Pore structure of the packing of fine particles

This paper presents a numerical study of the pore structure of fine particles. By means of granular dynamics simulation, packings of mono-sized particles ranging from 1 to 1000 microm are constructed. Our results show that packing density varies with particle size due to the effect of the cohesive van der Waals force. Pores and their connectivity are then analysed in terms of Delaunay tessellation. The geometries of the pores are represented by the size and shape of Delaunay cells and quantified as a function of packing density or particle size. It shows that the cell size decreases and the cell shape becomes more spherical with increasing packing density. A general correlation exists between the size and shape of cells: the larger the cell size relative to particle size, the more spherical the cell shape. This correlation, however, becomes weaker as packing density decreases. The connectivity between pores is represented by throat size and channel length. With decreasing packing density, the throat size increases and the channel length decreases. The pore scale information would be useful to understand and model the transport and mechanical properties of porous media.     

Effect of Young's modulus on the detachment force of 7 microm particles

The detachment force of ground, 7-microm-diameter polyester particles overcoated with clusters of silica having a cluster radius of approximately 100 nm from ceramer substrates with varying Young's moduli has been measured. It was found that the detachment force varied inversely with the Young's modulus of the ceramer. The results are attributed to the role of the silica, acting as asperities on the particles, and the degree of embedment of the particles into the substrate.     

Electrostatic double-layer interaction between spherical particles inside a rough capillary

Predictions of electrostatic double-layer interaction forces between two similarly charged spherical colloidal particles inside an infinitely long "rough" capillary are presented. A simple model of a rough cylindrical surface is proposed, which assumes the capillary wall to be a periodic function of axial position. The periodic roughness of the wall is characterized by the wavelength and amplitude of the undulations. The electrostatic double-layer interaction force between two spherical particles located axially inside this rough capillary is determined by solving the nonlinear Poisson-Boltzmann equation employing finite element analysis. The effect of surface roughness of the cylindrical enclosure on the interaction force between two particles is extensively studied on the basis of this model. The simulations are carried out for dimensionless amplitudes (amplitude/particle radii) ranging from 0.05 to 0.15 and scaled wavelengths (wavelength/particle radii) ranging from 0.4 to 4.0. The interaction force between the particles is significantly modified by the proximity of the rough capillary wall. Generally, the interaction force for rough capillaries oscillates around the corresponding interaction force in a smooth capillary depending on the magnitudes of the scaled amplitude and wavelength of the roughness. The influence of roughness on the electrostatic interactions becomes more pronounced when the surface potential of the cylinder wall is different from the sphere surface potentials. When the cylinder and the particle surfaces have large potential differences, the axial force experienced by a particle is dominated by the capillary roughness. There are dramatic oscillations of the force, which alternately becomes repulsive and attractive as the particle moves from the crest to the trough of the rough capillary wall. These results suggest that manipulation of colloidal particles in narrow microchannels may be subject to significant force variations owing to the roughness inherent in microfabricated channels etched on metal films.     

On the phase behavior of hard aspherical particles

We use numerical simulations to understand how random deviations from the ideal spherical shape affect the ability of hard particles to form fcc crystalline structures. Using a system of hard spheres as a reference, we determine the fluid-solid coexistence pressures of both shape-polydisperse and monodisperse systems of aspherical hard particles. We find that when particles are sufficiently isotropic, the coexistence pressure can be predicted from a linear relation involving the product of two simple geometric parameters characterizing the asphericity of the particles. Finally, our results allow us to gain direct insight into the crystallizability limits of these systems by rationalizing empirical data obtained for analogous monodisperse systems.     

Extended DLVO interactions between spherical particles and rough surfaces

An "extended DLVO" approach that includes Lifshitz-van der Waals, Lewis acid-base, and electrostatic double layer interactions is used to describe interaction energies between spherical particles and rough surfaces. Favorable, unfavorable, and intermediate deposition conditions are simulated using surface properties representing common aquatic colloids and polymeric membranes. The surface element integration (SEI) technique and Derjaguin's integration method are employed to calculate interaction energy. Numerical simulations using SEI demonstrate that nanometer scale surface roughness features can produce a distribution of interaction energy profiles. Local interaction energies are statistically analyzed to define representative interaction energy profiles-minimum, average, and maximum-for various combinations of simulated particles and surfaces. In all cases, the magnitude of the average interaction energy profile is reduced, but the reduction of energy depends on particle size, asperity size, and density of asperities. In some cases, a surface that is on average unfavorable for deposition (repulsive) may possess locally favorable (attractive) sites solely due to nanoscale surface roughness. A weighted average of the analytical sphere-sphere and sphere-plate expressions of Derjaguin reasonably approximates the average interaction energy profiles predicted by the SEI model, where the weighting factor is based on the fraction of interactions involving asperities.     

Depletion Interactions Produced by Nonadsorbing Charged and Uncharged Spheroids

The effect of macromolecule shape on the depletion attraction between two hard spherical particles in a solution with nonadsorbing hard spheroidal macromolecules of arbitrary size and aspect ratio was investigated using a modified form of the force-balance model of J. Y. Walz and A. Sharma (1994, J. Colloid Interface Sci. 168, 495). The macromolecules were represented as general spheroids, which could be either charged or uncharged. For the uncharged case, a set of analytical expressions describing the depletion attraction, valid for particles much larger than the characteristic macromolecule size, was developed. Comparisons with the case of spherical macromolecules were made under the condition of either constant macromolecule number density, rho(b), or constant volume fraction, phi. It was found that increasing the spheroidal macromolecule aspect ratio (major axis length/minor axis length) decreases the depletion attraction at constant rho(b), but increases the interaction at constant phi. In the latter case, the interaction produced by prolate macromolecules is greater than that produced by oblate macromolecules of equal axis lengths, while the opposite is true at constant rho(b). A simple scaling analysis is used to explain these trends. Surface charge is found to increase both the range and the magnitude of the depletion attraction; however, the general trends are the same as those found in the uncharged systems. Finally, the effect of the depletion attraction produced by spherical and spheroidal macromolecules on the stability of a dispersion of charged particles was examined. It was found that charged spheroids at concentrations of order 1% volume can produce secondary energy wells of sufficient magnitude to induce flocculation in a dispersion of charged spherical particles. Copyright 2000 Academic Press.     

Phase behavior of capillary bridges: towards nanoscale water phase diagram

Water capillary bridges often condense at contact spots between small particles or asperities. The capillary adhesion force caused by these bridges is a major component of the attractive adhesion force, and thus it significantly affects the nanotribological performance of contacting surfaces. Recent atomic force microscope (AFM) measurements indicate that phase behavior of water in these tiny bridges may be different from macroscale water behavior. In particular, a metastable state with a deeply negative pressure, boiling at low temperatures, and ice at room temperature have been reported. Understanding these effects can lead to a modification of the traditional water phase diagram by creating a scale-dependent or nanoscale phase diagram.     

DYNAMIC MONTE CARLO STUDY ON THE CORRELATION BETWEEN SHAPE AND SIZE OF LINEAR POLYMER CHAINS

The correlation between shape and size of linear chains on the simple cubic lattice is investigated using a dynamicMonte Carlo technique. A positive correlation between the asphericity parameter A and the square of the end-to-end distanceR~2, as well as that between A and the square of the radius of gyration S~2, is found for both RW and SAW chains, indicatingthat a chain conformation of small size is usually more spherical than one of large size. The result can explain why the shapeof the SAW chain deviate much more from a sphere than that of the RW chain, and can also explain the similar dependenceof size and shape on chain stiffness and on the distance of the first bead of a chain from an infinitely large flat surface.     

High velocity interparticle collisions driven by ultrasound

Ultrasonic irradiation of slurries produces high velocity impacts between solid metal particles that are sufficient to cause interparticle melting. Sonication of 5 mum Zn powder as a slurry in alkanes, for example, produces dense agglomerates 50 mum in diameter consisting of approximately 1000 fused particles. Particle size was found to be the most influential parameter in inducing local melting during interparticle collisions. Ultrasonic irradiation of mixed powders resulted in formation of agglomerates with larger Zn particles "soldered" by the smaller ones. A simple kinematic model of the ultrasound-driven interparticle fusion predicts a melting criterion that is nonmonotonically dependent on particle size and is shown to be in agreement with experiment.     

Adhesion as an interplay between particle size and surface roughness

Surface roughness plays an important role in the adhesion of small particles. In this paper we have investigated adhesion as a geometrical effect taking into account both the particle size and the size of the surface features. Adhesion is studied using blunt model particles on surfaces up to 10 nm root-mean-square (RMS) roughness. Measurements with particles both smaller and larger than surface features are presented. Results indicate different behavior in these areas. Adhesion of particles smaller than or similar in size to the asperities depend mainly on the size and shape of the asperities and only weakly on the size of the particle. For large particles also the particle size has a significant effect on the adhesion. A new model, which takes the relative size of particles and asperities into account, is also derived and compared to the experimental data. The proposed model predicts adhesion well over a wide range of particle/asperity length scales.     

计算平均力势的理论方法

A universal theoretical framework is proposed for calculating potential of mean force (PMF) between two solute particles immersed in a solvent bath, the present method overcomes all of drawbacks of previous methods. The only input required to implement the recipe is solvent density distribution profile around a single solute particle. The universal framework is applied to calculate the PMF between two large spherical particles immersed in small hard sphere solvent bath. Comparison between the present predictions and existing simulation data shows reliability of the present recipe. Effects of solvent-solute interaction detail, solvent bulk density, and solute size on the excess PMF are investigated. The resultant conclusion is that depletion of solvent component by the solute particle induces attractive excess PMF, while gathering of solvent component by the solute particle induces repulsive excess PMF, high solvent bulk density and large solute size can strengthen the tendency of attraction or repulsion. Relevance of transition from depletion attraction to gathering repulsion with the biomolecular interaction, i.e. hydrophobic attraction and hydration repulsion, is discussed.     

Aggregate formation and collision efficiency in differential settling

A new method of application of Stokesian dynamics, which can efficiently simulate movements of up to 500 particles with interparticle interactions in reasonable computational times, has been developed for the purpose of investigating particle-cluster aggregation in aqueous systems. The method is applied to monodisperse non-Brownian spherical particles aggregating in differential settling, while repulsive colloidal interaction is presumed to be negligible, so that a minimum separation distance can represent the attractive van der Waals force. The final aggregates formed by this algorithm, composed of 300 primary particles, have a common fractal dimension of approximately 2.0. The computed collision efficiency, defined as the product of a global and a capture efficiency, is about 5.77x10(-3). This value is significantly larger than the collision efficiency of primary particles colliding with an impermeable solid sphere of the same size as the aggregate, illustrating the important interplay between the permeability and the formation of aggregates.     

Micromechanical cohesion force measurements to determine cyclopentane hydrate interfacial properties

Hydrate aggregation and deposition are critical factors in determining where and when hydrates may plug a deepwater flowline. We present the first direct measurement of structure II (cyclopentane) hydrate cohesive forces in the water, liquid hydrocarbon and gas bulk phases. For fully annealed hydrate particles, gas phase cohesive forces were approximately twice that obtained in a liquid hydrocarbon phase, and approximately six times that obtained in the water phase. Direct measurements show that hydrate cohesion force in a water-continuous bulk may be only the product of solid-solid cohesion. When excess water was present on the hydrate surface, gas phase cohesive forces increased by a factor of three, suggesting the importance of the liquid or quasi-liquid layer (QLL) in determining cohesive force. Hydrate-steel adhesion force measurements show that, when the steel surface is coated with hydrophobic wax, forces decrease up to 96%. As the micromechanical force technique is uniquely capable of measuring hydrate-surface forces with variable contact time, the present work contains significant implications for hydrate applications in flow assurance.     

Adhesion between Nanoscale Rough Surfaces

In this investigation, the adhesion between particles and plates with root-mean-square, rms, surface roughness of 0.17-10.5 nm was measured by atomic force microscopy. Measurements obtained with particles both larger and smaller than the surface asperities are presented. Results indicate adhesion force decreases sharply with increasing surface roughness in the nanometer scale (<2 nm), followed by a gradual and slow decrease with further increase in roughness. Existing models were found to significantly underestimate adhesion force. Hence, a new model based on a geometry that considers both the height and breadth of asperities yielding an increased asperity radius compared to previous approaches, as detailed in Part I of this series, is applied using both van der Waals and elastic deformation/work of adhesion based approaches. For the system studied in this investigation, the adhesion forces predicted by the proposed model are considerably more accurate than those predicted by past models. Copyright 2000 Academic Press.     

Direct measurement of the viscous force between two spherical particles trapped in a thin wetting film

Here we present the first direct measurement of the viscous drag force between two spherical particles of millimeter size trapped in a thin wetting film. Each particle is constrained by the liquid/air interface and the solid substrate. The viscous force is counterbalanced by another known force, the attractive capillary immersion force between identical particles protruding from the film surface. The results of the measurements provide evidence for an increased hydrodynamic force due to a non-Stokesian resistance to the particle motion. Our findings can be applied to the self-assembly of colloidal particles in a two-dimensional array for coating and to the friction between small species and a solid. Received: 19 March 1999 Accepted in revised form: 11 May 1999     

球胶体颗粒之间相互作用能的求解

根据线性迭加近似方法,定义了一个修正电位项,较详细地推导出用于中等电位条件下球形胶体颗粒相互作用能和力的公式,该公式较为简单、实用,然而,对其所做的改进主要是针对相互作用能而不是力,对其原因也作了简单的讨论.     

Influence of polydispersity on the light scattering of concentrated dispersions of spherical particles

The light scattering of mixtures of hard spherical particles of two sizes is derived in the Percus-Yevick approximation, assuming that the light scattering intensity of each particle is proportional to its volume squared. A rather simple expression is obtained which is used to assess the influence of polydispersity on the light scattering of spherical particles at high concentrations.     

Characterization of laser‐treated Rene 41 surface due to B4C and SiC particles at surface prior to laser treatment

Laser surface treatment of Rene 41 high‐performance alloy is carried out with the presence of hard particles at the surface. B₄C and SiC particles are uniformly distributed within 40 µm carbon film at the workpiece surface prior to laser treatment process. The effect of hard particles on residual stress and microhardness variations is investigated at the treated surface. Morphological and metallurgical changes in the treated layer are examined by using electron microscopy, energy dispersive spectroscopy, and X‐ray diffraction. Residual stress formed at the surface is determined from the X‐ray data. It is found that the treated surface is free from asperities such as large size voids and cracks. A dense layer is formed in the surface region, which causes volume shrinkage while contributing to microhardness and residual stress enhancement at the surface. B₄C hard particles result in the highest residual stress and microhardness at the surface, which is attributed to its high thermal expansion coefficient. Copyright © 2013 John Wiley & Sons, Ltd.