Interparticle Van Der Waals Forces Research Articles

A CFD‐DEM study of the solid‐like and fluid‐like states in the homogeneous fluidization regime of Geldart A particles

AbstractThe mechanisms underlying homogeneous fluidization of Geldart A particles have long been debated. Recent experiments shed fresh insights that both a solid‐like and a fluid‐like state exist. Herein, 3D computational fluid dynamics‐discrete element method simulations with the incorporation of interparticle van der Waals forces were performed for five typical Geldart A particles to investigate the bed structure in the homogeneous fluidization regime. Simulation results reproduce the existence of the solid‐like and fluid‐like states, showing that in the solid‐like state, particles have enduring and strong interparticle van der Waals forces that stabilize the bed to keep the bed stationary; while in the fluid‐like state, the interparticle van der Waals forces are dominated by the drag force, and particles show global circulation. Both interparticle van der Waals forces and friction are found to be necessary for the existence of the solid‐like state, and increasing the former has an effect to widen the solid‐like state.

Improvement of the flowability of fine yttrium oxide powders by microwave oxygen plasma and evaluation of the dense coating layer

Ceramic powders such as yttrium oxide and aluminum oxide have been extensively used to coat the inner walls and components of chambers used in semiconductor fabrication processes. Fine powders are used to form a dense coating layer by techniques such as thermal spray coating. However, fine powders exhibit strong cohesive forces due to surface roughness and interparticle van der Waals forces. This results in low flowability, a nonuniform powder feed, and nonuniform coatings. Microwave O2 plasma is proposed to improve the flowability of fine Y2O3 powder for a uniform powder supply in the spray coating process. Microwave O2 plasma flames produce a high temperature with active oxygen species at atmospheric pressure. The active oxygen species of plasma are incorporated into the lattice of powder particles, electrically stabilizing the powder, decreasing the static electricity, and reducing interparticle van der Waals forces. In addition, the surface of the fine powder is melted and smoothed by the high temperature of the microwave plasma, so that the powder acts as a dry lubricant. The processed powder was employed to coat a surface using atmospheric thermal spray coating, and the coating was characterized in comparison to a coating produced using commercial powder. The plasma-treated powder produced a coating with higher density and strength and lower porosity and surface roughness. The plasma-treated powder has a high flowability of 2.38 g s−1 and an apparent density of 2.07 g cm−1. Accordingly, the coating layer has a higher adhesion strength of 8.75 MPa and hardness of 651 HV0.3. In addition, its surface roughness of 0.9 μm and porosity of 0.2% are lower than those of a commercial sample.

Effect of the argon/monomer flow-rate ratio on the flowability trend of PECVD-processed micropowder

Plasma-enhanced chemical vapor deposition of non-continuous coatings on the surface of micropowders can reduce the interparticle van der Waals forces and consequently improve the powder flowability. A quasi-Arrhenius relation holds between flowability improvement and energy delivered per mass of plasma monomer W/FM. This work reports the flowability trend of graphite micropowder processed with organosilicon glow-discharge plasma at high levels of energy per monomer mass (~10–100MJ/kg). A critical energy level is found, above which the argon/monomer flow-rate ratio becomes a significant additional parameter influencing the deposition process. A revised empirical quasi-Arrhenius relation is proposed to take into account this effect.

Applying the Macroscopic Kinetic Approach to Plasma Polymerization to the Plasma Surface Modification of Micropowders: Attempt of Correlating Powder Flowability and Plasma Process Parameters

The flowability of micropowders can be improved by depositing a non‐continuous coating on the micropowder surface, which reduces the interparticle van der Waals forces causing cohesion of the native powder. Such a coating can be achieved via a plasma enhanced chemical vapor deposition in a tubular, inductively‐coupled RF glow‐discharge‐plasma reactor fed by plasma‐polymerizable gases. Here we systematically study the influence of user‐set plasma parameters of feed gas flow rate and plasma power on the resulting powder flowability. We find a quasi‐Arrhenius relation between flowability factor and energy delivered per mass of monomer W/FM. These findings demonstrate that flowability measurements can be used to study the plasma polymerization processes and as a metric to assess surface coatings of powders that are otherwise difficult to characterize by standard metrologies.

Distinct element method investigation on mechanical behavior within shear bands in granulates under the Earth and the Moon conditions

This letter mainly aims to investigate the mechanical behavior within shear bands in regolith both under the Earth and the Moon conditions via the distinct element method, in which a novel contact model considering interparticle van der Waals forces and rolling resistance is employed. The results show that for regolith under both conditions the stress paths are almost identical inside and outside the shear bands but void ratio, average pure rotation rate, and strain paths are rather distinct with dilation, particle rotation and the second invariant of strain tensor mainly occurring within the bands. However, the regolith under the Moon condition has higher peak strength and more significant strain localization than those under the Earth condition.

Adjusting dissolution time and flowability of salicylic acid powder in a two stage plasma process

Many industrial products and intermediates in powder form suffer from low flowability and poor dissolution behavior. Current techniques to overcome such problems often include time-consuming admixture of additives or wet-chemical processes that are harmful to the environment. Therefore, the objective of this work was to develop a continuous non-equilibrium plasma process to modify the flowability and dissolution behavior of fine-grained, cohesive and temperature sensitive powders. As test substance salicylic acid was used due to its low melting point, cohesive nature and similarity to many pharmaceutical products.In the proposed process the powders were passed through a low-pressure discharge to modify the single particle surfaces. In a first process step SiOx nanostructures and thin films were formed in the plasma and deposited on the salicylic acid particles. The created nanostructures acted as spacers and reduced the prevailing interparticle van der Waals forces. Hence, the flowability of the dry powder was improved from the cohesive to the easy-flowing regime. The flowability was higher the more nanoparticles were deposited per surface area. On the other hand, the deposition of nanostructures retarded the dissolution, which was attributed to the partial coverage of the particle surfaces. Therefore, these already coated powders were surface activated in a second plasma-assisted process step in the same reactor. The particle surfaces were enriched with polar groups, which increased the surface free energy. This improved the powder wettability and the dissolution time was reduced even below the measured value of uncoated powder.Hence, with the introduced process both the flowability and dissolution time of the cohesive, temperature-sensitive and fine-grained test substance salicylic acid could be adjusted within a very short process time in the order of 1s.

Nanoparticle Synthesis in a Plasma Downstream Reactor – From Plasma Parameters to Nanoparticle Properties

In the presented study a tubular plasma reactor is investigated, which is normally used for the continuous plasma surface modification of fine-grained powders. The plasma reactor basically consists of a 1.5 m long glass tube with a gas and precursor feed unit at its top and a particle-gas separation unit at the lower end. The power is coupled inductively into the plasma via a coil which is wrapped around the reactor tube. Substrate powders normally pass the discharge tube with high velocity and are functionalized on their way through the plasma in approximately 0.1 s. Possible plasma surface functionalization processes for powders are illustrated in Figure 1.1. The wettability of powders is increased by the formation of polar groups on the surface. Films are deposited on particle surfaces to protect the substrate from harsh environments or for catalytic applications. In recent years, also a new plasma process, which increases the flowability of fine-grained powders, gained increasing attention. Nanostructured SiOx is formed in the plasma and directly deposited on the substrate particle surface. These nanoparticle structures increase the surface roughness of the substrate particles. Thus, the interparticle van der Waals forces are reduced, which leads to a major improvement of the powder flowability. This process shows promise for companies dealing with cohesive granular materials. The feasibility of this process was shown in the past, but at the same time the need for fundamental research in this field was recognized. Which ion density is required to yield in an effective surface modification? What is the thermal load of a substrate particle during the treatment? Which precursor should be used for a maximum improvement of the flowability? In order to answer such questions, we measured axial profiles of plasma parameters in this continuous reactor and studied the nanoparticle synthesis in detail. No substrate powder was fed during these investigations to facilitate probe measurements and to focus on the produced nanoparticles.Silica-like nanoparticles were produced from the four organosilicon monomers hexamethyldisiloxane (HMDSO), tetramethyldisiloxane (TMDSO), tetraethyl orthosilicate (TEOS), and tetramethyl orthosilicate (TMOS) in argon-oxygen gas mixtures. The chemical composition and morphology of the emerging particles and its production rate were studied as a function of process pressure (100 – 400 Pa), plasma power (200 – 350 W), gas velocity (5 – 16 m/s) and gas composition. Langmuir double probe and calorimetric probe measurements allowed determining the axial profiles of electron temperature, positive ion density, and energy influx along the vertical axis of this tubular reactor.

From Discrete Particle Model to a Continuous Model of Geldart A Particles

A reliable description of dense gas–solid two-phase flows of Geldart A particles in gas-fluidized beds at life-size scale is of great practical importance in process industries. The classical two-fluid model, based on the kinetic theory of granular flows (KTGF), provides a very promising theoretical framework for predicting large-scale gas–solid two-phase flows. However, thus far the two fluid model has not been successful in describing gas–solid flows of Geldart A particles. As the kinetic theory was originally developed for cohesiveless particles, it is essential to check if the theory can still work for Geldart A particles, which are slightly cohesive. In this research, a soft-sphere discrete particle model (DPM) is used to study the detailed particle–particle interactions in periodic boundary domains, where interparticle van der Waals forces are taken into account, with no gas phase present. In our simulations, we (1) compare the results for both the hard-sphere and the soft-sphere discrete particle model for cohesiveless particles, with the theoretical predictions obtained from the kinetic theory of granular flows, and (2) study the effect of the cohesive forces in the soft-sphere model and explore a way to modify the current kinetic theory according to the soft-sphere DPM simulation results. The information obtained from these simulations can be further incorporated into the KTGF based two-fluid model.

Rheology of concentrated SiC particles in silicon alkoxide sols

Silicon carbide (SiC) particles were mixed with silicon alkoxide precursors at the sol level, and their rheological behaviors were examined under various solids concentrations ( ϕ = 0.10–0.22 in volumetric ratios) and shear rates ( γ ˙ = 1 – 1000 s −1). The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC 2H 5) 4), ethyl alcohol (C 2H 5OH), H 2O and HCl in a constant [H 2O]/[TEOS] ratio of 11. The powdered mixtures all exhibited a shear-thinning flow behavior over most of the shear-rate range examined. This indicates that the concentrated suspensions were flocculated in character. Relative viscosity of the suspensions increased markedly as ϕ increased, suggesting that the particle interaction becomes more pronounced with the increased ϕ. No evidence was seen found for possible particle–precursor interactions at the sol level that might catalyze gelation of the sols. A theoretical maximum solids concentration ( ϕ m) of the suspensions was determined as ϕ m = 0.26–0.28 over a shear-rate range γ ˙ = 10 – 100 s −1. This reduced ϕ m value partially confirmed the dominance of attractive interparticle van der Waals forces in the particle–precursor mixtures.

The effects of particle and gas properties on the fluidization of Geldart A particles

We report on 3D computer simulations based on the soft-sphere discrete particle model (DPM) of Geldart A particles in a 3D gas-fluidized bed. The effects of particle and gas properties on the fluidization behavior of Geldart A particles are studied, with focus on the predictions of U mf and U mb , which are compared with the classical empirical correlations due to Abrahamsen and Geldart [1980. Powder Technology 26, 35–46]. It is found that the predicted minimum fluidization velocities are consistent with the correlation given by Abrahamsen and Geldart for all cases that we studied. The overshoot of the pressure drop near the minimum fluidization point is shown to be influenced by both particle–wall friction and the interparticle van der Waals forces. A qualitative agreement between the correlation and the simulation data for U mb has been found for different particle–wall friction coefficients, interparticle van der Waals forces, particle densities, particle sizes, and gas densities. For fine particles with a diameter d p < 40 μ m , a deviation has been found between the U mb from simulation and the correlation. This may be due to the fact that the interparticle van der Waals forces are not incorporated in the simulations, where it is expected that they play an important role in this size range. The simulation results obtained for different gas viscosities, however, display a different trend when compared with the correlation. We found that with an increasing gas shear viscosity the U mb experiences a minimum point near 2.0 × 10 - 5 Pa s , while in the correlation the minimum bubbling velocity decreases monotonously for increasing μ g .

A numerical study of fluidization behavior of Geldart A particles using a discrete particle model

This paper reports on a numerical study of fluidization behavior of Geldart A particles by use of a 2D soft-sphere discrete particle model (DPM). Some typical features, including the homogeneous expansion, gross particle circulation in the absence of bubbles, and fast bubbles, can be clearly displayed if the interparticle van der Waals forces are relatively weak. An anisotropy of the velocity fluctuation of particles is found in both the homogeneous fluidization regime and the bubbling regime. The homogeneous fluidization is shown to represent a transition phase resulting from the competition of three kinds of basic interactions: the fluid–particle interaction, the particle–particle collisions (and particle–wall collisions) and the interparticle van der Waals forces. In the bubbling regime, however, the effect of the interparticle van der Waals forces vanishes and the fluid–particle interaction becomes the dominant factor determining the fluidization behavior of Geldart A particles. This is also evidenced by the comparisons of the particulate pressure with other theoretical and experimental results.

Effect of vibration on the stability of a gas-fluidized bed of fine powder

We have investigated the effect of vibrations on the stability of gas-fluidized beds of fine powders (particle size approximately 10 microm). The powder is uniformly fluidized by an adjustable gas flow that enables us to control the average solid volume fraction straight phi(0). The fluidized bed is then subjected to a vertical oscillatory motion of controlled amplitude and frequency. The response of the fluidized bed depends essentially on the value of straight phi(0). For straight phi(0)>0.28 the fluidized bed is in a weak solidlike regime, it has a mechanical strength, and particles are static. In this regime vibration causes compaction of the loosely packed bed. For straight phi(0)<0.28 the mechanical strength vanishes and stresses are carried by interstitial gas and collisions. In this fluidlike regime the fluidized bed displays a diffusive dynamics and particles aggregate due to the strong interparticle van der Waals forces. When vibration is applied the powder expands due to the partial disruption of aggregates. However at a critical value of the vibration amplitude A=A(c) either surface (sloshing) or flow (bubbling) instabilities develop. The nucleation of gas bubbles has been correlated to the saturation in particle diffusivity measured elsewhere. The size of the bubbles increases as A is further increased above A(c) and as the vibration frequency is reduced. Moreover, as it should be expected from the predictions of hydrodynamic models, A(c) is independent of cohesivity for particles of the same size and density.

The Properties of Glacial Loess and the Formation of Loess Deposits

ABSTRACT Some loess is produced by direct glacial action. This is a two stage process: the fine quartz which forms the bulk of the loess is produced first as sand-size grains, and then these grains are crushed. The glacial crushing action tends to produce a wide range of particle sizes with concentrations in the sand-size range and the <200µ range; this product can be divided into two types of deposit, sand and loess. Quartz particles of about 80µ diameter are most easily raised by the wind and carried in suspension. Loess in deposits has a high tensile strength and forms vertical structures because the interparticle Van der Waals forces are of the same order of magnitude as the weight of the particles. Six events define the history of a loess deposit: two stages in the formatio of the material, transport by glacier, deposition, transport by the wind, and deposition again. Each of these events is important and affects the eventual nature of the deposit. The aeolian transportation process is quick and efficient, and the formation of the loess deposit can be correlated with the retreat of the glacier which produced the necessary detritus.