Particle-wall Interactions Research Articles

Analysis of interparticle forces and particle-wall interactions by powder bed pressure drops at incipient fluidization

A novel method is proposed to evaluate the interparticle forces and particle-wall interactions based on the pressure drop measurements at incipient fluidization. In the first experiment, we used the liquid bridge force as the model interparticle force of the glass ballotini (53μm–105μm). The theoretical relationship between the model liquid bridge force and the liquid addition volume was successfully presented. In the second part of experiment, the interparticle forces and the particle-wall interaction of cornstarch powders in a Perspex cylinder are determined. While the particle-wall interaction shows a linear relationship with the apparent bed-wall contact area, the interparticle force follows a logarithm relation with the bed mass for the Warren-Spring-type cornstarch powders. The results were also compared to the cohesion values obtained by a conventional shear tester. The results conclude that our method serves as an economically and accurate alternative to conventional shear testers.

Two-dimensional mixture of amphiphilic dimers and spheres: Self-assembly behaviour.

The emergence of supramolecular aggregates from simple microscopic interaction rules is a fascinating feature of complex fluids which, besides its fundamental interest, has potential applications in many areas, from biological self-assembly to smart material design. We here investigate by Monte Carlo simulation the equilibrium structure of a two-dimensional mixture of asymmetric dimers and spheres (disks). Dimers and disks are hard particles, with an additional short-range attraction between a disk and the smaller monomer of a dimer. The model parameters and thermodynamic conditions probed are typical of colloidal fluid mixtures. In spite of the minimalistic character of the interaction, we observe-upon varying the relative concentration and size of the two colloidal species-a rich inventory of mesoscale structures at low temperature, such as clusters, lamellæ (i.e., polymer-like chains), and gel-like networks. For colloidal species of similar size and near equimolar concentrations, a dilute fluid of clusters gives way to floating lamellæ upon cooling; at higher densities, the lamellæ percolate through the simulation box, giving rise to an extended network. A crystal-vapour phase-separation may occur for a mixture of dimers and much larger disks. Finally, when the fluid is brought in contact with a planar wall, further structures are obtained at the interface, from layers to branched patterns, depending on the nature of wall-particle interactions.

Numerical study of solid particle erosion in hydraulic spool valves

The null characteristics of hydraulic spool valves change significantly due to solid particle erosion. In the present work, a numerical model has been established to predict the material removal rate and the worn profile evolution with erosion in hydraulic spool valves. The particle trajectories are solved by a stochastic separated flow model based on eddy interaction, and the squeeze film is included as a part of a particle-wall interaction model. The erosion rates are then predicted by the particle impact characteristics, and the worn profiles are obtained by a time-discrete and spatial-discrete update method. The major influencing factors, including particle size, differential pressure, spool opening, and flow direction, are discussed. Through comparison with laboratory experiments and a field case, the model was verified. The present model and theoretical analysis are intended to elucidate the life-cycle management, life evaluation and degradation-resistant design of the spool valve.

Experimental characterization of particle-wall interaction relevant to entrained-flow gasification of biomass

The present study addresses particle–wall interaction phenomena relevant to entrained-flow gasifiers. Two types of biomass have been investigated, i.e. wood chips and corn stover. The dynamics of char and ash particles as they are impacted onto a flat surface in cold and hot conditions has been characterized by means of high speed imaging and tracking. Particle–wall collisions were described in terms of normal, tangential and global coefficients of restitution as well as deposition efficiency. The influence of carbon conversion and impact velocity on the dynamical pattern of rebound and deposition has been investigated. The results indicate that, even at ambient conditions, some plastic deformation occurs during the impact. Experiments performed under hot conditions demonstrate that a drop of the coefficients of restitution takes place at high temperature, especially at low impact velocity, for both the tested biomasses. This feature can be explained by considering the change of mechanical properties of particles as the temperature increases, and the effect of the adhesion energy during the impact. Results highlighted that ash of wood chips is not prone to form a slag layer, while ash of corn stover extensively contributes to formation of ash deposits and melts under hot conditions. Results from experiments with corn stover indicate that char particles deposit onto the molten ash surface. The dissipation of momentum associated with particle impact promotes the establishment of a dense-dispersed phase in the near-wall zone of entrained-flow slagging gasifiers.

Influence of different packing methods on the hydrodynamic stability of chromatography columns

It is well known that packing non-uniformity may cause peak asymmetry and limit the performance of packed-bed chromatographic columns. However, understanding of the reasons leading to packing non-uniformity is still limited. Therefore, the effect of different column packing methods, i.e. dynamic axial compression (DAC), flow packing, and combinations of both on the hydrodynamic packing heterogeneity and stability of packings composed of polymer-based compressible porous resins with a mean diameter of 90μm was investigated experimentally as well as in-silico. Deterministic Euler-Lagrange modeling of a small chromatographic column with a diameter of 9.6mm and a bed height of 30mm was applied by coupling Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM). Interparticle micromechanics as well as the fluid-particle and particle-wall interactions were taken into account. Experiments and simulations revealed substantial non-uniformity of compression force transmission and axial packing density distribution during both dynamic axial compression and flow packing which was related to wall support and interparticle friction. By combining both packing methods sequentially (dynamic axial compression followed by flow packing or vice versa), the compression forces were more homogeneous resulting in improved packing procedures. Repeated alternating application of flow packing and DAC (the so-called hybrid packing method) resulted in the most homogeneous packing density distribution and the highest packing stability which was kept nearly constant during long-term operation with cyclic hydrodynamic load. The hydrodynamic stability of the chromatographic column was evaluated by calculating the integral porosity deviation and packing induced flow velocity dispersion. The hybrid packing method gave the best results for both parameters.

On the importance of carrier fluid viscosity and particle–wall interactions in magnetic-guided assembly of quasi-2D systems

We demonstrate the influence of experimental conditions (carrier fluid viscosity and particle–wall interactions—friction) on the quasi-2D deterministic aggregation kinetics of carbonyl iron magnetic suspensions in rectangular microchannels. On the one hand, the carrier fluid viscosity determines the time scale for aggregation. On the other hand, friction strongly determines the aggregation rate and therefore the kinetic exponent (mean cluster size vs. time dependence). When particle–wall interactions are weak, the mean cluster size increases with a power of 0.65 ± 0.06, for open cavities (≥500 microns channel width), in very good agreement with theories and particle-level simulations. However, when the particle–wall interactions are strong, the kinetic exponent decreases and the aggregation is eventually arrested. This work suggests that particle–wall interactions may be one of the reasons for the discrepancies found in the experimental determination of the aggregation kinetic exponents in the literature.

Approximate One-Dimensional Models for Monoenergetic Neutral Particle Transport in Ducts with Wall Migration

ABSTRACTThe problem of monoenergetic neutral particle transport in a duct, where particles travel inside the duct walls, is treated using an approximate one-dimensional model. The one-dimensional model uses three-basis functions, as part of a previously derived weighted-residual procedure, to account for the geometry of particle transport in a duct system (where particle migration into the walls is not considered). Our model introduces two stochastic parameters to account for particle–wall interactions: an albedo approximation yielding the fraction of particles that return to the duct after striking the walls, and a mean-distance travelled in the walls by particles that re-enter the duct. Our model produces a set of three transport equations with a non-local scattering kernel. We solve these equations using discrete ordinates with source iteration. Numerical results for the reflection and transmission probabilities of neutron transport in ducts of circular cross section are compared to Monte Carlo results computed using the MCNP code.

Computer simulation of confined prolate hard spherocylinder liquids between hard walls

ABSTRACTIn present work, using canonical Monte Carlo simulation method, the packing fraction profiles and order parameters of prolate hard spherocylinder (HSC) fluid between hard walls are calculated. The hard particles and hard particle–wall interactions are considered. The confinement effects on transition packing fraction of HSCs with aspect ratios and 5 are calculated. In slit pore, while the bulk packing fraction increases, an orientationally ordered phase can form. We conclude that the required packing fraction for forming liquid crystal phases will decrease in confinement. Also while the molecular anisotropy increases, the isotropic–nematic transition packing fraction will decrease in confinement. We could not find the exact or simulation results for comparison with packing fraction profiles of confined HSCs. The results in the case of confined are compared with the simulation results of confined hard Gaussian overlap fluid of elongation . The results are corresponded, qualitatively. Also the confinement isotropic–nematic transition packing fractions are compared with the simulation and fundamental mixed measure theory (FMMT) results of bulk prolate HSCs.

Effect of relative humidity on the deposition and coagulation of aerosolized SiO2 nanoparticles

The temporal evolution of aerosolized SiO2 nanoparticles (NPs) released into an environmental test chamber has been investigated to interrogate the effect of relative humidity (RH) on the deposition and coagulation of the nanoparticles. The size-resolved deposition rate and Brownian coagulation coefficient for the particles at RH of ~10%, 27%, 40%, 54%, and 64% are estimated. The results show that the effect of RH on the deposition rate is size-dependent; for particle diameter (Dp)<70nm, the deposition rate reduces as the RH rises; while for Dp>70nm, it grows as the RH rises. Generally, both low and high RH tends to enhance the deposition rate, and the minimum rate appears at moderate RH (~54%). Electrostatic repulsion is probable for the inter-particles interaction at the low RH while the surface roughness due to water molecular adsorption is a main reason for the particle-wall interaction at higher RH. The increasing coagulation coefficient at high humidity correlates to the strong inter-particle adhesion, which may be caused by the water molecular adsorption on the hydrophilic surfaces of the SiO2 NPs due to the formation of nanometer-thick water film. This study suggests that air humidity plays unignorable roles in particle deposition and coagulation.

Impact experiments of char and ash particles relevant to entrained-flow coal gasifiers

The present study addresses particle–wall interaction phenomena relevant to entrained-flow coal gasifiers. The dynamics of coal, char and ash particles as they are impacted onto a flat surface in cold and hot conditions and undergo momentum transfer and rebound, has been characterized by means of high speed imaging and tracking. Particle–wall collisions were described in terms of normal, tangential and global restitution coefficients. The influence of carbon conversion, impact velocity and surface properties and structure of the target on the dynamical pattern of rebound has been scrutinized. The results indicate that, even at ambient conditions, some plastic deformation occurs during the impact. The restitution coefficients decreased as temperature and carbon conversion increased. This feature was more pronounced at large carbon conversion, confirming the criticality of the char/slag transition to particle deposition on the wall. The dissipation of momentum associated with particle impact may promote the establishment of a dense-dispersed phase in the near-wall zone of entrained-flow slagging gasifiers.

On the propagation and multiple reflections of a blast wave travelling through a dusty gas in a closed box

This paper concerns the propagation of shock waves in an enclosure filled with dusty gas. The main motivation for this problem is to probe the effect on such dynamics of solid particles dispersed in the fluid medium. This subject, which has attracted so much attention over recent years given its important implications in the study of the structural stability of systems exposed to high-energy internal detonations, is approached here in the framework of a hybrid numerical two-way coupled Eulerian-Lagrangian methodology. In particular, insights are sought by considering a relatively simple archetypal setting corresponding to a shock wave originating from a small spherical region initialized on the basis of available analytic solutions. The response of the system is explored numerically with respect to several parameters, including the blast intensity (via the related value of the initial shock Mach number), the solid mass fraction (mass load), and the particle size (Stokes number). Results are presented in terms of pressure-load diagrams. Beyond practical applications, it is shown that a kaleidoscope of fascinating patterns is produced by the “triadic” relationships among multiple shock reflection events and particle-fluid and particle-wall interaction dynamics. These would be of great interest to researchers and scientists interested in fundamental problems relating to the general theory of pattern formation in complex nonlinear multiphase systems.

Drag reduction in turbulent channel flow laden with finite-size oblate spheroids

We study suspensions of oblate rigid particles in a viscous fluid for different values of the particle volume fractions. Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle–particle and particle–wall interactions. With respect to the single-phase flow, we show that in flows laden with oblate spheroids the drag is reduced and the turbulent fluctuations attenuated. In particular, the turbulence activity decreases to lower values than those obtained by accounting only for the effective suspension viscosity. To explain the observed drag reduction, we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle–wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of oblate particles. These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.

Modeling of dielectrophoretic particle motion: Point particle versus finite-sized particle.

Dielectrophoresis (DEP) is a very popular technique for microfluidic bio-particle manipulation. For the design of a DEP-based microfluidic device, simulation of the particle trajectory within the microchannel network is crucial. There are basically two approaches: (i) point-particle approach and (ii) finite-sized particle approach. In this study, many aspects of both approaches are discussed for the simulation of direct current DEP, alternating current DEP, and traveling-wave DEP applications. Point-particle approach is implemented using Lagrangian tracking method, and finite-sized particle is implemented using boundary element method. The comparison of the point-particle approach and finite-sized particle approach is presented for different DEP applications. Moreover, the effect of particle-particle interaction is explored by simulating the motion of closely packed multiple particles for the same applications, and anomalous-DEP, which is a result of particle-wall interaction at the close vicinity of electrode surface, is illustrated.

Particle–wall interaction in entrained-flow slagging coal gasifiers: Granular flow simulation and experiments in a cold flow model reactor

Entrained-flow slagging coal gasifiers display large conversion efficiencies and small levels of unconverted carbon at the exhaust. Both features are apparently at odds with the fairly small “space-time” of the particle-laden gas feeding, as compared with the time scale of heterogeneous gasification of carbon. This apparent inconsistency can be explained by considering that fuel residence times are longer than the “space-time” due to segregation of fuel particles in the near-wall region of the gasifier. Segregation is promoted by swirl flow, by particle–wall interaction as the wall is covered by a molten layer of slag and by the establishment of a dense-dispersed flow of granular solids in the proximity of the wall.This study presents results of granular flow simulations of the interaction of a dense-dispersed particle flow with the confining wall. Simulations consider that both the particles and the wall may be either “sticky” or “non sticky”, based on the prevailing elastic vs plastic behaviour upon collision. The effect of drag forces exerted on particles by swirled gas flow is simulated in a simplified manner. Particle–particle collisions are modelled with a Hertzian approach that includes torque and cohesion. The extent and time scale of segregation of a lump of particles loaded into a cylindrical vessel are assessed. Results clearly indicate the different structure of the layer of particles establishing at the wall surface in the different interaction regimes.Results of simulations are qualitatively compared with results of an experimental campaign performed in a reactor representing a cold flow model of the entrained-flow gasifier, where solid, molten or semi-molten particles have been simulated by atomized wax as surrogate material. Altogether, the results confirm the importance of the particle–particle and particle–wall micromechanical interactions for a correct prevision of the segregation of fuel particles in entrained-flow slagging gasifiers.

Computer Simulation of Three Particles Sedimentation in a Narrow Channel

The settling of three particles in a narrow channel is simulated via the lattice Boltzmann direct-forcing/fictitious domain (LB-DF/FD) method for the Reynolds number ranging from 5 to 200. The effects of the wall and the Reynolds number are studied. It is interesting to find that at certain Reynolds numbers the left (right) particle is settling at 0.175 (0.825) of the channel width irrespective of its initial position or the channel width. Moreover, numerical results have shown that the lateral particles lead at small Reynolds numbers, while the central particle leads at large Reynolds numbers due to the combined effects of particle-particle and particle-wall interactions. The central particle will leave the lateral ones behind when the Reynolds number is large enough. Finally the effect of the Reynolds number on the trajectory of the lateral particles is presented.

Fluid-particle energy transfer in spiral jet milling

Spiral jet milling is a size reduction process driven by the fluid energy of high velocity gas jets. Inter-particle and particle-wall interactions are responsible for size reduction. The process is energy intensive, but inefficient. The underlying mechanisms for size reduction in the mill are also not very well understood. The optimum grinding conditions are still currently found by trial and error experimentation. In this work, the Discrete Element Method coupled with Computational Fluid Dynamics is used to investigate the effects of different parameters on the particle collisional behaviour in a spiral jet mill. These include the particle concentration in the grinding chamber, the particle size, and the fluid power input. We report on our work analysing the efficiency of energy transfer and how it can be improved by changing the milling conditions and particle properties.

Colloidal diffusion in confined geometries.

Colloidal dispersions in confined geometries exhibit rich diffusive dynamics governed by an interplay between particle-particle and particle-wall interactions. This perspective reviews recent selected computational and experimental studies on diffusion of dense liquid-like dispersions in spatial confinement, with an emphasis on general physical concepts rather than system-specific details. The common thread is to analyse colloidal diffusion in confined geometries in terms of the local density experienced by the colloidal particles, viz. at the level of anisotropic pair densities, which have recently become experimentally accessible.

Brownian Dynamics Simulations of Magnetic Nanoparticles Captured in Strong Magnetic Field Gradients

The behavior of spherical single-domain magnetic nanoparticles in strong inhomogeneous magnetic fields is investigated through Brownian dynamics simulations, taking into account magnetic dipole–dipole interactions, repulsive hard-core Yukawa potential, hydrodynamic particle-wall interactions, and the mechanism of magnetic dipole rotation in the presence of a magnetic field. The magnetic capture process of nanoparticles in prototypical magnetic field gradients generated by a sudden reversal in perpendicular magnetization of a flat substrate (defining a “capture line”) is studied as a function of strength of the magnetic field and volume fraction of the magnetic nanoparticles. Capture curves show a regime where capture follows a power law model and suggest that particles with the Brownian relaxation mechanism are captured at a slightly faster rate than particles with the Néel relaxation mechanism under similar conditions of the field gradient. Additionally, evaluation of the shape of the aggregates of captured particles suggests that greater dipole–dipole interactions result in aggregate structures that are flatter/wider than in the case of negligible dipole–dipole interactions. These results can help guide the design of systems for magnetically directed assembly of nanoparticles into complex shapes at a substrate.

Numerical simulation of polygonal particles moving in incompressible viscous fluids

A two-dimensional coupled lattice Boltzmann immersed boundary discrete element method is introduced for the simulation of polygonal particles moving in incompressible viscous fluids. A collision model of polygonal particles is used in the discrete element method. Instead of a collision model of circular particles, the collision model used in our method can deal with particles of more complex shape and efficiently simulate the effects of shape on particle–particle and particle–wall interactions. For two particles falling under gravity, because of the edges and corners, different collision patterns for circular and polygonal particles are found in our simulations. The complex vortexes generated near the corners of polygonal particles affect the flow field and lead to a difference in particle motions between circular and polygonal particles. For multiple particles falling under gravity, the polygonal particles easily become stuck owing to their corners and edges, while circular particles slip along contact areas. The present method provides an efficient approach for understanding the effects of particle shape on the dynamics of non-circular particles in fluids.

Boundary-induced autophoresis of isotropic colloids: anomalous repulsion in the lubrication limit

When suspended in a liquid solution, chemically active colloids may self-propel due to an asymmetry in either particle shape or the interfacial distribution of solute absorption. We here consider a chemically homogeneous spherical particle which undergoes self-diffusiophoresis due to the presence of nearby inert wall. In particular, we focus upon the near-contact limit where it was recently observed (Yariv,Phys. Rev. Fluids, vol. 1 (3), 2016, 032101) that the solute-concentration profile within the narrow gap separating the particle and the wall cannot be uniquely determined by a gap-scale analysis. We here revisit this near-contact limit using matched asymptotic expansions, the inner region being the gap domain and the outer region being on the particle scale. Asymptotic matching with the Hankel-transform representation of the outer distribution of solute concentration serves to determine both the scaling and magnitude of the corresponding inner profile. The ensuing gap-scale pressure field, set by a lubrication mechanism, gives rise to an anomalous particle–wall interaction, scaling as an irrational power of the gap clearance.