Showing papers on "Particle published in 1977"

Thermal or electrical conduction through a granular material

Abstract: The material under investigation consists of particles of relatively large conductivity embedded or immersed in a matrix, the volume fraction of the particles being so high that they are in, or nearly in, contact. The particles are arranged randomly, and the material is statistically homogeneous. A general formula gives the effective conductivity of the material in terms of the average thermal (or electrical) dipole strength of a particle. The thermal flux across the surface of a particle is concentrated in areas near points of contact with another particle, and the dipole strength is approximately equal to a weighted sum of the fluxes across the areas near contact points. It is thus necessary to calculate the flux between two adjoining particles at different temperatures, and we do this by solving numerically an integral equation for the distribution of temperature over the (locally spherical) surface of one of the particles near the contact point. The flux between the two particles is found to be proportional to loge ah when a2 2h/a ≫ 1 and to log e a when a 2h/a ≪ 1, where h is the minimum gap between the particle surfaces, a~ 1 the mean of their local curvatures, and a the ratio of the conductivities of the particles and the matrix. In the case of two particles pressed together to form a circular flat spot of radius p , the flux occurs almost wholly in the particle material, and is proportional to p when ap/a ≫ 1. Explicit approximate results are obtained for the effective conductivity of the granular material in the case of uniform spherical particles. For a close-packed bed of particles making point contact the effective conductivity is found to be 4.0 k log e a where k is the matrix conductivity. This asymptotic relation (applicable when a ≫ 1) is seen to be consistent with the available measurements of the conductivity of packed beds of spheres. Values of the effective conductivity for packed beds of particles of different shape are not expected to be greatly different.

Self-diffusion of particles in shear flow of a suspension

Abstract: Self-diffusion coefficients were determined experimentally for lateral dispersion of spherical and disk-like particles in linear shear flow of a slurry at very low Reynolds number. Using a concentric-cylinder Couette apparatus, recurrent observations were made of the lateral position of a particular radioactively labelled particle. The self-diffusion coefficient D was calculated by means of random-walk theory, using the ergodic hypothesis. Owing to great experimental difficulties, the calculated values of D are not of high accuracy, but are correct to within a factor of two. In the range 0 < ϕ < 0·2, D/a2ω increases from zero linearly with ϕ up to D/a2ω ≅ 0·02 (where ϕ = volumetric concentration of particles, a = particle radius, ω = mean shear rate of suspending fluid). In the range 0·2 < 0·5, the trend of D/a2ω is not clear because of experimental scatter, but in this range D/a2ω ≅ 0·025 to within a factor of two. Within the experimental accuracy, spheres and disks have the same value of D/a2ω.

An averaged-equation approach to particle interactions in a fluid suspension

Abstract: Earlier ideas are combined to produce a systematic approach both to forming the bulk equations of motion of a dilute suspension and to calculating the overall hydrodynamic interactions between the suspended particles. Equations governing averaged field quantities are derived by taking ensemble averages of the conservation laws and constitutive relations. The bulk equations thus produced contain a term in which the averaging is performed holding one particle fixed. If now the same prescription is applied to fields averaged with one particle fixed, equations are produced containing a term averaged with two particles fixed, and so on up an infinite hierarchy. The hierarchy can be truncated in an asymptotic analysis for small particle concentrations. This approach to the mechanics of suspensions is illustrated by applying it to three problems which have already been well studied by different methods. The problems concern the first effects of hydrodynamic interactions on the bulk stress and sedimontation velocity of a free suspension, and on the permeability of a fixed bed. Earlier results are recovered in a new light. Multiparticle effects, which before have occurred as divergent sums, are seen to arise because the suspension described by the averaged equations assumes a viscosity and density different from the solvent, or in the case of the fixed bed because the suspension starts behaving as a porous medium instead of as a Newtonian solvent. A close connexion is thus revealed between the averaged-equation description of the interactions and a self-consistentfield model.

Properties of aerosols in the Martian atmosphere, as inferred from Viking lander imaging data

Abstract: Observations of the Martian sky, Phobos, and the sun were taken with the Viking lander imaging cameras to obtain information on the properties of the atmospheric aerosols. Atmospheric optical depths were derived from the observations of the brightness of the celestial objects. Information on the absorption coefficient, mean size, and shape of the aerosols was derived from studies of the sky brightness. For this purpose we used a multiple-scattering computer code that employed a recently developed technique for treating scattering by nonspherical particles. By monitoring the brightness of the twilight sky we obtained information on the vertical distribution of the particles. Three types of aerosols are inferred to have been present over the landers during the summer and fall season in their hemisphere. A ground fog made of water ice particles was present throughout this period. It formed late at night during the summer season and dissipated during the morning. We infer that during the summer the frost point temperature was 195°K and the water vapor volume mixing ratio equaled about 1× 0−4 near the ground at VL-2. Assuming that condensation occurs only on suspended soil particles, we estimate that the average particle radius of the fog was about 2 μm and that the fog's depth equaled approximately 0.4 km. A higher-level ice cloud was prominent only during the fall season, when it was a sporadic source of atmospheric opacity at VL-2. The formation of upper level water ice clouds during the summer may have been inhibited by dust heating of the atmosphere. Suspended soil particles were present throughout the period of observation. During the summer they constituted the only major source of opacity in the afternoon and most of the night. The cross-section weighted mean radius of these aerosols is about 0.4 μm. They have a nonspherical but equidimensional shape and rough surfaces. These soil particles have a scale height of about 10 km, which is comparable to the gas scale height, and they extend to an altitude of at least 30 km. The principal opaque mineral in these particles is magnetite, which constitutes 10%±5% by volume of this material. We propose that soil particles, as well as any associated water ice, are eliminated from the atmosphere, in part, by their acting as condensation sites for the growth of CO2 ice particles in the winter polar regions. The resultant CO2-H2O-dust particle is much larger and therefore has a much higher fallout velocity than an uncoated dust or water ice particle.

On the dispersion of small particles suspended in an isotropic turbulent fluid

Abstract: A solution to the dispersion of small particles suspended in a turbulent fluid is presented, based on the approximation proposed by Phythian for the dispersion of fluid points in an incompressible random fluid. Motion is considered in a frame moving with the mean velocity of the fluid, the forces acting on the particle being taken as gravity and a fluid drag assumed linear in the particle velocity relative to that of the fluid. The probability distribution of the fluid velocity field in this frame is taken as Gaussian, homogeneous, isotropic, stationary and of zero mean. It is shown that, in the absence of gravity, the long-time particle diffusion coefficient is in general greater than that of the fluid, approaching with increasing particle relaxation time a value consistent with the particle being in an Eulerian frame of reference. The effect of gravity is consistent with Yudine's effect of crossing trajectories, reducing unequally the particle diffusion in directions normal to and parallel to the direction of the gravitational field. To characterize the effect of flow and gravity on particle diffusion it has been found useful to use a Froude number defined in terms of the turbulent intensity rather than the mean velocity. Depending upon the value of this number, it is found that the particle integral time scale may initially decrease with increasing particle relaxation time though it eventually rises and approaches the particle relaxation time. It is finally shown how this analysis may be extended to include the extra forces generated by the fluid and particle accelerations.

Mechanism of emulsion polymerization of styrene in soap‐free systems

Abstract: The formation and growth of monodisperse polystyrene latex particles in the absence of added surfactant has been studied by sampling polymerization reactions at different times and determining the surface and bulk properties of the latex. A large number of nuclei in excess of 5 × 1012/ml were generated during the first minute of reaction, but this fell due to coagulation until a constant number (1011−1012/ml) was reached. The rate of polymerization per particle was then found to be proportional to the particle radius. Gel-permeation chromatography has shown that the initial particles consist mainly of material of MW 1000 with a small amount of polymer up to MW 106, and the presence of this low molecular weight polymer, which in many cases can still be detected after 100% conversion, is taken as being indicative of particle formation via a micellization-type mechanism involving short-chain (MW 500) free-radical oligomers. Mn values determined for the latex particles throughout the course of reactions show that the molecular weight increases to a maximum of about 105 as the particles grow. The presence of anomalous regions within the particles has been confirmed by transmission electron microscopy, scanning electron microscopy, and gas adsorption studies. It has also been found possible to re-expose these regions within apparently homogeneous particles by stirring with styrene monomer; this is indicative of a molecular weight heterogeneity within the latex particles. The presence of sulfate, carboxyl, and hydroxyl groups upon the latex particle surfaces has been determined by conductometric titration.

Properties of silicon carbide for nuclear fuel particle coatings

Abstract: The properties of pyrolytic silicon carbide (SiC) that are important to its use in nuclear fuel particle coatings are reviewed. The structure of material deposited under different conditions varies...

Physical factors affecting recovery rates in flotation

Abstract: This paper reviews the physical factors that influence the rate of flotation of particles with emphasis on the hydro dynamic interactions between particles and bubbles. The flotation process is first order with respect to the particle concentration. The rate constant is strongly dependent on the particle size and the bubble diameter.

Ultrastructure of the sodium pump. Comparison of thin sectioning, negative staining, and freeze-fracture of purified, membrane-bound (Na+,K+)-ATPase.

Abstract: Purified (Na+, K+)-ATPase was studied by electron microscopy after thin sectioning, negative staining, and freeze-fracturing, particular emphasis being paid to the dimensions and frequencies of substructures in the membranes. Ultrathin sections show exclusively flat or cup-shaped membrane fragments which are triple-layered along much of their length and have diameters of 0.1-0.6 μm. Negative staining revealed a distinct substructure of particles with diameters between 30 and 50 A and with a frequency of 12,500 +/- 2,400 (SD) per μm(2). Comparisons with sizes of the protein components suggest that each surface particle contains as its major component one large catalytic chain with mol wt close to 100,000 and that two surface particles unite to form the unit of (Na+,K+)-ATPase which binds one molecule of ATP or ouabain. The further observations that the surface particles protrude from the membrane surface and are observed on both membrane surfaces in different patterns and degrees of clustering suggest that protein units span the membrane and are capable of lateral mobility. Freeze-fracturing shows intramembranous particles with diameters of 90-110 A and distributed on both concave and convex fracture faces with a frequency of 3,410 +/- 370 per μm(2) and 390 +/- 170 per μm(2), respectively. The larger diameters and three to fourfold smaller frequency of the intramembranous particles as compared to the surface particles seen after negative staining may reflect technical differences between methods, but it is more likely that the intramembranous particle is an oliogomer composed of two or even more of the protein units which form the surface particles.

Atomic data for controlled fusion research

Abstract: Presented is an evaluated graphical and tabular compilation of atomic and molecular cross sections of interest to controlled thermonuclear research. The cross sections are tabulated and graphed as a function of energy for collision processes involving heavy particles, electrons, and photons with atoms and ions. Also included are sections on data for particle penetration through macroscopic matter, particle transport properties, particle interactions with surfaces, and pertinent charged particle nuclear cross sections and reaction rates. In most cases estimates have been made of the data accuracy.

Influence of particles on AC and DC electrical performance of gas insulated systems at extra-high-voltage

Abstract: High-voltage breakdown measurements were made in two similar particle contaminated coaxial test systems, one with AC and the other with DC voltages. Information is presented on the effects of particle size, shape, and material for both SF 6 and N 2 gases at pressures up to 15 atm in a plain coaxial gap and a coaxial gap including a post-type support spacer. Particle motion and location were found to strongly influence insulation performance. Measured values of electric fields which lifted and drove the particles, so that they bounced vertically and laterally, compare favorably with calculated levels. Movement into the the higher stress region at the center conductor was correlated with the initiation of sparkover. These breakdowns could be at levels more than a factor of five lower than those obtained when contamination was not introduced. Large variations in breakdown voltage of as much as 3 to 1 encountered under DC correspond to conditions where particle motion could be restricted, presumably by corona discharge, to motion near the outer electrode. AC sparkover levels were typically at the lower limits of the DC range. Both free and attached particles on the dielectric spacer surface would trigger flashover at the same low levels as were measured in the gas gap.

Diagnostic method employing a mixture of normally separable protein-coated particles

Abstract: A mixture of two different kinds of particles having distinctive, different properties is employed for determining the presence of a select protein in, or the absence of a select protein from, a solution. The first kind of particle provides a property facilitating separation, while the second kind of particle provides a property facilitating detection. The particles are coated with the same protein, a protein able to interact specifically with the select protein.

Particle heating in a radio-frequency plasma torch

Abstract: A numerical model was developed to predict the behavior of a particle in a radio‐frequency (rf ) plasma. The analysis obtained the heat, mass, and momentum transfer of a single particle injected into an rf plasma. The governing equations for vaporization of a liquid particle were taken from the model which was used to simulate the vaporization process of liquid droplets in a rocket combuster. All the thermodynamical and transport properties were calculated as functions of temperature in order to simulate the actual behavior more precisely. The boundary conditions are a finite temperature at the particle center and a heat balance at the particle surface. Thermodynamical equilibrium at the particle surface was always assumed. ’’Moving‐boundary problems’’ in the case of melting or vaporizing processes were solved by the use of a ’’moving grid system’’. Four sizes of iron particles injected into the rf argon plasma have been computed: r0=20, 40, 60, and 80 μ. The iron particles with radii less than 70 μ could be melted under the assumed plasma conditions. It was found that the potentiality as concerns powder processing of an rf plasma was overestimated by previous models.

The energy dissipation in sheared coagulated sols

Abstract: The observed energy dissipation in coagulated sols subjected to a shear flow can be explained by assuming that the basic units of flow are elastic flocs which are slightly deformed during collisions because of the stretching of the particle bonds within a floc by a few tenths of a nanometer. Liquid inside perforated structures is generally considered immobile, but during the deformation of the flocs some internal liquid movement occurs. Although the amount of internal liquid movement is rather small, of the order of 1%, it is sufficient to account for the observed energy dissipation.

Magnetic particles for immunoassay

Abstract: This invention relates to magnetic particles and to the use thereof. Each particle comprises a low density core and a component therewith, at least a portion of the surface of the core being coated with magnetic material.

The motion of a small sphere in a rotating velocity field: A possible mechanism for suspending particles in turbulence

Abstract: The diffusion and transport of particles suspended in turbulent flows depend on the interaction between the particles and the turbulence. To investigate a possible particle-turbulence interaction, we observed particle trajectories in a rotating fluid with a solid body velocity profile, as might occur in the core of a vortex or an eddy. The experiments were conducted with a series of single small spheres (with low particle Reynolds numbers) sinking or rising in a horizontal cylinder rotating about its central axis at a constant speed. Each sphere was found to follow a nearly circular orbit in a vertical plane perpendicular to the cylinder axis. The orbit center lay very near the horizontal plane through the axis of the cylinder and was at the point in the fluid where the velocity of the fluid was equal and opposite to the terminal velocity of the particle. The particle trajectories also evolved slowly, spiraling either inward or outward. A theoretical description of the particle motion shows that the two principal forces on the particle, fluid drag and gravity-buoyancy, account for the circular motion. A force due to particle inertia (a centrifugal force from the center of its orbit), a small position-dependent wall effect on drag, a very small force (also affected by the walls), and a very small centrifugal buoyancy force account for the long-term inward or outward spirals. This kind of systematic interaction in which particles seek and remain in fluid closely opposing their own motion could have a role in the suspension of small particles in some turbulent flows.

Anchorage for highly stressed endoprostheses

Abstract: An implantable prosthesis anchorage which contains a non-porous outer coating in the regions which is positioned in contact with bone. The non-porous outer coating consists of (i) at least one bioactive resorbable ceramic material which is a calcium phosphate and (ii) at least one polymer which is mechanically and chemically stable in the body. The ceramic material is in particulate form having particle diameters between 0.5 and 1 mm. The ceramic material particles is incorporated in the polymer in such a way that resorption of the ceramic material leads to a polymer structure with continuous pores filled with living bone tissue. Bioactivating bonding residues of the ceramic material are left on the inner surfaces of the filled pores which were created by the resorption of the ceramic material.

A low resolution model for the chromatin core particle by neutron scattering

Abstract: Neutron scattering studies have been applied to chromatin core particles in solution, using the contrast variation technique. On the basis of the contrast dependance of the radius of gyration and the radial distribution function it is shown that the core particle consists of a core containing most of the histone around which is wound the DNA helix,following a path with a mean radius of 4.5 nm,in association with a small proportion of the histones. Separation of the shape from the internal structure, followed by model calculations shows that the overall shape of the particle is that of a flat cylinder with dimensions ca. 11x11x6 nm. Further details of the precise folding of the DNA cannot be deduced from the data, but detailed model calculations support concurrent results from crystallographic studies(25).Images

The influence of particle size on the tensile strength of particulate — filled polymers

Abstract: The tensile strengths of a particulate-filled rigid polyurethane resin are presented at varying volume fractions and a wide range of particle sizes. These results are compared with exisiting theories of the strength of particulate-filled composite systems. A linear relationship is proposed to exist between the mean particle diameter and the tensile strength at a given volume fraction. A method of normalizing data is presented which removes the stress-concentration effects of finite particle sizes and allows comparison of the data with a simple equation relating tensile strength and volume fraction. The effects of particle size and volume fraction in relation to crack propagation are discussed, and the proposed method of analysis is shown to give similar results when applied to published data.