Showing papers on "Particle published in 1990"

Particle response and turbulence modification in isotropic turbulence

Abstract: The effect of turbulence on particle concentration fields and the modification of turbulence by particles has been investigated using direct numerical simulations of isotropic turbulence. The particle motion was computed using Stokes’ law of resistance and it was also assumed the particle volume fraction was negligible. For simulations in which the particles do not modify the turbulence field it was found that light particles collect preferentially in regions of low vorticity and high strain rate. For increased mass loading the particle field attenuated an increasing fraction of the turbulence energy. Examination of the spatial energy spectra showed that the fraction of turbulence kinetic energy in the high wave numbers was increased relative to the energy in the low wave numbers for increasing values of the mass loading. It was also found that the turbulence field was modified differently by light particles than by heavy particles because of the preferential collection of the light particles in low‐vorticity, high‐strain‐rate regions. Correlation coefficients between the second invariant of the deformation tensor and pressure showed little sensitivity to increased loading while correlations between enstrophy and pressure were decreased more by the light particles than by the heavy particles for increased mass loading.

Inertial effects of a Dirac particle.

Abstract: Stationary laboratories on Earth accelerate and rotate relative to the local inertial frames. Any experiment precise enough would detect and/or need to take into account the effects due to acceleration and rotation. We derive these inertial effects for a Dirac particle in a straightforward and unified way within the framework of special relativity. The effects found include the Bonse-Wroblewski phase shift due to acceleration, the Sagnac-type effect, the rotation-spin effect, and the redshift of the kinetic energy.

Role of sub-micrometre particles in the ocean

Abstract: PARTICULATE matter plays an important part in biogeochemical cycles in the ocean; as particles settle out of the water column, they carry with them carbon and other adsorbed chemicals. Because particles of sub-micrometre size (at the boundary between 'particulate' and 'dissolved' materials) do not have appreciable settling rates in natural waters, they have generally been considered unimportant in the downward flux of particles and the nature and origin of the particles are largely unknown1–5. Here we present results from epifluorescence microscopy and from particle counting which have allowed us to determine the vertical distribution of sub-micrometre particles (size range 0.38–1 µm). We find that >95% of these particles are non-living and occur in the upper layers of the ocean (50 m) in concentrations of the order of 10 million per millilitre. Many of the non-living particles seem to be fragile and flexible, and seem to have a high water content and to be composed largely of organic material. The size distribution of these sub-micrometre particles leads us to conclude that a significant portion (at least 10%) of 'dissolved' organic material may in fact be in the form of these small particles, as suggested by Sharp6.

Particle trapping phenomena in radio frequency plasmas

Abstract: Particles generated in an argon plasma and suspended at the plasma/sheath boundary are localized by lateral trapping fields. In the commercial rf etching reactor used in this work, the particles and their motion in real time are observed by laser light scattering with the laser beam rapidly rastered in a plane parallel to the rf electrode. Repulsion between individual, relatively large particles is observed, verifying that there is significant negative charge on the particles. Two types of trapping regions are commonly seen: rings of particles around the outside edge of silicon wafers, and domes of particles over the centers of the wafers. It is shown that these effects are influenced by the topography of the electrode. In addition, particle densities >107 cm−3 for particles of diameter 0.2 μm are inferred from transmission studies for certain plasma conditions.

Measurements of Kinetic Energy Loss for Particles Impacting Surfaces

Abstract: Incoming and rebounding particle velocities were measured to within several particle diameters of the impaction surface using laser Doppler velocimetry. Impacts occurred normal to the surface and ranged from 1 m/s, near the threshold for particle bounce, to 100 m/s, well into the plastic damage regime. Monodisperse ammonium fluorescein spheres, 2.6–6.9 μm in diameter, impacted target surfaces including polished molybdenum and silicon, cleaved mica, and a fluorocarbon polymer. The incident kinetic energy recovered on rebound depended on particle size and target composition at low velocity (< 20 m/s), where the adhesion surface energy is important. No dependence on target composition was found at higher velocities where up to half of the impact energy was lost to plastic deformation. Plastic deformation was a significant component of energy loss even at impact velocities near critical velocity. Critical velocities for the onset of bounce decreased with a stronger power-law dependence on particle diameter th...

Mass-Dimensional Relationships for Ice Particles and the Influence of Riming on Snowfall Rates

Abstract: The masses, dimensions, and habits of over 2800 natural ice particles precipitating from orographic winter storms in the central Sierra Nevada were obtained using photomicrographs. Ice particles that could be unambiguously classified were used to generate empirical expressions relating snow particle masses and dimensions. Many of the ice particle types had not been investigated previously. The influence of riming and aggregation on ice particle masses was examined. When possible, comparisons are made between these results and those of other experimental observations. By incorporating these mass-dimensional relationships into an expression for the ice mass content in a snowstorm, it was possible to estimate the mass fraction of the fresh snowpack resulting from accreted supercooled cloud water. The results from two storms analyzed suggest that about 30 to 40 percent of the deposited snow is composed of accreted cloud water during moderately rimed snowfall.

Polymer-filler interactions in rubber reinforcement

Abstract: The reinforcement of elastomers by finely divided fillers, particularly carbon black and silica, is fundamental to the rubber industry. Optimal reinforcement appears to involve both physical and chemical interactions. From a consideration of the effects of particle size as such, it appears that reinforcement, in the sense of tensile enhancement, will occur with any very finely divided filler. Physical factors prevent escape of the polymer from the filler surface (vacuole formation) but allow stress delocalization through interfacial slippage. Occasional stronger bonds may be introduced advantageously to facilitate dispersion, reduce particle/ particle interactions, and optimize practical properties relating to resilience and durability. Several lines of evidence suggest that only a minor amount of strong bonding is necessary or desirable, such that polymer/filler slippage can occur, under stress, over most of the interfacial area.

Morphology development of polymeric microparticles in aqueous dispersions. I. Thermodynamic considerations

Abstract: A thermodynamic analysis of polymer particle morphology highlights the role of interfacial tensions in controlling particle structure. The influence of the surfactant and the nature of the incompatible polymers is seen through their individual and collective effects upon these interfacial tensions. It has been found that by simply changing the type of surfactant used in the emulsion the particle morphology can change from core-shell to hemispherical, in agreement with thermodynamic predictions. Several apparently different morphologies (hemispherical, sandwich, multiple lobes) have been found to coexist at the same time within a single emulsion, suggesting that they may be simply different states of phase separation and not thermodynamically stable, unique morphologies. The thermodynamic analyses are independent of particle size and method of emulsion processing. Experimental evidence shows that the morphology of particles formed via in situ polymerization (as in a synthetic latex) is controlled by interfacial tensions in the same manner as those particles formed via solvent evaporation from a solution of an incompatible polymer pair (as in an artificial latex or microencapsulation).

Kinetic theory for a monodisperse gas–solid suspension

Abstract: The fluid‐dynamic and solid‐body interactions among a suspension of perfectly elastic particles settling in a viscous gas are studied. The Reynolds number of the particles, Re≡ρfUa/μ, is small but their Stokes number St≡mŪ/(6πμa2) is large, indicating that particle inertia and viscous forces in the fluid are important. Here, ρf is the density of the fluid, m is the mass of a particle, U is the average velocity of the particles, a is their radius, and μ is the fluid viscosity. Equations for the particle velocity distribution and averages of the fluid and particle velocities are derived. For very large Stokes numbers, St≫φ−3/2, where φ is the particle volume fraction, solid‐body collisions lead to a nearly Maxwellian velocity distribution. On the other hand, at smaller Stokes numbers, St≪φ−3/2, fluid‐dynamic interactions play a more important role in determining the particle velocity distribution and the distribution is not Maxwellian. The amount of energy contained in the particle velocity fluctuations is...

Condensation of DNA by trivalent cations. 1. Effects of DNA length and topology on the size and shape of condensed particles.

Abstract: In vitro condensation of DNA by multivalent cations can provide useful insights into the physical factors governing folding and packaging of DNA in vivo. We have made a detailed study of hexammine cobalt (III) induced condensation of 2700 and 1350 base pair (bp) fragments of plasmid pUC12 DNA by electron microscopy and laser light scattering. The condensed DNA takes the form of toroids and rods. Both are present in all condensates, but the proportion of toroids is higher with the larger fragments. The intact, closed circular plasmid produces smaller particles than the linear fragments. The size of a particle is independent of DNA fragment length. Two hours after adding the condensing agent, a typical toroid is about 800 A in diameter; the outer radius (R1) is approximately 400 A, and the inner radius (R2) is approximately 140 A for both sets of fragments. These dimensions are relatively stable, but there is sufficient change in both R1 and R2 to produce approximately 50% increase in volume from 2 to 24 h. A typical rod at 2 h is about 1800 A long and 300 A wide. The distribution of rod lengths is similar to that of mean toroid circumferences pi (R1 + R2), and the distribution of rod widths is similar to that of toroidal widths (R1-R2). The 2700-bp fragments show a significantly higher ratio of toroids to rods than the 1350-bp fragments. Both types of particle are multimolecular. The average number of molecules/particle, calculated from the above dimensions, assuming hexagonally packed B-form DNA with a center-to-center spacing of 27 A, is 13 +/- 4 for condensates of 2700-bp fragments and 26 +/- 11 for those of 1350-bp fragments. Monomolecular condensates of much larger DNAs have similar dimensions, suggesting that size is governed primarily by the balance of attractive and repulsive intermolecular forces rather than by the entropic factors associated with incorporation of a number of small particles into a larger one. The similar dimensions and volumes of toroids and rods indicate that the free energy cost of continual bending in toroids, minus that gained by extra net attraction in a cyclic particle, is comparable to that of abrupt bending or kinking in rods. Although the condensed particles are multimeric, their distinct toroidal or rodlike shapes distinguish them from the random aggregates that would be generally expected from the multimolecular association of large, flexible polymers.

Method for gravel packing a well

Abstract: A method of packing a well, particularly an oil, gas or water well comprises injecting a particle/liquid slurry into a wellbore wherein the particles comprise particles of at least two different densities independently selected from the density range of about 0.1 to about 4.0. The at least two different density particles may be injected as blend of different density particles or in a sequential series of slugs, each slug having a single density particle suspended therein. This technique gives substantially equivalent packing efficiency throughout the annulus (14), perforations (4) and perforation chambers (6) when compared with gravel packing using specialized low to medium density materials.

Measurements of Solid Spheres Bouncing Off Flat Plates

Abstract: Recent years have seen a substantial increase of interest in the flows of granular materials whose rheology is dominated by the physical contact between particles and between particles and the containing walls. Considerable advances in the theoretical understanding of rapid granular material flows have been made by the application of the statistical methods of molecular gas dynamics (e.g., Jenkins and Savage (1983), Lun et al. (1984)) and by the use of computers simulations of these flows (e.g., Campbell and Brennen (1985), Walton (1984)). Experimental studies aimed at measurements of the fundamental rheology properties are much less numerous and are understandably limited by the great difficulties involved in trying to measure velocity profiles, solid fraction profiles, and fluctuating velocities within a flowing granular material. Nevertheless, it has become clear that one of the most severe problems encountered when trying to compare experimental data with the theoretical models is the uncertainty in the material properties governing particle/particle or particle/wall collisions. Many of the theoretical models and computer simulations assume a constant coefficient of restitution (and, in some cases, a coefficient of friction). The purpose of the present project was to provide some documentation for particle/wall collisions by means of a set of relatively simple experiments in which solid spheres of various diameters and materials were bounced off plates of various thickness and material. The objective was to provide the kind of information on individual particle/wall collisions needed for the theoretical rheological models and computer simulations of granular material flows: in particular, to help resolve some of the issues associated with the boundary condition at a solid wall. For discussion of the complex issues associated with dynamic elastic or inelastic impact, reference is made to Goldsmith (1960) and the recent text by Johnson (1985).

Morphology and magnetic properties of ultrafine ZnFe2O4 particles

Abstract: Well-crystallized ultrafine ZnFe2O4 particles of several nanometers in size have been prepared by the coprecipitation method, and their particle morphology and magnetic properties, especially at low temperatures, examined. Room-temperature X-ray diffraction, transmission electron microscopy, magnetization measurements at various temperatures from 300 K to 4.2 K, and Mossbauer spectroscopy at various temperatures from 300 K to 4.2 K, and at 4.2 K with a longitudinal magnetic field of 16.4 kOe applied have been employed. The formation of short-range and long-range magnetic order in small ZnFe2O4 particles above and below approximately 30 K is discussed. Below 30 K, the appearance of spontaneous magnetization and its hysteretic property is confirmed for small ZnFe2O4 particles.

Transient bleaching of small lead sulfide colloids: influence of surface properties

Abstract: Small PbS colloids with a particle diameter of 40 {angstrom} were prepared in aqueous solution, and their absorption spectra exhibit several maxima. Injection of electrons into these particles was achieved by using the pulse radiolysis technique. Excess electrons trapped on the surface lead to a blue shift in the absorption edge of colloids. The appearance of this shift depends critically on the method of colloid preparation. PbS and CdS colloids prepared at pH 8). The existence of a hydroxide ion on the particle surface most likely removes surface defects on which electrons are trapped. PbS colloids prepared in the presence of 3-mercapto-1,2-propanediol have an unstructured absorption spectrum, which is due to a wide particle size distribution (10-50 {angstrom}).

Dispersion polymerization of styrene in polar solvents

Abstract: The true steric stabilizer in hydroxypropyl cellulose (HPC)-stabilized dispersion polymerization of styrene in polar solvents is grafted HPC-polystyrene, formed in situ , which ends up on the particle surface. The pyrene-labeled HPC experienced a molecular weight increase similar in magnitude to the molecular weight of polystyrene formed by dispersion polymerization. The pyrene-containing polymer also had solubility properties consistent with graft. Kinetic analysis for grafting via chain transfer in solution polymerization predicts a grafting probability of about 1 in 200 polystyrene chains, so the particles must be large (on the order of 0.1 μm) before being sterically stabilized. Precipitation experiments were designed to separate nucleation and stabilization from concurrent polymerization. Precipitation of dispersion-polymerized particles from dioxane with methanol produced new particles with the same morphology as the original particles. The size of these precipitated particles was inversely proportional to the addition rate of nonsolvent to the 0.4 power, but the size distributions were very wide. In contrast, precipitation of commercially available polystyrene from dioxane in the presence of HPC did not produce stable particles. These experiments also suggest that nuclei coalesce extensively until they are sterically stabilized by graft.

Fate of Metals in Waste Combustion Systems

Abstract: The mechanisms which control the emission of trace metals from waste combustion systems were examined. Important phenomena include particle entrainment, chemical interactions, vaporization, condensation, particle coagulation and particle collection by flue gas cleaning equipment. A model based on these phenomena was developed to estimate their relative importance and to assess the potential impact of waste combustor operating parameters on metals emissions. The results of this assessment were used to develop a more accurate method of assessing the ability of waste combustion devices to control the emission of toxic metals than is currently proposed by the Environmental Protection Agency. The method is based on use of metals spiking and was tested in a pilot scale rotary kiln incinerator. The tests indicated that the spiking method could effectively be used to estimate metals emissions.

Energetic Particles from Impulsive Solar Flares

Abstract: Observations of solar energetic particles from impulsive flares are reviewed. Consideration is given to observations of electron events, He-3 rich events, and heavy-nuclei-rich events. It is found that these observations can be unified into a description of the particles from impulsive flares. The observations are compared with observations of gamma-ray line in impulsive flares and particles in flares and compared with particles in space. A model for accelerating the unique particle abundances of energetic particles is proposed.