Particle Sticking Research Articles

A micro-polar theory for binary media with application to phase-transitional flow of fiber suspensions

A phase-transitional flow takes place during the filling stage by injection molding of short-fiber reinforced thermoplastics. The mechanical properties of the final product are highly dependent on the flow-induced distribution and orientation of particles. Therefore, modelling of the flow which allows to predict the formation of fiber microstructure is of particular importance for analysis and design of load bearing components. The aim of this paper is a discussion of existing models which characterize the behavior of fiber suspensions as well as the derivation of a model which treats the filling process as a phase-transitional flow of a binary medium consisting of fluid particles (liquid constituent) and immersed particles-fibers (solid-liquid constituent). The particle density and the mass density are considered as independent functions in order to account for the phenomenon of sticking of fluid particles to fibers. The liquid constituent is treated as a non-polar viscous fluid, but with a non-symmetric stress tensor. The state of the solid-liquid constituent is described by the antisymmetric stress tensor and the antisymmetric moment stress tensor. The forces of viscous friction between the constituents are taken into account. The equations of motion are formulated for open physical systems in order to consider the phenomenon of sticking. The chemical potential is introduced based on the reduced energy balance equation. The second law of thermodynamics is formulated by means of two inequalities under the assumption that the constituents may have different temperatures. In order to take into account the phase transitions of the liquid-solid type which take place during the flow process a model of compressible fluid and a constitutive equation for the pressure are proposed. Finally, the set of governing equations which should be solved numerically in order to simulate the filling process are summarized. The special cases of these equations are discussed by introduction of restricting assumptions.

Rotational Diffusion in Iron Ferrofluids

The dynamic magnetic susceptibility of relatively monodisperse iron ferrofluids was measured from 1 Hz to 100 kHz for different sizes of the iron particles (all with a 7-nm-thick organic surface layer, dispersed in Decalin). In the case of particles with an iron core of 6-nm radius, the orientation of the magnetic dipole moment thermally rotated inside the particles (Néel rotation). In the case of particles with a slightly larger iron core, the orientation of the magnetic dipole moment was blocked inside the particles but could still change by rotational diffusion of the particles themselves (Brownian rotation). With even larger particles (above 7-nm iron core radius), aggregates were formed: the rotational diffusion rate was lower than that of single particles by more than 1 order of magnitude. This sudden appearance of aggregates above a certain size of the iron particles agrees with previous observations in two dimensions, by cryogenic transmission electron microscopy of ultrathin ferrofluid films. Here, it is found that the threshold for aggregation is practically the same in three dimensions. Moreover, the rotational diffusion rate of the aggregates is seen to increase upon dilution, indicating a decrease in aggregate size. This suggests that a dynamic equilibrium exists between the sticking of particles to each other and unsticking, especially when the particles are sufficiently small so that the sticking energy is not more than a few times the thermal energy.

Deposition of micron-sized particles on flat surfaces: effects of hydrodynamic and physicochemical conditions on particle attachment efficiency

An experimental study of micron-sized particle deposition on flat surfaces is presented, aimed at delineating the effects of hydrodynamic and physicochemical interactions on particle transport and attachment efficiency, and at obtaining a better understanding of the particle sticking probability, a concept employed in modelling particulate fouling of industrial heat exchangers. Dilute particle suspensions are employed in a parallel-plate-laminar-flow channel, and hydrodynamic and physicochemical conditions are systematically varied. Deposition rates are determined by optical microscopy and image analysis techniques. It is observed that if gravity forces are present (in a horizontal channel) they control deposition at low wall shear stresses. As the hydrodynamic wall shear stress increases particle deposition rates are significantly reduced due to the effect of hydrodynamic lift or drag forces inhibiting transport or attachment. In general, for hydrodynamic conditions similar to those encountered in industrial heat exchangers, it appears that the particle sticking probability is significantly lower than unity.

Hydrocarbon transport in the MkIIa divertor of JET

This contribution presents two-dimensional Monte-Carlo simulations of the local transport of hydrocarbons that are chemically eroded in the JET MkIIa divertor. The effect of a given background carbon flux flowing from the main plasma down to the divertor is also taken into account. The influence of local plasma temperatures and densities and the influence of different assumptions for the sticking of re-deposited particles hitting the tiles is analysed. Even under the assumption that the sticking of re-deposited hydrocarbon fragments is zero, a large amount (about 75%) of the eroded particles is re-deposited in the form of ionized carbon on the tiles. A reasonable agreement between the simulation and the experimental observation of the carbon deposition at the inner louvers in the MkIIa JET divertor is achieved, if high chemical erosion yields of about 20% together with a negligible effective sticking of hydrocarbons are assumed. Although there are experimental observations indicating such high erosion yields, such yields cannot be applied as a stationary value for the effective erosion since it would turn the inner divertor into a net erosion area in contrast to the experimental findings. The possibility of applying different erosion rates for (re)-deposited layers and the substrate material is discussed. To further improve the modelling a better knowledge about the formation and erosion of re-deposited layers is required. A first conclusion concerning the erosion and re-deposition behaviour in ITER will be drawn.

Effect of Crystallization Behavior of Polyethylene Glycol 6000 on the Properties of Granules Prepared by Fluidized Hot-Melt Granulation (FHMG)

The objective of this study was to investigate the effect of the crystallization behavior of Macrogol 6000 (polyethylene glycol 6000; PEG 6000), used as a binder, during the solidification process on the properties of mononucleic granules prepared by the fluidized hot-melt granulation (FHMG) technique. Crystallization of PEG 6000 from molten liquid was investigated using differential scanning calorimetry (DSC) and hot stage microscopy. The results obtained from the measurement of isothermal crystallization demonstrated that crystallization of PEG 6000 was either slow or rapid. Analysis based on solid-state decomposition showed that slow crystallization was due to the two-dimensional growth of nuclei mechanism, while rapid crystallization was due to the three-dimensional growth of nuclei mechanism. Observation of the crystallization of PEG 6000 by hot stage microscopy supported the existence of two different crystallization mechanisms. Granules containing PEG 6000 that underwent rapid crystallization during FHMG showed a significantly higher fraction powder under 150 microm in diameter. This was caused by the loss of powder particles from the surface of mononucleic granules during the solidification process, because many cracks were observed after crystallization of PEG 6000 with a short isothermal crystallization time (ICT) due to the reduced of sticking of particles. The results of this study suggested that the crystallization behavior of the binder during the solidification process of FHMG can influence the properties of the resultant granules, such as particle size distribution, content uniformity or taste masking. It was also indicated that measuring the ICT using DSC was a useful method to classify PEG 6000.

Influence of Paper Dust on Electrophotographic Images in Mono-component Impression Development Process

The fog image shaped as the fiber-type paper dust on the background occurs under the combination of the monocomponent impression development with the cleaner-less system. Occurrence of fiber-shaped fog images was studied by using two types of printers. The microscope observation of the fiber-shaped fog image indicates that many toner particles stick to the fiber-type paper dust on the photoreceptor and there are many toner particles without paper dust on the paper. The occurrence condition of the fiber-shaped fog image is different between the two different processes. The fiber-shaped fog image has periodicity in with-rotation printer, while no periodicity in counter-rotation printer. The fiber-type paper dust is collected by the development roller and this collected paper dust causes the fiber-shaped fog images in counter-rotation printer. The toner pressure pushes the fiber-type paper dust with toner out of the development room and the fibershaped fog image occurs in counter-rotation printer. The fiber-type paper dust is not caught by the development roller and kept turning with rotation of the photoreceptor in with-rotation printer. The paper dust is corona charged with positive polarity after scorotron charging device. However, it should be charged negatively, if it can not be caught by development roller. The styrene-acrylic polymerized toner can be considered to makes the paper dust tribocharged negatively at the place where the photoreceptor rubs against the development roller in with-rotation printer.

Influence of Some Test Parameters on Dry Sliding Wear Characteristics of a Zinc-11.5% Aluminum Alloy

This study analyzes observations made pertaining to the sliding wear response of a Zn-11.5% Al alloy in different test conditions. The wear rate and frictional heating increased with sliding speed and pressure. Seizure pressure of the samples reduced with the test speed. Frictional heating, the severity of surface and subsurface damage, and debris size increased with speed and pressure. Sticking of debris particles on the specimen surface was noted at low sliding speed. Specimen seizure was caused by the application of high pressure in view of excessive adherence of the specimen material on to the disk surface. Considerable wear-induced subsurface deformation took place in general; the degree of deformation decreased as the depth below the wear surface increased. Wear behavior of the samples is discussed in terms of the lubricating nature and thermal stability of its various microconstituents and specific characteristics of wear surfaces, subsurface regions, and debris particles.

Conditioning of Freshly Dredged Heavy Metal-Polluted Aquatic Sediment with Reed Canary Grass (Phalaris arundinacea L.)

The remediation of heavy metal-polluted sediment by solid-bed bioleaching, using the percolation principle, requires a material well permeable to air and water. Freshly dredged sediment is nearly impermeable, unsuitable for solid-bed leaching, and therefore needs preliminary conditioning. During conditioning, the sediment underwent physicochemical changes such as oxidation and acidification, which were significantly accelerated by the presence of reed canary grass (Phalaris arundinacea L.) in comparison to the processes taking place in sediment without plant cover. P. arundinacea transpired large amounts of water followed by the formation of cavities in the sediment package which were then penetrated by atmospheric oxygen. Furthermore, P. arundinacea actively transported oxygen into the sediment via the roots. Oxygen and root exudates stimulated the growth of microbes, which together with hair roots made mineral sediment particles stick together, forming larger aggregates and changing the sediment structure from muddy-pasty to crumbly and soil-like. The structural changes markedly improved the permeability of the sediment to water by a factor of 5,000. Sediment conditioned with P. arundinacea consisted of larger and more stable particles and therefore it was twice as permeable as unplanted sediment. Comparative solid-bed bioleaching experiments on a pilot scale demonstrated that the removal of heavy metal occurred at nearly the same rate and efficiency with conditioned (for six months with P. arundinacea) and spontaneously ripened (stored for six years in the open) sediment.

The Effect of Molecular Weight of Nonadsorbing Polymer on the Structure of Depletion-Induced Flocs

This work explores the structural compactness of depletion-induced particle flocs with respect to the molecular weight of nonadsorbing polymer flocculants. Small-angle static light scattering was used to monitor the structural characteristics of the flocs, which were formed by the addition of nonadsorbing poly(acrylic acids) to a stable colloidal polystyrene latex dispersion. It was found that the floc mass fractal dimension, considered to be a measure of structural compactness, was dependent upon both the molecular weight and the concentration of the polyacid. In particular, reducing the molecular weight of the polymer at a fixed polyacid concentration resulted in higher mass fractal dimensions, despite the highly polydisperse nature of the polymer samples. This structural behavior was attributed to the lower particle sticking efficiencies upon collision. This reduced sticking ability is the result of the shallowing in the secondary potential energy well with decreasing polymer chain length, which was directly supported by atomic force microscopy data. Our results suggest that the formation of a shallower attraction well with a lower molecular weight nonadsorbing polymer is the result of the insufficiency of the increased osmotic pressure to counter-balance the short-ranged nature of the depletion interaction.

Synthesis of Mixed Conducting Ceramic Oxides SrFeCo0.5Oy Powder by Hybrid Microwave Heating

Mixed conducting ceramic oxide SrFeCo0.5Oy powders were prepared by a conventional solid‐state reaction method and two types of hybrid microwave heating methods. XRD, SEM/EDS (energy‐dispersive spectroscopy), and TEM/EDX (energy‐dispersive X‐ray microanalysis) were used to characterize the products. The conventional heated sample is a three‐phase mixture composed of a predominant orthorhombic Sr4(Fe1−xCox)6O13±δ phase, a cubic perovskite SrFe1−xCoxO3−δ phase, and a CoO phase. Phase components of the direct hybrid heated samples are similar to those of the conventional heated sample, except that its predominant phase is a cubic perovskite SrFe1−xCoxO3−δ phase other than the orthorhombic Sr4(Fe1−xCox)6O13±δ phase. The indirect hybrid heated sample is mainly composed of a cubic perovskite Sr(Fe,Co)1.5Oy phase, with a trace amount of orthorhombic phase and spinel (Fe,Co) oxides. The cobalt content of the Sr(Fe,Co)1.5Oy phase is much higher than that of the SrFe1−xCoxO3−δ phase. Particle size distribution of the conventional heated sample is very wide. The indirect hybrid heated sample has a relatively narrow particle size distribution, with an average particle size of 3–4 μm, but the particles stick to form agglomerates. The morphology of the direct hybrid heated sample is similar to that of the conventional heated sample, except that submicrometer grade particles were rarely observed.

Experiments on depolarized optical scattering to sense in situ the onset of early agglomeration between nano-size soot particles

Soot aerosol particles generated in hydrocarbon flames are investigated in this paper by laser light scattering techniques. The contribution of co- and cross-polarized scattered light, each for vertical and horizontal linear polarization states of the incident laser light at 514.5nm are measured at variable polar angles θ. Main results here reported are the following: (1) the vertical depolarization ratio ρv measured against the scattering angle is very low and flat in the case of smaller chain-like (aggregate fractal dimension Df≈1.3), whereas, in the case of larger branched-chain aggregates (Df≈1.7), ρv exhibits a maximum at about 90° (2) in contrast to the reciprocity theorem, the measured ratio IHV/IVH between the two depolarized contributes for each of the two possible linear (vertical and horizontal) polarization states of the incident light, is found to be different than unity when first sticking of the primary particles occurs. Therefore, the measurement of the reciprocity ratio IHV/IVH seems a powerful and reliable tool to establish the onset of the early aggregation mechanism between nano-size soot particles formed in a flame.

Particle aggregation in volcanic eruption columns

We develop a model to describe the formation of aggregates in a volcanic eruption column. The model combines a description of the rate of collision and sticking of particles with a model of the vertical transport in the eruption column. We thereby determine the evolution of the grain size distribution as a function of height in the eruption column. We consider aggregates in which liquid water provides the binding agent. For sufficiently large eruption columns we find that this limits the vertical extent of the zone where aggregates may form since near the source, all water is in the vapor form, while in the upper part of the column the mixture becomes very cold and freezes. However, in many cases, we find that the particles spend sufficient time in the central region, where the water is in the liquid form, that a substantial amount of aggregation occurs. Furthermore, we predict that owing to the reduction in the binding efficiency of water as particle size increases [Gilbert and Lane, 1994], there is a relatively narrow size distribution of aggregates at the plume top. We also model the airfall deposits associated with this aggregate‐rich distribution of particles which is injected to the top of the eruption column. We show that the relatively narrow size distribution of particles at the top of the column, coupled with the gravitational settling and transport by ambient winds, may lead to enhanced deposition close to the source and in some cases secondary thickening of the deposit. The relatively near‐source deposition of fine ash in these deposits is associated with the fallout of aggregates. As a simple application, we show that the present dynamical aggregation model is consistent with the secondary thickening of the deposit from the May 18, 1980, eruption of Mount St. Helens.

Sticking of protein-coated particles in a shear field

The shear-driven formation of particle doublets at a wall, of latex and emulsion systems, has been studied using a colloidal particle scattering apparatus. In this technique, one particle is fixed on the wall while a second, mobile free particle is made to collide with the fixed particle by the application of the shear field. The effects of pH (in the range 5.5–4.8) and ionic strength (in the range 0.01–0.5 mol dm −3) on the efficiency of irreversible sticking (capture) of the mobile particle to the fixed particle have been studied for systems stabilised by adsorbed films of sodium caseinate, pure α s1-casein or pure β-casein. At low ionic strength (≤0.2 mol dm −3), the efficiency of capture increases with decreasing pH and increasing ionic strength in ways that are consistent with decreasing net charge on the protein and increased screening of the electrostatic repulsion between the particles. The increase in capture efficiency also agrees well with a previously observed increase in viscoelasticity of the corresponding concentrated protein-stabilised emulsions. However, a pronounced drop in the capture efficiency was found at higher ionic strength (0.5 mol dm −3). This effect at high ionic strength, combined with qualitative visual assessment of the behaviour of particles during collisions, conforms to the additional role of tangential interparticle forces in determining the sticking behaviour. It seems that, if the adsorbed protein layer becomes sufficiently thick, even if it has a very low net charge, then particle sticking can be prevented. The sticking phenomenon probably depends on entanglement of the adsorbed layers and on the strength of the protein–protein attractive interactions, as well as on the net particle charge.

Modeling of particle growth and morphology in the gas phase polymerization of butadiene. II. Simulation and discussion

AbstractThe improved multigrain model was used to simulate the gas phase polymerization of butadiene catalyzed by low‐, medium‐, and high‐activity catalysts, respectively. For the low‐activity catalyst, the mass and heat transfer resistances in the particle were negligible. The morphology of the polymeric particles was uniform. For the medium‐activity catalyst, the overall mass transfer effectiveness was > 90%, the maximal temperature rise was 8K, and the heat transfer resistance in the particle was negligible. Mass transfer resistance does not affect the morphology of product particle significantly. For the high‐activity catalyst, the overall mass transfer effectiveness was within the range of 70–96%, the morphology of the product particle was affected by the mass transfer resistance to some extent. The maximal temperature rise was 21K; the heat transfer resistance in the particle was negligible as well. However, there was some severe mass transfer resistance in the particle, and the maximal temperature rise was ≤ 30K for the large catalyst particle with the same activity. Thus, the polymeric particle morphology was comparatively poor, with the occurrence of particle softening and sticking. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 730–741, 2001

Focused sticking of light mass particles in physisorption

In the scattering of atoms and molecules from surfaces, focusing effects can give rise to a variety of significant features in both the experimentally measured elastic and inelastic scattering intensities. The study of focusing is extended to the case of sticking of atoms upon scattering into the physisorption potential well. It is demonstrated that this focused sticking (FS) should give rise to enhancements of the sticking coefficient for certain well-defined incident conditions.

HEATING BEHAVIOR OF ALUMINUM COATINGS OBTAINED IN FRICTION CONTACT

Aluminizing is a well-studied process. There are various methods for depositing aluminum coatings with their own specific features, advantages, disadvantages, and fields of application. Aluminum coatings are chiefly used for raising the corrosion and wear resistances of metal parts. A comparatively new method for depositing aluminum coatings consists of surface plastic deformation and simultaneous aluminizing (SPDA). The coating is deposited by a rotating metallic brush of disc shape (RMB), which is in friction contact with the part and the donor aluminum rod simultaneously. Aluminum particles stick to the ends of the RMB hairs and are transferred to the treated part and coat the surface of the substrate in the zone of the friction contact [1]. The aim of the present work consisted in studying the effect of heating on aluminum coatings obtained by the method of SPDA.

Deposit growth dynamics: particle sticking and scattering phenomena

A common phenomenon in deposition processes is the impaction of a particle on a pre-deposited particle in a deposit (e.g., as in thin film growth via aerosol routes or in gas-side fouling of heat exchange equipment). The fate of an incident particle (i.e., whether it sticks on the deposit or it escapes) affects the net deposition rate, as well as the resulting deposit microstructure, phenomena that we have been studying, using discrete particle computer simulations. Here, we summarize the sticking behavior of impacting particles in terms of appropriate macroscopic “boundary conditions” that can be used in continuum level simulations of the dynamics of deposit growth. In addition, we study the properties of rebounding/scattered particles from a “rough” particulate deposit surface, in terms of rebounding linear and angular velocities and scattering angle distributions. Interestingly enough, the rebounding velocity distributions exhibit a multimodal character which becomes less pronounced with the degree of “rigidity” of the deposit, reflecting the influence of local microstructural details (coordination number distribution) on the scattering process. Scattering angle distributions are unimodal, resembling a “distorted” sinusoidal. The remaining challenge is to develop, with further parametric studies, appropriate “boundary conditions” for the scattering quantities of interest as well, and open the door to continuum level simulations of macroscopic systems in realistic geometries.

Series expansion and computer simulation studies of random sequential adsorption

We discuss two important techniques; series expansion and Monte Carlo simulation, for a random sequential adsorption study. Random sequential adsorption is an idealization for surface deposition where the time scale of particle relaxation is much longer than the time scale of deposition. Particles are represented as extended objects which are adsorbed onto a continuum surface or lattice sites. Once landed on the surface, the particles stick to the surface. We review in some detail, various methods of computing the coverage θ( t) and present some of the recent and new results in random sequential adsorption.

Relationship between interaction forces and the structural compactness of depletion flocculated colloids

The addition of non-adsorbing polymers to a colloidal dispersion can induce particle aggregation through depletion flocculation. Small-angle static light scattering has been used to show that the structure of aggregates formed upon the addition of non-adsorbing poly(acrylic acid) to a dilute aqueous dispersion of colloidal polystyrene latex particles exhibited power-law scattering behaviour. The observed floc mass fractal dimension, d F, was found to be dependent upon the polyacid concentration. Increasing the polyacid concentration resulted in lower fractal dimensions, with d F levelling off at high polymer concentrations. This structural behaviour was attributed to the deepening in the secondary potential energy well and hence higher particle sticking efficiencies upon collisions when the polymer concentration was raised. This is directly supported by atomic force microscopy data, which showed that the interaction force curves between charged surfaces for solutions of the non-adsorbing poly(acrylic acid) exhibited attractive secondary minima, the depths of which increased with increasing polymer concentration, saturating at high polyacid concentrations. In contrast to the much lower values obtained via salt-induced aggregation ( d F=1.78 to 2.20), significantly higher mass fractal dimensions and hence greater structural compactness were observed for these depletion-flocculated flocs.

試料導入室における試料汚染要因

The sample contamination in a loadlock chamber, caused by particle sticking, adsorption of gas and moisture in a loadlock chamber and back stream from pumping systems, was investigated. In order to eliminate the first two sources of contamination, a clean silicon sample was prepared in a glove box filled with particle-free nitrogen gas, and the sample was transferred to the loadlock chamber without exposing to air. To observe the contamination during pumping, the last source of contamination, a clean sample was set in the loadlock chamber and after evacuation surface contaminant was evaluated by C1s intensity of XPS spectra. Several evacuation procedures were compared and we concluded that oil-free pumping system is essentially important for contaminant-free introduction of a clean sample.