Showing papers on "Particle published in 1983"

Absorption and Scattering of Light by Small Particles

Abstract: BASIC THEORY. Electromagnetic Theory. Absorption and Scattering by an Arbitrary Particle. Absorption and Scattering by a Sphere. Particles Small Compared with the Wavelength. Rayleigh--Gans Theory. Geometrical Optics. A Potpourri of Particles. OPTICAL PROPERTIES OF BULK MATTER. Classical Theories of Optical Constants. Measured Optical Properties. OPTICAL PROPERTIES OF PARTICLES. Extinction. Surface Modes in Small Particles. Angular Dependence of Scattering. A Miscellany of Applications. Appendices. References. Index.

A theory for the rapid flow of identical, smooth, nearly elastic, spherical particles

Abstract: We focus attention on an idealized granular material comprised of identical, smooth, imperfectly elastic, spherical particles which is flowing at such a density and is being deformed at such a rate that particles interact only through binary collisions with their neighbours. Using general forms of the probability distribution functions for the velocity of a single particle and for the likelihood of binary collisions, we derive local expressions for the balance of mass, linear momentum and fluctuation kinetic energy, and integral expressions for the stress, energy flux and energy dissipation that appear in them. We next introduce simple, physically plausible, forms for the probability densities which contain as parameters the mean density, the mean velocity and the mean specific kinetic energy of the velocity fluctuations. This allows us to carry out the integrations for the stress, energy flux and energy dissipation and to express these in terms of the mean fields. Finally, we determine the behaviour of these fields as solutions to the balance laws. As an illustration of this we consider the shear flow maintained between two parallel horizontal plates in relative motion.

Particle Systems—a Technique for Modeling a Class of Fuzzy Objects

Abstract: This paper introduces particle systems--a method for modeling fuzzy objects such as fire, clouds, and water. Particle systems model an object as a cloud of primitive particles that define its volume. Over a period of time, particles are generated into the system, move and change form within the system, and die from the system. The resulting model is able to represent motion, changes of form, and dynamics that are not possible with classical surface-based representations. The particles can easily be motion blurred, and therefore do not exhibit temporal aliasing or strobing. Stochastic processes are used to generate and control the many particles within a particle system. The application of particle systems to the wall of fire element from the Genesis Demo sequence of the film Star Trek H: The Wrath of Khan [10] is presented.

Cooling Neutral Atoms in a Magnetic Trap for Precision Spectroscopy

Abstract: A configuration of magnetic fields is exhibited which can harmonically trap paramagnetic particles in a shallow field minimum, superposed on a nearly uniform field which simplifies spectroscopic interactions with the particles. The potential energy of a trapped neutral particle depends on the particle's internal quantum number, as well as its position in the trap, permitting a combined rf-laser optical pumping cycle which can cool particles by a factor of 2 per cycle. Application of these ideas to the trapping of Na atoms is discussed.

The effects of crossing trajectories on the dispersion of particles in a turbulent flow

Abstract: The effects of ‘crossing trajectories’ and inertia on the dispersion of particles suspended in a field of grid-generated turbulence were investigated experimentally. The effect of particle trajectories crossing the trajectories of fluid elements, under the influence of a potential field (usually gravity), is to force the particles from one region of highly correlated flow to another. In this manner, particles lose velocity correlation more rapidly than the corresponding fluid points and as a result disperse less.A homogeneous decaying turbulent field was created behind a square biplanar grid in a wind tunnel. Particles were charged by a corona discharge then passed into the test section through a small plastic tube. A uniform electric field within the test section was used to simulate the effect of gravity, forcing the charged particles out of regions of correlated fluid at a higher than normal rate, therefore inducing the effects of crossing trajectories. Two sizes of glass beads (5 μm and 57 μm diameter) were employed in order to observe inertial effects. Laser-Doppler anemometry was used to measure particle mean-square displacement, autocorrelation coefficient, and mean-square velocity, from which dispersion coefficients were calculated.For the two particle sizes used in the tests, it was found that the particle diffusion coefficient, after a suitably long time from their release, was influenced primarily by the effect of crossing trajectories. Only in the particle mean-square velocity was the particle inertia seen to have any effect. The ratio of the particle relaxation time to the Kolmogoroff timescale was found to be a good indicator for the effects of particle inertia.

Strength of mineral absorption features in the transmitted component of near‐infrared reflected light: First results from RELAB

Abstract: Bidirectional reflectance measurements are the only type of reflectance data available to the remote observer. For compositional interpretations, data are desired not only for identification of possible mineral components but also for modal abundance. The latter requires detailed information about the strength of absorption features. Using a new laboratory facility, the RELAB, laboratory data in the near infrared are presented that document effects of particle size, mineral mixtures, and viewing geometry for selected materials with well-developed absorption bands. The commonly observed increase in reflectance with decrease in particle size is also observed for absorption bands as well as a related decrease in absorption strength. For small particles in parts of the spectrum of maximum reflectance, however, a minor decrease in reflectance with a decrese in particle size is sometimes observed. Small particles dominate the observed characteristics of particulate surfaces, which contain a range of particle sizes. The mean optical path length (transmission through particles) of reflected radiation measured for a variety of particle sizes has an apparent upper limit of about 2 mm for particles of less than 250 microns. The typical number of particles involved in the optical path is less than 50.

How can a particle absorb more than the light incident on it

Abstract: A particle can indeed absorb more than the light incident on it. Metallic particles at ultraviolet frequencies are one class of such particles and insulating particles at infrared frequencies are another. In the former strong absorption is associated with excitation of surface plasmons; in the latter it is associated with excitation of surface phonons. In both instances the target area a particle presents to incident light can be much greater than its geometrical cross‐sectional area. This is strikingly evident from the field lines of the Poynting vector in the vicinity of a small sphere illuminated by a plane wave.

A direct method for studying particle deposition onto solid surfaces

Abstract: An experimental technique has been developed to study the deposition of colloidal particles under well controlled hydrodynamic conditions. The deposition process is observed under a microscope and recorded on video tape for further analysis. Fluid flow conditions in the experimental set-up were determined by numerical solution of the Navier-Stokes equations. Mass transfer equations were solved numerically (taking into account hydrodynamic, gravitational, electric double layer, and dispersion forces) for the stagnation point region. Also, some analytical solutions are presented. Deposition has been studied of 0.5μm polystyrene latex particles on cover glass slides used as collectors. From an analysis of the shape of the coating density vs. time curves and independently from the distribution of the particles on collector surfaces, it was found that one particle is able to block an area of about 20 to 30 times its geometrical cross-section. The initial flux of particles to the collector for a given salt concentration was found to depend strongly on the method of cleaning the collector surface. In general the flux and the escape of particles to and from the collector surface are sensitive to the interaction energy at small separations. The direct method of observing particle deposition and detachment could lead to important insights into the nature of particle-wall interactions at near contact.

Densification of powders by particle deformation

Abstract: Based on a previous experimental study of particle deformation during powder compaction, a model is developed for describing the densification behaviour of an irregular packing of spherical particles. Using the radial density function of a ‘random dense packing’, the increase in both the average size and the number of contact faces are calculated. A simple criterion for local yielding allows the compaction pressure to be determined for relative densities up to 90%. In the final stage of compaction, particle deformation, now constrained by neighbouring contacts, is modelled by extrusion into the remaining pore space. A compaction equation encompassing both stages is presented; its application to non-spherical powders elucidates the role of particle shape during powder densification. PM/0150

Biological control of the removal of abiogenic particles from the surface ocean.

Abstract: Concurrent measurements of particle concentrations in the near-surface water and of particle fluxes in the deep water of the Sargasso Sea show a close coupling between the two for biogenic components. The concentrations of suspended matter appear to follow an annual cycle similar to that of primary production and deepwater particle flux. Although the concentration of particulate aluminum in the surface water appears to vary randomly with respect to that cycle, the removal of aluminum to deep water is intimately linked to the rapid downward transport of organic matter.

Plasma—particle momentum and heat transfer: Modelling and measurements

Abstract: Measurements were made of the two-dimensional flow and temperature field in a dc plasma jet under different operating conditions The particle velocity and the in-flight particle temperature were also measured for narrow cuts of alumina powders, of mean particle diameters of 18, 23, 39 and 46 mum, injected in the jet The results are compared with the predictions of a one-dimensional mathematical model The measured and computed particle velocities are in good agreement This is, however, not the case for the particle temperature where considerable differences are observed An attempt is made to determine the parameters which are often unduly neglected in modelling work and to identify the areas where further work is needed

Diffusion in a dilute polydisperse system of interacting spheres

Abstract: When m different species of small particles are dispersed in fluid the existence of a (small) spatial gradient of concentration of particles of type j is accompanied, as a consequence of Brownian motion of the particles, by a flux of particles of type i The flux and the gradient are linearly related, and the tensor diffusivity Dij is the proportionality constant When the total volume fraction of the particles is small, Dij is approximately a linear function of the volume fractions ϕ1, ϕ2, …s, ϕm, with coefficients which depend on the interactions between pairs of particles The complete analytical expressions for these coefficients given here for the case of spherical particles are a linear combination of the second virial coefficient for the osmotic pressure of the dispersion (measuring the effective force acting on particles when there is a unit concentration gradient) and an analogous virial coefficient for the bulk mobility of the particles Extensive calculations of the average velocities of the different species of spherical particles in a sedimenting polydisperse system have recently been published (Batchelor & Wen 1982) and some of the results given there (viz those for small Peclet number of the relative motion of particles) refer in effect to the bulk mobilities wanted for the diffusion problem It is thus possible to obtain numerical values of the coefficient of ϕk in the expression for Dij, as a function of the ratios of the radii of the spherical particles of types i, j and k The numerical values for ‘hard’ spheres are found to be fitted closely by simple analytical expressions for the diffusivity; see (46) and (47) The dependence of the diffusivity on an interparticle force representing the combined action of van der Waals attraction and Coulomb repulsion in a simplified way is also investigated numerically for two species of particles of the same size The diffusivity of a tracer particle in a dispersion of different particles is one of the many special cases for which numerical results are given; and the result for a tracer ‘hard’ sphere of the same size as the other particles is compared with that found by Jones & Burfield (1982) using a quite different approach

Plasmon resonance broadening in small metal particles

Abstract: A quantum mechanical method based on the Kramers–Heisenberg dispersion relation is used to evaluate the dielectric response of small metal particles, and thereby to determine the influence of particle size on the widths of the plasmon resonance line shapes. Several different particle shapes are considered (sphere, cylinder, rectangular prism, spherical shell, and cylindrical shell) and for each shape a free electron Schrodinger equation is used to determine conduction band energies and dipole matrix elements. The main emphasis in this work is on particle sizes large enough that only the first order deviations from the infinite size limit are important, and for such sizes we find that the size dependent contribution to the width can be expressed in terms of an effective length Leff. This effective length is found to depend on the direction of the external field relative to the particle symmetry axes, and on the shape of the particle. For compact shapes, Leff is accurately approximated by 0.65 Lav along eac...

Direct Observation of the Local-Field-Enhanced Surface Photochemical Reactions

Abstract: The enhancement of photochemical reactions on surface-supported metallic particles is reported. The result is obtained by uv photodissociation of organometallic molecules adsorbed on the supporting surface. Transmission electron-microscope micrographs of the resulting photodeposited film reveal enhanced deposition on and near the metal particles, and a clear interference pattern resulting from the mixing of the incident and reradiated light. The importance of the particle plasma resonance is described.

Turbulent coagulation of particles dispersed in a viscous fluid.

Abstract: A kinetic equation for the turbulent coagulation of particles in a viscous fluid in which the hydrodynamic interaction between colliding particles is taken into account is proposed. The theoretical prediction of the time-dependent behavior of particle concentration is compared with results of experiments in which latex particles in KC1 solutions are coagulated in a stirred tank. It is found that the effect of the hydrodynamic interaction is not negligible, and that changes of particle concentration under various experimental conditions are quantitatively predicted by the present theory. On the other hand, the Saffman-Turner theory is found to overestimate the coagulation rate considerably and to yield a systematic error in the dependence of coagulation rate on the dissipation energy.

On the fracture of brittle-matrix/ductile-particle composites

Abstract: A mechanism for the toughening of brittle matrices with ductile particles is presented. It is shown that the constraints imposed on the particles by the rigid matrix suppress plastic deformation of the particles at crack tips so that the main contribution to composite toughness comes from ligament formation in the matrix and their fracture behind the advancing crack front. The incorporation of inherently tough second-phase particles into a brittle matrix does not therefore automatically lead to a large toughness improvement of the composite, if other requirements are not satisfied. As far as toughness is concerned, the most desirable composite is one consisting of a soft (low yield strength) particle strongly bonded to a brittle matrix and free from internal stresses. The mechanism of stress relaxation in a spherical particle embedded in a matrix of smaller thermal expansion coefficient is also described.

Theory of Agglomeration of Ferromagnetic Particles in Magnetic Fluids

Abstract: For some magnetic fluids, large agglomerates of magnetic particles are formed when a weak magnetic field is applied. In this paper we regard this phenomenon as a kind of gas-liquid phase transition induced by the magnetic field, and study the condition for the formation of the agglomerates by a simple mean field theory. It is found that if the particle is small enough, no phase separation occurs even if the magnetic field is infinitely large, while if the particle is large, the liquid phase appears at certain magnetic field. The van der Waals attraction between the particles enhances the agglomeration.

Turbidimetric determination of particle size distributions of colloidal systems

Abstract: Application of the Mie theory of light scattering to measurements of the turbidity ratio and the wavelength exponent provides an easy method for estimating particle size distributions of nonabsorbing isotropic spheres in the micrometer to submicrometer range. Combining both these lightscattering techniques not only allows one to analyze particle sizes which are too large for quasi-elastic light scattering and too small for optical microscopy, but can be accomplished with only two turbidity measurements and no prior knowledge of the particle volume fraction. An algorithm is presented for constructing turbidity spectra, for any system of known optical constants and known distributional form, which can be used to easily determine the mean diameter and standard deviation of an unknown distribution. Using this algorithm, size-distribution curves were obtained from turbidity measurements at two widely separated wavelengths. These distributions are in agreement with distributions determined from scanning electron microscopic analysis.

Light scattering of colloidal dispersions in non-polar solvents at finite concentrations. Silic spheres as model particles for hard-sphere interactions

Abstract: Static and dynamic light-scattering studies are reported for spherical model particles in non-polar solvents. The model particles have a core of silica and a dense surface layer of octadecylalcohol chains which makes them ‘oil soluble’. The refractive-index difference between particles and solvent is very small and so dispersions can be studied by means of light scattering up to high concentrations. Multiple-scattering effects are considered briefly. In cyclohexane the particles show repulsive forces which can be described by a hard-sphere interaction. The small refractive-index differences can also be used to detect differences in optical density in the silica core. The periphery of the core is found to be more dense than the centre. Furthermore, the small natural spread in refractive index of the particles can be used to differentiate between collective-diffusion and self-diffusion processes. Differences in refractive indexes can also be obtained by variations in the particle synthesis. In this way it is possible to study self-diffusion by following the motion of tracer particles.

The formation of “inverted” core‐shell latexes

Abstract: It has been found that when hydrophobic monomers are polymerized in the presence of highly hydrophilic polymer seed particles, the second-stage hydrophobic polymers form cores surrounded by the first-stage hydrophilic polymers, resulting in “inverted” core-shell latexes. The formation of core-shell morphology by this inversion process has been found to be dependent on the hydrophilicity and molecular weight of the first-stage hydrophilic polymers and the extent of phase separation between the two polymers involved. Particle morphology has been examined by electron microscopy, surface acid titration, alkali swelling of particles, and surface reactivity.