Showing papers on "Particle published in 1975"

Transfer of Particles in Nonisotropic Air Turbulence

Abstract: The inertia of particles is the basis of the present model of particle “dry deposition” from a turbulent fluid. The basic physical concepts of particle thermal motion as described by Einstein are herein extended to describe the particle motion in nonisotropic turbulence in terms of a “turbophoretic” velocity. The particle transfer from the turbulent fluid to a collecting black surface is therefore described in terms of a differential motion describing the chain of “inertial flights” undergone by the particle under the gradient of turbulence velocity component in the direction of the collecting surface. This model of dry deposition shows satisfactory agreement with experimental values of particle deposition velocity as a function of particle size.

Disposable absorbent articles containing hydrogel composites having improved fluid absorption efficiencies and processes for preparation

Abstract: Hydrogel composites are provided having improved fluid absorption efficiencies which render them especially useful in disposable absorbent articles, such as diapers, catamenial devices and the like. The hydrogels are in particulate form, each particle or agglomerate of particles being substantially coated with fibers, a portion of which fibers extend from the particle. The fibers enhance the rate at which the hydrogel absorbs aqueous fluids and also serve to maintain the fluid in close proximity to the hydrogel. The extended fibers serve to anchor the hydrogel particle when it is contained in a fibrous or cellular matrix.

Application of the Langevin equation to fluid suspensions

Abstract: Brownian motion of particles suspended in a fluid is studied, and expressions derived for the particle diffusivity and velocity autocorrelation function. The theory of thermal noise in a general linear system is applied to both the particles and the fluid in a novel formulation. This enables the recent modification of the Langevin equation to include the effect of fluid inertia to be seen as just a necessary but simple reinterpretation of the original analysis, without introducing the theory of non-Markovian processes.

The slow motion of slender rod-like particles in a second-order fluid

Abstract: The motion of a slender axisymmetric rod-like particle is investigated theoretically for translation through a quiescent second-order fluid and for rotation in a simple shear flow of the same material. The analysis consists of an asymptotic expansion about the limit of rheologically slow flow, coupled with an application of a generalized form of the reciprocal theorem of Lorentz to calculate the force and torque on the particle. It is shown that an arbitrarily oriented particle with fore-aft symmetry translates, to a first approximation, at the same rate as in an equivalent Newtonian fluid, but that the motion of particles with no fore-aft symmetry may be modified at the same level of approximation. In addition, it is found that freely translating particles with fore-aft symmetry exhibit a single stable orientation with the axis of revolution vertical. In simple shear flow at small and moderate shear rates, the non-Newtonian nature of the suspending fluid causes a drift through Jeffery orbits to the equilibrium orbit C = 0 in which the particle rotates about its axis of revolution. At larger shear rates, the particle aligns itself in the direction of flow and ceases to rotate. Comparison with the available experimental data indicates that the measured rate of orbit drift may be used to determine the second normal stress difference parameter of the second-order fluid model. Finally, in an appendix, some preliminary observations are reported of the motion of slender rod-like particles falling through a quiescent viscoelastic fluid.

Optical levitation of liquid drops by radiation pressure.

Abstract: Charged and neutral liquid drops in the diameter range from 1 to 40 microns can be stably levitated and manipulated with laser beams. The levitation technique has been extended toward smaller particles (about 1 micron), lower laser power (less than 1 milliwatt), and deeper traps (greater than ten times the particle's weight). The techniques developed here have particular importance in cloud physics, aerosol science, fluid dynamics, and optics. The interactions of the drops with light, the electric field, the surrounding gas, and one another can be observed with high precision.

Low-angle neutron scattering from chromatin subunit particles

Abstract: Monomer chromatin particles containing 140 base pairs of DNA and eight histone molecules have been studied by neutron scattering. From measurements in various H2O/D2O mixtures, radii of gyration and the average scattering density of the particle were determined. The radius of gyration under conditions when scattering from the DNA dominates is 50A, and when scattering from the protein dominates, 30A. Consequently the core of the particle is largely occupied by the histones while the outer shell consists of DNA together with some of the histone.

A theory of aerosol deposition in the human respiratory tract.

Abstract: The deposition of inhaled aerosol particles in the human respiratory tract is due to the mechanisms of inertia impaction, Brownian diffusion, and gravitational settling. A theory is developed to predict the particle deposition and its distribution in human respiratory tract for any breathing condition. A convection-diffusion equation for the particle concentration with a loss term is used to describe the transport and deposition of particles. In this equation, an apparent diffusion coefficient due to the velocity dispersion in the lung is present and found to be the dominant diffusion mechanism for the cases considered here. Expressions for deposition by various mechanisms are also derived. The governing equation is solved numerically with Weibel's lung model A. The particle concentration at the mouth is calculated during washin and washout and compared favorably with experimental recordings for 0.5-mum diameter di(2-ethylhexyl) sebacate particles. The total deposition in the lung for particle size ranging from 0.05 to 5 mum is also computed for a 500-cm-3 tidal volume and 15 breaths/min. The results in general agree with recent measurements of Heyder et al. However, a particle size of minimum deposition is found to exist theoretically near 0.3 mum.

Hot-pressing behaviour of silicon carbide powders with additions of aluminium oxide

Abstract: The hot-pressing behaviour of different silicon carbide powders (average particle sizes ranging from ∼ 0.5 to 9 μm) with aluminium oxide additions ranging from 0.01 to 0.15 volume fractions was investigated. Using powders with an average particle size < 3 μm, densities ≥ 99% theoretical could be achieved at 1950° C (1 h) with 28 MN m−2 for volume fractions of AI2O3 $$\bar > $$ 0.02. A liquid phase forms at high temperatures which dissolves the silicon carbide particles to promote densification by a solution-reprecipitation mechanism.

Observation of new-particle production by high-energy neutrinos and antineutrinos

Abstract: We have observed fourteen events in which two muons are produced by high-energy neutrino and antineutrino interactions. The absence of trimuon events and the observed characteristics of the dimuon events require the existence of one or more new massive particles that decay through the weak interaction. The new particle mass is estimated to lie between 2 and 4 GeV.

Particle formation by resonant laser light in alkali-metal vapor

Abstract: Resonant laser light causes micron-sized particles to form in cesium vapor when a small amount of hydrogen gas is present in the vapor. The particles are identified as cesium-hydride crystals. This is the first observation of light-induced particle formation in metal vapors. The phenomenon has relevance to lasers and nonlinear optical devices based on dense alkali vapors, and it may also be of interest for laser isotope-separation schemes.

The dynamics of interacting Brownian particles

Abstract: Suspensions of microscopic particles in a liquid occur widely in nature and are important in many industrial processes. In very dilute suspensions, the particles can be regarded as effectively non-interacting; thus a particle can take up any position in, and diffuse through, the suspension without interference from other particles. At higher concentrations, however, interactions (for example, short-ranged “hard-sphere” or longer-ranged Coulombic) between the particles become important and the positions and motions of different particles become correlated. Detailed theoretical understanding of the structure and dynamics of concentrated suspensions constitutes an interesting and challenging many-body problem to which modern statistical mechanical and hydrodynamical theories can be applied.

Failure of brittle polymers by slow crack growth

Abstract: The mechanical properties of a silica particle-filled epoxy resin composite system have been investigated in air as a function of volume fraction of particles for volume fractions ranging from 0 to 0.52. The Young's modulus and the compressive yield stress both increase as the volume fraction of silica particles is increased and various models of particle strengthening have been used to explain this behaviour. Slow crack growth in the various particulate composites has been studied using a fracture mechanics approach. The variation of crack velocity (V) with stress intensity factor (KI) has been measured for each of the compositions investigated. In each case, a unique relationship between V and KI has been found with KI increasing with volume fraction of particles at a given value of V. The failure mechanisms and the variation of other fracture mechanics parameters, for example, crack opening displacement and plastic zone size with increasing particle volume fraction have been discussed.

Coagulation of Colloidal Silica by Calcium Ions, Mechanism, and Effect of Particle Size

Abstract: It is found that the critical concentration of calcium ion in solution required to coagulate colloidal silica, increases with decreasing particle size. The effect was studied with particles from 4 to 130 nm in average diameter in the pH range 8.1 to 9.5 and was most marked at lower pH. By careful control of conditions, larger particles in a mixture can be preferentially coagulated and separated from smaller ones. For coagulation, a critical number of calcium ions must be absorbed per square nanometer of silica surface, independent of particle size; but to attain this degree of adsorption, a higher concentration of calcium must be maintained in solution when the particles are smaller. On the silica surface, which already contains negative charges, each adsorbed calcium ion liberates only one hydrogen ion, creating one additional negative charge on the surface, so that each adsorbed calcium ion retains one positive charge. On the more highly curved surface of smaller particles, each calcium ion, adsorbed outside the particle surface, is repelled by its neighbors with a resultant force away from the surface so that a higher concentration of calcium in solution is required to maintain the critical concentration of adsorbed calcium for coagulation. Coagulation is probably due to attraction between surfaces bearing a mosaic of positive and negative sites.

Augmentation of heat transport in laminar flow of polystyrene suspensions. II. Analysis of the data

Abstract: It has been experimentally demonstrated in the preceding paper, I, that heat transport is augmented in the laminar flow of suspensions of 100‐ and 50‐μ polystyrene spheres. The augmentation has been seen to be as much as 200% and depends on the shear rate and several other parameters. Here the experimental data are analyzed by proposing a model based on the particle rotations and the entrained fluid, and by presenting similarity arguments. It is shown that the augmentation in heat transport resides in the inertia of the entrained fluid rotating with the particle which is manifest in two parameters, namely, Reynolds (ωa2/νf) and Peclet (ωa2/αf) numbers, where ω is the angular velocity of the particle, a is the radius of the sphere, and νf and αf, respectively, are the kinematic viscosity and the thermal diffusivity of the suspending fluid. The other parameters on which the augmentation in heat transport depends are the particle volume fraction, tube‐radius–to–particle‐radius ratio, tube‐length–to–particle‐...

Experimental testing of the hydrodynamic collision model of fine particle flotation

Abstract: Flotation rates of glass beads and of latex particles have been measured as a function of particle size using very small bubbles. With glass beads the observed rate versus size relationship agreed quite well with the prediction of a simple hydrodynamic collision model, but that found with latex particles did not. It is suggested that electrical forces may have to be taken into account when the particles have a significant zeta potential. With both types of particle, the relationship between flotation rates measured at two different bubble sizes is consistent with the model's predictions.

The Melting of Small Particles. II. Bismuth

Abstract: The melting of thin films of bismuth, consisting of individual crystallites, has been investigated in the electron microscope. These films showed an unexpected behaviour in the neighbourhood of the melting point: when an aggregate of particles was held at a constant temperature, the number of particles remaining solid gradually decreased with time, although the actual melting process for each individual particle was extremely fast. This reduction in the number of solid particles with time is approximately exponential. The dependence of the characteristic time for the melting of an aggregate of particles on the temperature and morphology of the particles has been studied in detail. In particular, a platelet form had a sufficiently large time delay, even above the bulk melting point, to allow some of the crystallites to be superheated by up to 7 K.

High gradient magnetic separation: Theory versus experiment

Abstract: The experimental performance of a high gradient magnetic separator has been previously reported by other workers in some detail for a CuO/Al 2 O 3 slurry. Less detailed results were also reported for slurries of Mn 2 O 3 , Al, and α-Fe 2 O 3 particles with Al 2 O 3 representing a 20:1 range in particle sizes and a 200:1 range in magnetic susceptibility. Examination of these results indicates that many layers of particles build up on each filter fiber. Accordingly, in this paper we extend the original particle trajectory model for the calculation of filter performance, to include the build-up of multiple layers of particles on the fibers. Good agreement is obtained between the calculated recoveries and purities for all of the particles and the experimentally reported values using a filter packing efficiency and a mechanical trapping term, derived from the CuO data, as adjustable parameters.

Stratospheric Aerosol Particles and Their Optical Properties

Abstract: This review emphasizes those aspects of aerosol particles in the stratosphere that have not been covered by other reviews. This approach is not severely limiting because of the almost explosive increase in our knowledge of such particulate matter during the last few years. Furthermore, this review places special emphasis on the optical properties of the particles. Stratospheric particles have been investigated by remote optical means for at least 80 years, but not until about 1960 were such particles collected and examined. The particles consist largely of volcanic ash for periods of possibly a few months following major explosive volcanic eruptions, but at other times they consist largely of impure sulfuric acid which may be in the form of droplets or crystals. The sulfuric acid is believed to be formed largely by the oxidation and hydration of sulfur dioxide, much of which is introduced into the stratosphere by volcanic eruptions. There is considerable uncertainty as to whether the particle size distributions are unimodal or polymodal or whether the nature of the distribution varies markedly with time and space. The mixing ratio of the particles, at least of the larger ones in the 0.1- to 1µm-radius interval, is almost always greatest a few kilometers above the tropopause. However, this ‘layer’ is often highly structured. The particles may serve as catalysts for, or otherwise participate in, stratospheric chemical reactions. Stratospheric particles, by absorbing and scattering radiation from the sun and earth, can affect the atmosphere's radiation balance, thereby affecting the climate. Some numerical results concerning the amount of energy lost from an incident light beam as it propagates through a layer of uniformly distributed particles are presented. The results are shown as contours of constant values of the total percentage loss of energy due to extinction, scattering, and absorption for a vertically incident solar beam over a wide range of particle radii and imaginary refractive indices for real refractive indices of 1.4 and 1.5. The application of Mie scattering theory to the use of laser radar (lidar) is also described. Another major effect of the stratospheric aerosols is the denial of radiation to the region below the stratosphere. At the same time, they add energy to the stratosphere by the absorption of radiation. Various types of analytical models of climate change due to aerosols are discussed, and the energetic equilibrium of small particles in the atmosphere is also considered in some detail.

Calibration of the Pollak Counter with Monodisperse Aerosols.

Abstract: The photoelectric condensation nucleus counter of Pollak with convergent light beam has been compared with an electrical aerosol detector using monodisperse aerosols with particle diameters between 0.025 and 0.15 μm, particle concentrations between 127 and 260,800 cm−3, and particles of two different chemical constituencies, e.g., NaCl and material volatilized from a heated nichrome wire. Very good agreement has been obtained. The discrepancy between these two methods was found to be less than 9% at concentration levels below 104 particles cm−2 and 17% at 2.5 × 105 particles cm−3. This discrepancy is well within the combined uncertainties in the two independent aerosol concentration measuring methods.

Evidence for an Elemental Sulfur Component of the Clouds from Venus Spectrophotometry

Abstract: The decrease in the reflectivity of Venus in the near-UV can be explained if the clouds contain particles of elemental sulfur in addition to sulfuric acid. The low-resolution McDonald-Pittsburgh spectrum can be fitted by two sulfur-containing, multiple-scattering cloud models: (1) a mixed cloud consisting of one particle of elemental sulfur of radius 10 microns for every 670 particles of sulfuric acid of radius 1 micron, and (2) a layered cloud of optical thickness tau = 1.0 consisting of one-micron particles of sulfuric acid overlying a thick cloud of elemental sulfur particles of radius 3.6 microns. Some of the sulfur is incompletely polymerized. The source of the sulfur is photo-dissociation of COS, although some may also be recycled from the lower atmosphere. The sulfur plays a crucial role in the planetary meteorology of Venus since it is responsible for the bulk of the absorption of solar energy.