Showing papers on "Particle published in 1971"

Surface and Colloid Science

Abstract: 1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Water and waste water filtration. Concepts and applications

Abstract: H A conceptual model for water and waste water filtration processes is presented and compared with the results of laboratory experiments. Efficient filtration involves both particle destabilization and particle transport. Destabilization in filtration is similar to destabilization in coagulation; effective coagulants are observed to be effective “filter aids.” Particle transport in filtration is analogous to transport in flocculation processes. A particle size with a minimum contact opportunity exists ; smaller particles are transported by diffusion while larger particles are transported by interception and settling. Applications of these concepts to water and waste water filtration are presented.

The stress generated in a non-dilute suspension of elongated particles by pure straining motion

Abstract: In a pure straining motion, elongated rigid particles in suspension are aligned parallel to the direction of the greatest principal rate of extension, provided the effect of Brownian motion is weak. If the suspension is dilute, in the sense that the particles are hydrodynamically independent, each particle of length 2l makes a contribution to the bulk deviatoric stress which is of roughly the same order of magnitude as that due to a rigid sphere of radius l. The fractional increase in the bulk stress due to the presence of the particles is thus equal to the concentration by volume multiplied by a factor of order l2/b2, where 2b is a measure of the linear dimensions of the particle cross-section. This suggests that the stress due to the particles might be relatively large, for volume fractions which are still small, with interesting implications for the behaviour of polymer solutions. However, dilute-suspension theory is not applicable in these circumstances, and so an investigation is made of the effect of interactions between particles. It is assumed that, when the average lateral spacing of particles (h) satisfies the conditions b [Lt ] h [Lt ] l, the disturbance velocity vector is parallel to the particles and varies only in the cross-sectional plane. The velocity near a particle is found to have the same functional form as for an isolated particle, and the modification to the outer flow field for one particle is determined by replacing the randomly placed neighbouring particles by an equivalent cylindrical boundary. The resulting expression for the contribution to the bulk stress due to the particles differs from that for a dilute suspension only in a minor way, viz. by the replacement of log 2l/b by log h/b, and the above suggestion is confirmed. The relative error in the expression for the stress is expected to be of order (log h/b)−1. Some recent observations by Weinberger of the stress in a suspension of glass-fibre particles for which 2l/h = 7·4 and h/2b = 7·8 do show a particle stress which is much larger than the ambient-fluid stress, although the theoretical formula is not accurate under these conditions.

Influence of Particle Size on the Electrical Resistivity of Compacted Mixtures of Polymeric and Metallic Powders

Abstract: Powder samples of high‐density polyethylene and nickel of particle sizes Rp and Rm , respectively, were mixed and compacted at room temperature under a pressure of 1000 kg/cm2. Microscopic examination of polished sections of the compact, by reflected light, showed that the metallic particles did not penetrate the polymeric particles and that this resulted in a segregated distribution of metallic particles at high ratios of Rp/Rm . The electrical resistivity of the compacts had a value of 1016 Ω cm unless the composition of metal reached a critical value, beyond which the resistivity decreased markedly by as much as twenty orders‐of‐magnitude. This critical composition was found to decrease with an increase in the ratio Rp/Rm throughout the range studied of from 1 to 16. The general features of the dependence of electrical resistivity on composition of metal could be rationalized by reference to a model according to which small particles of nickel form a monolayer on the large particles of polymer in the mixture of powders. This arrangement was supposed to be but little changed during compaction and to result in a segregated distribution of metal which can be visualized as approximating to the accommodation of metallic particles on three mutually perpendicular sets of lattice planes. The critical composition for a sudden decrease in electrical resistivity was assumed to correspond to the first nonzero probability for infinitely long chains of contiguously occupied lattice sites.

Theory of particle formation and growth in oxide synthesis flames

Abstract: This work discusses those factors which determine ultimate particle size in the synthesis of pyrogenic silica. Pyrogenic or fumed silicas are produced commercially through the combustion of a premixed stream of silicon tetrachloride, hydrogen and air. Because of their minute size, silica particles assume translational velocities and collision frequencies characteristic of large gas molecules. When chemical reaction and nucleation times are short relative to the total formation period; growth, theoretically, approaches a rate which is independent of the early history of the system and is determined solely by the frequency of Brownian collisions. Under these conditions, the logarithm of particle size is directly proportional to the logarithm of growth time. In commercial flames, secondary air is inducted into the flame jet, causing particles to cool below their fusion temperature. Under such circumstances, the growth time and thus the ultimate particle size is a strong function of initial flame tem...

The effect of weak Brownian rotations on particles in shear flow

Abstract: Axisymmetric particles in zero Reynolds number shear flow execute closed orbits. In this paper we consider the role of small Brownian couples in establishing a steady-state probability distribution for a particle being on any particular orbit. After presenting the basic equations, we derive an expression for the equilibrium distribution. This result is then used to calculate some bulk properties for a suspension of such particles, and these predicted properties are compared with available experimental observation.

Molecular Theory of Brownian Motion for Several Particles

Abstract: A molecular derivation is presented for the coupled Langevin equations that describe the motion of heavy particles in a fluid. In contrast to the case of a single heavy particle, the friction tensors which appear depend upon the instantaneous separations between the particles. A Fokker–Planck equation describing the reduced distribution function for the heavy particles is also obtained. Calculations with these equations require evaluation of the friction tensors. The friction tensors are evaluated in two ways, by an approximate macroscopic hydrodynamic calculation and an approximate hydrodynamic fluctuation calculation. Both calculations lead to identical expressions for the friction tensors which are shown to have a long‐range character at large interparticle separations. Finally, it is shown that these long‐range effects cancel in the calculation of the diffusion constant for each particle but remain in the calculation of the relative diffusion constant.

Particle Formation in Polymer Colloids, III: Prediction of the Number of Particles by a Homogeneous Nucleation Theory

Abstract: The formation of particles in polymer colloids ordinarily is accomplished by the free radical polymerization of an organic monomer in a liquid which is a non-solvent (diluent) for the polymer. A surface active material, such as a soap or other amphipathic molecule, is usually added to stabilize the colloidal particles as they are formed. The particle size distribution varies from very narrow to extremely broad depending primarily upon the solubility of the monomer in the diluent, the stabilizer concentration and the ionic strength.

Mechanically-alloyed aluminum-aluminum oxide

Abstract: A mechanically alloyed composite powder wherein individual particles include an aluminum matrix having equiaxed aluminum oxide dispersoid particles having a size of about 100A to 2,000A uniformly distributed therethrough at particle spacings of about 500 to 3,000A. Products made by consolidating the mechanically alloyed powder exhibit improved strength properties at relatively low dispersoid contents.

Polymerization in expanding catalyst particles

Abstract: Four models are presented to describe polymerization in expanding catalyst particles. The globules are presumed to be expanding with accumulating polymer, and catalytic reaction sites are dispersed throughout the polymer matrix which is forming about them. Monomer must diffuse through the polymer to polymerize at the catalyst sites. The Thiele modulus (α), , is a ratio of characteristic diffusion time to reaction time which is a measure of the importance of diffusion relative to reaction. Polymerization rates are predicted by the models which are generally dependent on the controlling mechanism. Broad molecular weight distributions are predicted for cases of diffusion control (large α) for those models in which catalyst sites are not equally accessible to monomer. Polymerization rates decline toward an asymptotic final value as the particles expand in diffusion-controlled cases. Most of the decline which would be readily observable in a laboratory experiment would have occurred by the time the particle radii had increased to about three times their original value.

Low temperature magnetic hysteresis of fine particle aggregates occuring in some natural samples

Abstract: Hysteresis loops of basaltic rocks which usually have aggregates of magnetic particles have been studied in the temperature range 25° to −190°C. Some samples showed coercive forces as high as 930 oe at −190°C. Blocking temperature studies of the magnetic grains in these samples revealed that they contain particles, most of which are superparamagnetic at or around room temperature. The studies are consistent with the inference that very fine titanomagnetite particles are responsible for the observed behaviour of the samples.

Relation of particle size of colloidal silica to the amount of a cationic polymer required for flocculation and surface coverage

Abstract: This is a study of the flocculation of colloidal silicas of known uniform particle sizes by a quaternary ammonium substituted methacrylate polymer, of which the cations are adsorbed on the anionic sites of the silica surface by ion exchange. The unusual observation is reported that as increasing amounts of polymer are added to a silica sol of a given particle size, all of the polymer is precipitated, carrying down with it a constant ratio of silica to polymer. When sufficient polymer has been added to precipitate all of the silica, further amounts of polymer are then adsorbed on the precipitate until the silica surface is saturated, after which free polymer remains in solution. For particles smaller than 40 nm, the amount of polymer required to precipitate 1 unit weight of silica varies inversely with particle diameter; for larger silica particles, it varies inversely with the square of the diameter. These different relationships are consistent with calculations based on two models: (1) With particles smaller than 40 nm diameter, only a segment of a polymer chain, which is 150 nm long, is required to bridge one pair of particles; thus a single chain can extend through the bridging points of several pairs of particles. (2) With larger particles, one or more polymer chains are required to form the bridge at one contact point between a single pair of particles. A new spot test technique for detecting either excess silica or polymer in the supernatant liquid is employed, based on adsorption of a monolayer on a black glass test plate, and making its presence or absence visible by the interference color developed by a subsequently applied monolayer of 150 nm silica particles.

Settling velocity and porosity of large suspended particle

Abstract: The settling velocity of large suspended particles which were formed in turbid water due to re-coagulation was measured in the laboratory, and the relation to the particle diameter which was taken as that of a sphere with the same volume, was obtained. This is, however, different from Stokes' law in regard to the relation to the particle diameter, though the Reynolds numbers are almost in the region of Stokes' law. This may be caused by the change of apparent density with the particle diameter. Stokes' law for a porous particle was applied to the settling velocity and the diameter found was compared with the real diameter to obtain an expression for the porosity.

Particle size distributions in the Eastern Equatorial Pacific

Abstract: Size distributions for particles larger than 2.22 μ were measured at selected depths in the eastern equatorial Pacific along with the standard hydrographic variables. The particle size distributions were approximately exponential in shape, but a better fit is provided by the Weibull distribution. Measured, particle concentrations at the surface averaged 5×103cc. Near-surface maxima were found in both number and mean size, especially in regions with sharp thermal gradients. Below the thermocline, the concentration decreased rapidly to a typical value of 5×102/cc. The volume scattering function at 45° and the particle size distribution as measured at several stations were combined to create an optical vector consisting of the specific light scattering of the average particle and the cross section of the mean particle. Plots of the optical vector show a characteristic form, with a decrease in mean area accompanying an increase in specific scattering in the water above the oxygen minimum and little change below the minimum. The change in the optical vector with depth appears to be directly related to the oxidation of the decomposable parts of the nepheloid material.

Particle diffusivities and deposition velocities over a horizontal smooth surface

Abstract: Particle effective eddy diffusion coefficients, ϵ/ν, [(inertial + diffusion)/viscous forces], were calculated from experimental deposition fluxes to a smooth horizontal surface. The ϵ/ν are a function of both particle inertia (τ+), the distance from the surface (y+), and the air friction velocity ( u ∗ ) which is contained in both dimensionless terms. These ϵ/ν are greater than the ϵ/ν for particle deposition along a vertical surface. Both correlations show that particle effective eddy diffusion coefficients are much greater than the eddy diffusion of air momentum for distances from the surface of y+ 350. These ϵ/ν are similar to those for air momentum transfer. The effective eddy diffusivities are combined with Brownian diffusion and gravity settling to predict particle deposition velocities for particle size from 10−3 to 102 μm diameter for particle densities of 1 and 10 gm/cm3. Each of these mechanisms of particle transport controls the predicted deposition in some range of particle size. The deposition velocities show minima of 1.8 × 10−4 to 7.7 × 10−3 cm/sec for particle sizes from 0.066 to 0.54 μm. The minima as well as all deposition velocities are equal to or much greater than predicted from gravitational settling.

Combustion and ignition of particles of finely dispersed aluminum

Abstract: 1. It has been established that the burning time of individual aluminum particles in a high-temperature gas flow is almost independent of the pressure (P>20 atm) and temperature (T>2000° K) of the medium, but is a very strong function of the oxidizing properties of the flow relative to the concentration of active oxygen-containing products (H2O, CO2, etc). The formula τb0==0.67(d1.5/(akr)0.9) is suggested for calculation of the particle burning time. This gives satisfactory agreement with experiment. 2. The ignition time for aluminum particles is insensitive to the composition of the oxidizing medium and pressure, but depends strongly on the temperature of the gas flow. The increase in ignition time as the particle diameter increases is proportional to d2.

The role of polymer particles in the emulsion polymerization of vinyl acetate

Abstract: The emulsion polymerization of vinyl acetate was carried out at 50°C using sodium lauryl sulfate as an emulsifier and potassium persulfate as an initiator, and the role of the polymer particles produced in this system was studied on the basis of a previous theory. The low value of the average number of radicals per particle obtained in this work, i.e. in the range 0.01 - 0.5, is explained by introducing a mechanism of a rapid escape of monomeric radicals produced by the chain-transfer reaction which occurs dominantly in the polymer particles. Semiempirical equations are proposed for the estimation for the average number of radicals per particle over the whole range of monomer conversion.

Photoelectrophoretic imaging process employing electrically photosensitive particles and inert particles

Abstract: A photoelectrophoretic imaging system wherein an imaging suspension made up of electrically photosensitive particles and inert particles dispersed in an insulating carrier liquid are exposed to a light image and a uniform electrical field. An image is formed by particle migration in image configuration. Either the electrically photosensitive particles or the inert particles migrate to form an image depending on the polarity of the field applied.