbles wave dynamics of shocks and detonations in condensed media with phase transitions dynamics of two-velocity flows of disperse media (gas-particle suspensions) gas and wave dynamics of combustion and detonation of gas particle suspensions and powders. Part 2 Volume 2: wave dynamics of bubbly liquids hydrodynamics and thermodynamics of stationary one-dimensional gas-liquid and vapor-liquid flows in channels theory of inertia-free and percolation flows of heterogeneous media.
1,101 citations
TL;DR: In this article, a range of disaggregating treatments from gentle to vigorous were applied to different particle sizes and the results indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates.
Abstract: An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 µm diameter breaking down to microaggregates 20-250 µm diameter before particles <20 µm were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 µm diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 µm but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.
1,046 citations
01 Jan 1991
TL;DR: In this paper, a detailed analysis of particle inception and surface growth in laminar premixed hydrocarbon flames is presented, which predicts the classical picture of particle formation and the classical description of soot particle structure.
Abstract: Detailed modeling of soot particle nucleation and growth in laminar premixed hydrocarbon flames is presented. The model begins with fuel pyrolysis, followed by the formation of polycyclic aromatic hydrocarbons, their planar growth and coagulation into spherical particles, and finally, surface growth and oxidation of the particles. The computational results are in quantitative agreement with experimental results from several laminar premixed hydrocarbon flames. A detailed analysis of soot particle inception and surface growth processes is presented. Surface growth was described in terms of elementary chemical reactions of surface active sites. The density of these sites was found to depend on the chemical environment. The model predicts the classical picture of soot particle inception and the classical description of soot particle structure.
1,012 citations
TL;DR: In this paper, the energy spectrum of a system of two particles enclosed in a box with periodic boundary conditions is determined by the scattering phases at these energies, and exact exact formulae are derived which can be used to compute the energy levels given the scattering phase.
968 citations
TL;DR: In this paper, a direct numerical simulation of isotropic turbulence was used to investigate the effect of turbulence on the concentration fields of heavy particles, and it was shown that the particles collect preferentially in regions of low vorticity and high strain rate.
Abstract: Direct numerical simulation of isotropic turbulence was used to investigate the effect of turbulence on the concentration fields of heavy particles. The hydrodynamic field was computed using 643 points and a statistically stationary flow was obtained by forcing the low‐wave‐number components of the velocity field. The particles used in the simulations were time advanced according to Stokes drag law and were also assumed to be much more dense than the fluid. Properties of the particle cloud were obtained by following the trajectories of 1 000 000 particles through the simulated flow fields. Three values of the ratio of the particle time constant to large‐scale turbulence time scale were used in the simulations: 0.075, 0.15, and 0.52. The simulations show that the particles collect preferentially in regions of low vorticity and high strain rate. This preferential collection was most pronounced for the intermediate particle time constant (0.15) and it was also found that the instantaneous number density was as much as 25 times the mean value for these simulations. The fact that dense particles collect in regions of low vorticity and high strain in turn implies that turbulence may actually inhibit rather than enhance mixing of particles.
853 citations
TL;DR: In this paper, a theory of the application of phase equilibrium was presented as an example for the pore forming mechanism in the solvent casting method, and the growth rate of nucleus by diffusion to the primary particle and the time of attainment of phase equilibria of whole system were determined under the assumption of local equilibrium between nucleus and sorrounding sphere.
Abstract: In order to give better and quantitative understanding of the pore forming mechanism in the solvent casting method, a theory was presented as an example of the application of phase equilibrium. Mechanism of formation of critical nucleus from homogeneous polymer solution in the metastable region of the phase diagram was investigated in accordance with the activation energy of nucleation. The growth rate of nucleus by diffusion to the primary particle and the time of attainment of phase equilibria of whole system were determined under the assumption of local equilibrium between nucleus and sorrounding sphere. Formation of secondary particle by amalgamation was tried to explain by use of the particle Monte–Carlo simulation approach.
637 citations
TL;DR: In this paper, the kinetics of O 2 reduction accompanying oxidation of organic compounds on photocatalytic semiconductor particles are analyzed and the rate-controlling processes are defined and the electron reactivities are derived for two cases: A, electrons moving freely in the semiconductor particle and reacting with O 2 anywhere on its surface and B, electrons trapped at or near the surface of the polysilicon particle and transferred from the traps only to nearby O 2 molecules
Abstract: The kinetics of O 2 reduction accompanying oxidation of organic compounds on photocatalytic semiconductor particles is analyzed. The rate-controlling processes are defined and the electron reactivities are derived for two cases: A, electrons moving freely in the semiconductor particle and reacting with O 2 anywhere on its surface and B, electrons trapped at or near the surface of the semiconductor particle and transferred from the traps only to nearby O 2 molecules
619 citations
TL;DR: The background aerosol in the boundary layer over the remote oceans is not aged continental aerosol but, rather, is largely of marine origin this article, and the principal gas-toparticle conversion mechanisms are particle formation by homogeneous nucleation of lowvolatility gas-phase reaction products, condensation of these products on existing particles, and SO2-to-sulfate conversion in cloud droplets.
513 citations
TL;DR: In this paper, the formation of uniform silica particles is investigated and rates of silicon alkoxide reactions are reported and linked to particle growth rates, which are consistent with a particle growth mechanism involving reactions occurring in solution that proceed independently of the presence of particles.
TL;DR: In this article, an improved theory of the temperature and composition dependence of deliquescence was proposed to predict the size distribution of the inorganic components of atmospheric aerosols, which can account for both therthermodynamic properties of the aerosol particles and transport between the gas and aerosol phases.
TL;DR: In this article, an aggregative growth model for the formation of uniform particles was developed and tested with rate and particle size distribution data gathered on spheres precipitated from silicon alkoxides.
TL;DR: The rapid expansion of supercritical solutions (RESS) is a new and promising method of particle formation as discussed by the authors, which can be used to comminute shock sensitive solids, to produce intimate mixtures of amorphous materials, to form polymeric microspheres, and to deposit thin films.
01 Apr 1991
TL;DR: In this paper, the diffusion of Brownian submicrometer particles from a point source in the vicous sublayer of a turbulent shear flow near a solid smooth wall is considered.
Abstract: Diffusion of Brownian submicrometer particles from a point source in the vicous sublayer of a turbulent shear flow near a solid smooth wall is considered in this paper. The equation of motion of particles including the Brownian effect is considered. Ensembles of 500 particle trajectories are evaluated, compiled, and statistically analyzed. Effects of particle size, density ratio, and source distance from the wall on particle concentration profile and wall deposition rate are studied. The results are compared with those obtained from the exact solution to the corresponding convective diffusion equation in the absence of turbulent fluctuations. The effect of turbulence near a wall is also considered. The results show that the Brownian effects play a significant role in the diffusion of submicrometer particles at distances less than 2 wall units from the solid surface. The effect of turbulence, however, could become significant during the ejection/inrush process.
TL;DR: In this article, the authors examined the functional dependence of particle migration in concentrated suspensions undergoing shear flow in a wide gap, annular Couette flow, and found that the particle concentration near the outer wall approaches maximum packing for randomly distributed spheres at steady state, and velocity profiles reveal that the suspension is almost stagnant in these regions.
Abstract: Nuclear magnetic resonance (NMR) imaging was used to observe the evolution of radial concentration and velocity profiles of initially well‐mixed concentrated suspensions of spheres in viscous Newtonian liquids undergoing flow between rotating concentric cylinders (wide‐gap, annular Couette flow). In Couette flow, particles migrate from the high shear‐rate region near the inner rotating cylinder to the low shear‐rate region at the outer wall. The particle concentration near the outer wall approaches maximum packing for randomly distributed spheres at steady state, and velocity profiles reveal that the suspension is almost stagnant in these regions. For unimodal suspensions of spheres, the shear‐induced migration of large particles results in concentric two‐dimensional, circular sheets of particles arranged in hexagonal close‐packed arrangements extending inward from the outer wall. This paper examines the functional dependence of particle migration in concentrated suspensions undergoing shear flow in a wid...
TL;DR: In this article, a model for the initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation is presented, together with a comparison with previously published experimental data, and the model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength.
Abstract: A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for free-radical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.
TL;DR: In this paper, the generation and behavior of particles in a low-pressure silane-argon discharge have been analyzed under continuous and pulsed radio-frequency (rf) excitation conditions.
Abstract: The generation and behavior of particles in a low‐pressure silane‐argon discharge have been analyzed under continuous and pulsed radio‐frequency (rf) excitation conditions. In the continuous rf excitation regime, the influence of parameters such as gas temperature and silane partial pressure are determined. By using rf pulsed excitation, it is shown that gas‐flow effects play a predominant role for particle dynamics when the excitation is stopped. Radio‐frequency regimes with short and adjustable rf off sequences are used to study both the inhibition of particle formation and the elimination of particles from the dusty plasmas. The electrical properties of the discharge are shown to be sensitive to the presence of the particles. Simple models for particle trapping in the plasma edge and for particle dynamics when the discharge is turned off are presented.
TL;DR: In this article, the effect of particle inertia and drift due to body forces on particle dispersion has been investigated in the presence of particle drift and particle drift in the absence of body forces.
Abstract: Measurements of heavy particle dispersion have been made using direct numerical simulations of isotropic turbulence. The parameters affecting the dispersion of solid particles, namely particle inertia and drift due to body forces were investigated separately. In agreement with the theoretical studies of Reeks, and Pismen & Nir, the effect of particle inertia is to increase the eddy diffusivity over that of the fluid (in the absence of particle drift). The increase in the eddy diffusivity of particles over that of the fluid was between 2 and 16%, in reasonable agreement with the increases reported in Reeks, and Pismen & Nir. The effect of a deterministic particle drift is shown to decrease unequally the dispersion in directions normal and parallel to the particle drift direction. Eddy diffusivities normal and parallel to particle drift are shown to be in good agreement with the predictions of Csanady and the experimental measurements of Wells & Stock.
TL;DR: In this article, effective elastic moduli and thermal expansion coefficient of spherical particle composites with imperfect interfaces are evaluated on the basis of the composite spheres assemblage and generalized self-consistent scheme models.
Abstract: Effective elastic moduli and thermal expansion coefficient of spherical particle composites with imperfect interfaces are evaluated on the basis of the composite spheres assemblage and generalized self-consistent scheme models. Imperfect interface is defined in terms of interface displacement discontinuities which are linearly related to interface tractions in terms of spring constant parameters. In the case of presence of interphase these parameters are evaluated in terms of interphase characteristics.
TL;DR: In this article, the presence of nuclear magnetic resonance can be detected noninductively by coupling the nuclear spin to the motion of a mechanical oscillator by applying a large gradient magnetic field, which exerts a mechanical force on the particle by virtue of its magnetic moment.
Abstract: The presence of nuclear magnetic resonance can be detected noninductively by coupling the nuclear spin to the motion of a mechanical oscillator. The coupling is obtained by applying a large‐gradient magnetic field, which exerts a mechanical force on the particle by virtue of its magnetic moment. The coupling increases in strength as the oscillator mass is decreased and the gradient length scale is made shorter. Oscillator‐based detection is thus only marginally effective for macroscopic samples, but can be quite effective for single protons interacting with a micron‐scale oscillator. This letter describes the physics of devices in which single‐nucleon magnetic resonance is detected by monitoring the excitation of a mechanical oscillator.
TL;DR: A light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres shows the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed.
Abstract: This paper describes a light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres. The observed phase behavior, fluid, crystal, and glass, is consistent with an essentially hard-sphere interaction between the particles. Metastable fluid states were obtained upon shear melting the crystalline phases by tumbling the samples. Their intermediate scattering functions, measured by dynamic light scattering, showed the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed. The overall forms of the intermediate scattering functions are consistent with the predictions of mode-coupling theories for the glass transition. Supplementary studies of the static structure factors indicated only short-ranged spatial order for particle concentrations ranging from the equilibrium fluid through the metastable fluid to the glass.
TL;DR: In this article, a two-dimensional particle size distribution model is presented, for the first time, describing the evolution of both particle size and shape during gas phase powder production, which leads to a quantitative characterization of powder morphology as well as the average size and polydispersity of the powders.
TL;DR: In this paper, it was shown that the size of the particle rather than the presence of nonstoichiometry or impurities of reduced nickel determines the magnetic moment of a particle.
Abstract: It has been known for some time that particles of nickel oxide of less than about 100 nm in size show superparamagnetism that increases as the particle size decreases. The origin of the particle magnetic moment responsible for this behavior has never been fully explained. This research indicated that the size of the particle rather than the presence of nonstoichiometry or impurities of reduced nickel determines the moment. The critical experiment was the measurement of magnetization versus magnetic field for a sample of nickel oxide prepared under conditions that preclude metallic nickel. Almost identical results were found for the original sample, which was black in color and thus nonstoichiometric, and after mild reduction in hydrogen at 400 K, which produced stoichiometry and changed the color to green. The magnetic susceptibility was inversely proportional to the particle size for a given method of preparation. This is consistent with a simple model of incomplete edges on the bounding planes of the crystallite and provides a possible basis for a practical method for measuring particle size in nickel oxide‐containing samples.
TL;DR: In this article, the defect concentration in the TiO2 crystallites strongly affects the transformation rate, and anatase particles produced at 1,173 K are transformed to rutile more rapidly than those produced by 1,373 K. The model can be applied to such particle production processes as collision, sintering and crystal transformation occurring simultaneously.
Abstract: Rate process concerning the formation of TiO2 fine crystalline particles by the gas-phase reaction of TiCl4 and O2 are studied using aerosol reactors. Chemical reaction of TiCl4, sintering of particles, mixing of reactants, and transformation from anatase to rutile are evaluated as the system parameters of the simulation model proposed. The crystallite size in the range of 55–65 nm at 1,273 K is predicted well by a model that assumes the maximum fusible particle size, 15 nm in this case.
The defect concentration in the TiO2 crystallites strongly affects the transformation rate, and anatase particles produced at 1,173 K are transformed to rutile more rapidly than those produced at 1,373 K. The transformation is simulated quantitatively by the model with the coordinates for elapsed time, particle size and rutile fraction. The model can be applied to such particle production processes as collision, sintering and crystal transformation occurring simultaneously.
01 Jan 1991
TL;DR: It has been pointed out that efficiencies of microbial decomposition of fast-sinking large particles and nonsinking small particles may be different and competition for organic particles between animals and microbes will be determined by the slow but continuous microbial component and, on the other hand, by pulse-feeding activities of animals.
Abstract: Three general pathways of organic matter degradation exist in natural aquatic environments. These are based on predation, particle feeding, and dissolved organic matter (DOM) uptake. Bacteria are involved in the latter two in that they are able to hydrolyze nonliving particles thereby competing with particle feeders, and take up small organic molecules, which is their exclusive domain. Particle hydrolysis is mediated by extracellular enzymes in the intestines of animals and by the enzymatic activity of attached bacteria. Therefore, successful competition for organic matter among tropic levels is also a question of extra-cellular enzymatic efficiencies. The decomposition of dissolved organic macromolecules is mediated mainly by the enzymes of free-living bacteria, which subsequently incorporate the small molecules resulting from enzymatic hydrolysis. Therefore, bacterial activity has a strong influence on the concentration and speciation of dissolved organic molecules in the water. A major fraction of the DOM-pool in the water can be expected to consist of dissolved macromolecules since extracellular hydrolysis is a relatively slow process in comparison to the uptake of low-molecular-weight organic matter (LMWOM). The efficiency of animals feeding on particles may vary considerably, depending on various factors. Microbial hydrolysis of particles will depend greatly on the chemical composition and the size of the particles. Thus competition for organic particles between animals and microbes will be determined on the one hand, by the slow but continuous microbial component and, on the other hand, by pulse-feeding activities of animals (Joint and Morris, 1982). In detail, it has been pointed out that efficiencies of microbial decomposition of fast-sinking large particles and nonsinking small particles may be different (Cho and Azam, 1988; Karl et al., 1988).
TL;DR: In this paper, the effects of particle size and aspect ratio on the hysteresis in controlled arrays of small magnetic particles were investigated using an alternating gradient magnetometer for particles ∼5−0.1 μm.
Abstract: We have investigated the effects of particle size and aspect ratio on the hysteresis in controlled arrays of small magnetic particles. The arrays of permalloy particles were fabricated via electron‐beam lithography. Each array consists of ∼ 106 identical, uniformly spaced particles. Hysteresis loops measured with an alternating‐gradient magnetometer for particles ∼5–0.1 μm are presented. We find an increase in the coercive force as the particle width decreases below 0.3 μm due to a change in the switching mechanism from domain‐wall nucleation and wall motion to vortex nucleation and vortex motion. A novel angular dependence of the loops is described in detail. Results from ab initio micromagnetic calculations on isolated rectangular Permalloy particles are compared, where applicable, with the measurements. We find excellent qualitative and, in selected cases, quantitative agreement between the experiments and calculations.
TL;DR: In this article, the critical volume fraction of inclusions required to form the first percolative cluster strongly depends on the inclusion to matrix particle size ratio and the fraction of the inclusions belonging to a connective cluster is a size-ratio-insensitive function of the inclusion-inclusion coordination number.
Abstract: Deformation processing of composite materials is often affected by the percolation of inclusions, which may form a connective network supporting a significant part of the applied pressure and to hinder the densification of the matrix powder. By numerical simulation it was found that the critical volume fraction of inclusions required to form the first percolative cluster strongly depends on the inclusion to matrix particle size ratio. Moreover the fraction of inclusions belonging to a connective cluster is a size-ratio-insensitive function of the inclusion-inclusion coordination number. Using this correlation and an analytical model for particle coordination, a simple scheme is proposed to predict the percolation within bimodal powder mixtures. Interest of these results for understanding pressure transmission during processing of particulate composites is also discussed.
TL;DR: In this article, the distribution of particle movement in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles was examined using the compound Poisson model of Einstein-Hubbell-Sayre and simple gamma function model.
Abstract: Distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles. The compound Poisson model of Einstein-Hubbell-Sayre and a simple gamma function model were compared with observed distributions of moved particles, and of all particles. Both models fit the data reasonably well for small mean displacements, but notable misfits occurred in an event with large mean displacement. When mean particle travel distance approaches the scale of bar spacing, trapping in the bars interrupts particle progress and the dispersion process. The data remain very noisy, so definitive discrimination of suitable models will require trials with more than 103 particles.