Aggregation and clogging phenomena of rigid microparticles in microfluidics

We developed a numerical tool to investigate the phenomena of aggregation and clogging of rigid microparticles suspended in a Newtonian fluid transported through a straight microchannel. In a first step, we implement a time-dependent one-way coupling Discrete Element Method (DEM) technique to simulate the movement and effect of adhesion on rigid microparticles in two- and three-dimensional computational domains. The Johnson–Kendall–Roberts (JKR) theory of adhesion is applied to investigate the contact mechanics of particle–particle and particle–wall interactions. Using the one-way coupled solver, the agglomeration, aggregation and deposition behavior of the microparticles is studied by varying the Reynolds number and the particle adhesion. In a second step, we apply a two-way coupling CFD–DEM approach, which solves the equation of motion for each particle, and transfers the force field corresponding to particle–fluid interactions to the CFD toolbox OpenFOAM. Results for the one-way (DEM) and two-way (CFD–DEM) coupling techniques are compared in terms of aggregate size, aggregate percentages, spatial and temporal evaluation of aggregates in 2D and 3D. We conclude that two-way coupling is the more realistic approach, which can accurately capture the particle–fluid dynamics in microfluidic applications.     

Blood cell transport and aggregation using discrete ellipsoidal particles

A computational model is presented for efficient mesoscale simulation of the transport, collision and aggregation of blood cells, which can be applied to examine red blood cells (RBCs), leukocytes, or platelets in various types of blood flows in which the fluid length scale is substantially larger than the particle length scale. This method is intended to be intermediate between microscale models, which examine deformation and flow around a small number of individual blood cells, and more phenomenological continuum models. The computational model utilizes a particle approximation for the blood cells and introduces other physically-justifiable approximations in order to accommodate computations with large numbers of cells. For instance, the non-spherical RBC and platelet shape is incorporated into the model by use of ellipsoidal particles. A novel method based on particle level-surfaces is presented for rapid identification of particle collision. It is shown that receptor–ligand binding between the cells can be modeled under certain conditions using a formulation that is mathematically similar to van der Waals adhesion of particles, but in which the surface energy density is variable in time. The method is demonstrated to provide computations of the interaction and adhesion of over 13,000 red-blood-cell particles on an ordinary workstation. These computations exhibit formation of chain-like rouleaux aggregates, modification of rouleaux structure due to shear flow, and capture and/or breakup of colliding rouleaux. The model predictions are examined for rouleaux size distribution in channel flow in comparison to experimental data, as well as for the effect of RBC aggregation on margination of white blood cells and platelets in channel flows.     

A DEM-DLM/FD method for direct numerical simulation of particulate flows: Sedimentation of polygonal isometric particles in a Newtonian fluid with collisions

An efficient direct numerical simulation method to tackle the problem of particulate flows at moderate to high concentration and finite Reynolds number is presented. Our method is built on the framework established by Glowinski and his co-workers [Glowinski R, Pan TW, Hesla TI, Joseph DD. A distributed lagrange multiplier/fictitious domain method for particulate flow. Int J Multiphase Flow 1999;25:755-94] in the sense that we use their Distributed Lagrange Multiplier/Fictitious Domain (DLM/FD) formulation and their operator-splitting idea but differs in the treatment of particle collisions. Compared to our previous works [Yu Z, Wachs A, Peysson Y. Numerical simulation of particle sedimentation in shear-thinning fluids with a fictitious domain method. J Non Newtonian Fluid Mech 2006;136:126-139; Yu Z, Shao X, Wachs A. A fictitious domain method for particulate flow with heat transfer. J Comput Phys 2006;217:424-52; Yu Z, Wachs A. A fictitious domain method for dynamic simulation of particle sedimentation in Bingham fluids. J Non Newtonian Fluid Mech 2007;145:78-91], the novelty of our present contribution relies on replacing the simple artificial repulsive force based collision model usually employed in the literature by an efficient Discrete Element Method (DEM) granular solver. The use of our DEM solver enables us to consider particles of arbitrary shape (at least convex) and to account for actual contacts, in the sense that particles actually touch each other, in contrast with the repulsive force based collision model. We validate GRIFF,¹ our numerical code, against benchmark problems and compare our predictions with those available in the literature. Results, which, to the best of our knowledge, have never been reported elsewhere, on the 2D sedimentation of isometric polygonal particles with collisions are presented.     

Improved coupling of time integration and hydrodynamic interaction in particle suspensions using the lattice Boltzmann and discrete element methods

This paper introduces improvements to the simulation of particle suspensions using the lattice Boltzmann method (LBM) and the discrete element method (DEM). First, the benefit of using a two-relaxation-time (TRT) collision operator, instead of the popular Bhatnagar–Gross–Krook (BGK) collision operator, is demonstrated. Second, a modified solid weighting function for the partially saturated method (PSM) for fluid–solid interaction is defined and tested. Results are presented for a range of flow configurations, including sphere packs, duct flows, and settling spheres, with good accuracy and convergence observed. Past research has shown that the drag, and consequently permeability, predictions of the LBM exhibit viscosity-dependence when used with certain boundary conditions such as bounce-back or interpolated bounce-back, and this is most pronounced when the BGK collision operator is employed. The improvements presented here result in a range of computational viscosities, and therefore relaxation parameters, within which drag and permeability predictions remain invariant. This allows for greater flexibility in using the relaxation parameter to adjust the LBM timestep, which can subsequently improve synchronisation with the time integration of the DEM. This has significant implications for the simulation of large-scale suspension phenomena, where the limits of computational hardware persistently constrain the resolution of the LBM lattice.     

Deposition of suspensions in the entrance of a parallel-plate channel with gravity effect

The deposition of aerosols near the inlet region of a horizontal parallel channel (for a distance of 7 channel widths) was investigated by solving numerically the governing equations for both the fluid and particulate phases with boundary layer assumptions. Surface adhesion between the channel walls and the aerosol particles along with the gravitational influence was varied.This analysis took into consideration the simultaneous development of both the fluid and particle phases. The deposition for the present analysis for high surface adhesion was found to be greater for $\text{X ≥ 2}$ than the deposition obtained by Ingham for Plug flow who used only the diffusion equation with zero particulate density at the wall.     

Pattern recognition problems in the study of carbon black

This paper reviews certain pattern recognition problems associated with the study of carbon black. Two methods are described in which various properties are estimated by visual comparison with two-way charts. For studying fused aggregates, a new method of shape characterization has been developed, which should be of quite general utility. This method is based on representation of the silhouette of the aggregate as an ellipse with equivalent radii of gyration. Computer simulation is used to provide realistic models of aggregates, from which a relation is derived between the projected area of the silhouette (relative to that of a particle) and the volume of the aggregate. Automated methods are discussed for the determination of mean aggregate size and certain particle size parameters.     

Coupled solid (FVM)–fluid (DSMC) simulation of micro-nozzle with unstructured-grid

The flow field and temperature distributions of free molecular micro-electro-thermal resist jet (FMMR) were studied resorting to DSMC–FVM coupled method. Direct simulation Monte Carlo (DSMC) method is the most useful tool to simulate the flow field of FMMR and unstructured grid is suitable for the flow simulation in a complicated region with tilted wall surface. DSMC code based on unstructured grid system was developed and the result was compared with that of structured grid and analytical solution to validate the reliability of the developed code. The DSMC method was then used to simulate the fluid flow in the micro-nozzle (Kn > 0.01) and the temperature distribution in the nozzle wall was obtained by the finite volume method (FVM). The Dirichlet–Neumann method was used to couple the wall heat flux and temperature between flow field and solid area. The effect of different income pressure was studied in detail and the results showed that the temperature of solid area changed drastically at different income pressure, so the commonly-adopted method of pre-setting boundary temperature before simulation was unreasonable.     

A PZT-driven atomizer based on a vibrating flexible membrane and a micro-machined trumpet-shaped nozzle array

This paper presents the design, and fabrication of a PZT-driven atomizer based on a flexible membrane and a micro-machined trumpet-shaped nozzle array. Tests were conducted to demonstrate that the developed atomizer can produce fine droplets. The atomizer uses a PZT bimorph plate attached to a liquid-proof HDPE membrane with a low Young’s modulus to generate a pressure wave in the liquid reservoir. The trumpet-shaped micro-nozzle array is fabricated using a surface micromachining technique and an electroplating process. The fabrication process allows the use of a low resolution photomask to fabricate a high feature-sized trumpet-nozzle array. The SMD values of the ejected droplets and the flow rate of the fabricated atomizer are measured experimentally as a function of the operating frequency and the nozzle diameter for liquids of various viscosities. The relationship between the droplet size distribution and the SMD value is also explored. The experimental results show that the atomizer is capable of generating droplets with an SMD of 4.6 μ at a flow rate of 2.5 g/min. Hence, the atomizer has the potential for use in many applications.     

气固两相流模拟的随机离散模型

§1.引言 气固两相流动形式是最复杂的两相流动实例,其系统中的颗粒浓度较高,颗粒间的碰撞经常发生,从而导致细观层次上的颗粒运动具有复杂性,对于两相流动系统,拟流体模型以其大规模模拟的可行性在数值模拟领域中居重要地位。但是,拟流体模型的连续性假设     

Mesoscale simulations of particulate flows with parallel distributed Lagrange multiplier technique

Fluid particulate flows are common phenomena in nature and industry. Modeling of such flows at micro and macro levels as well establishing relationships between these approaches are needed to understand properties of the particulate matter. We propose a computational technique based on the direct numerical simulation of the particulate flows. The numerical method is based on the distributed Lagrange multiplier technique following the ideas of Glowinski et al. [16] and Patankar [30]. Each particle is explicitly resolved on an Eulerian grid as a separate domain, using solid volume fractions. The fluid equations are solved through the entire computational domain, however, Lagrange multiplier constrains are applied inside the particle domain such that the fluid within any volume associated with a solid particle moves as an incompressible rigid body. Mutual forces for the fluid-particle interactions are internal to the system. Particles interact with the fluid via fluid dynamic equations, resulting in implicit fluid-rigid body coupling relations that produce realistic fluid flow around the particles (i.e., no-slip boundary conditions). The particle-particle interactions are implemented using explicit force-displacement interactions for frictional inelastic particles similar to the DEM method of Cundall et al. [10] with some modifications using a volume of an overlapping region as an input to the contact forces. The method is flexible enough to handle arbitrary particle shapes and size distributions. A parallel implementation of the method is based on the SAMRAI (Structured Adaptive Mesh Refinement Application Infrastructure) library, which allows handling of large amounts of rigid particles and enables local grid refinement. Accuracy and convergence of the presented method has been tested against known solutions for a falling particle as well as by examining fluid flows through stationary particle beds (periodic and cubic packing). To evaluate code performance and validate particle contact physics algorithm, we performed simulations of a representative experiment conducted at the U.C. Berkeley Thermal Hydraulic Lab for pebble flow through a narrow opening.     

Blood flow through a stenosed catheterized artery: Effects of hematocrit and stenosis shape

The problem of blood flow through a narrow catheterized artery with an axially nonsymmetrical stenosis has been investigated. Blood is represented by a two-phase macroscopic model, i.e., a suspension of erythrocytes (red cells) in plasma (Newtonian fluid). The coupled differential equations for both fluid (plasma) and particle (erythrocyte) phases have been solved and the expression for the flow characteristics, namely, the flow rate, the impedance (resistance to flow), the wall shear stress and the shear stress at the stenosis throat have been derived. It is found that the impedance increases with the catheter size, the hematocrit and the stenosis size (height and length) but decreases with the shape parameter. A significant increase in the magnitude of the impedance and the wall shear stress occurs even for a small increase in the catheter size. The flow resistance increases and the shear stress at the stenosis throat decreases with the increasing catheter size and assume an asymptotic value at about the catheter size half of the artery size.     

Lattice Boltzmann simulation of the dispersion of aggregated particles under shear flows

The lattice Boltzmann method (LBM) for multicomponent immiscible fluids is applied to simulations of the deformation and breakup of a particle-cluster aggregate in shear flows. In the simulations, the solid particle is modeled by a droplet with strong interfacial tension and large viscosity. The van der Waals attraction force is taken into account for the interaction between the particles. The ratio of the hydrodynamic drag force to cohesive force, I, is introduced, and the effect of I on the aggregate deformation and breakup in shear flows is investigated. It is found that the aggregate is easier to deform and to be dispersed when I is over 100.     

Slow viscous flow around two particles in a cylinder

This article describes the motion of two arbitrarily located free moving particles in a cylindrical tube with background Poiseuille flow at low Reynolds number. We employ the Lamb’s general solution based on spherical harmonics and construct a framework based on cylindrical harmonics to solve the flow field around the particles and the flow within the tube, respectively. The two solutions are performed in an iterated framework using the method of reflections. We compute the drag force and torque coefficients of the particles which are dependent on the distances among the cylinder wall and the two particles. In addition, we provide detailed flow field in the vicinity of the two particles including streamlines and velocity contour. Our analysis reveals that the particle–particle interaction can be neglected when the separation distance is three times larger than the sum of particles radii when the two particles are identical. Furthermore, the direction of Poiseuille flow, the particle position relative to the axis and the particle size can make the two particles attract or repel. Unlike the single particle case, the two particles can move laterally due to the hydrodynamic interaction. Such analysis can give insights to understand the mechanisms of collision and aggregation of particles in microchannels.     

Cluster formation and growth in microchannel flow of dilute particle suspensions

The lifetime of microfluidic devices depends on their ability to maintain flow without interruption. Certain applications require microdevices for transport of liquids containing particles. However, microchannels are susceptible to blockage by solid particles. Therefore, in this study, the phenomenon of interest is the formation and growth of clusters on a microchannel surface in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particle-laden flows in microchannels with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel are presented. Although the cluster size seems to be independent of location, more clusters are found at the inlet/outlet regions than in the microchannel center. Similarly as for an individual cluster, as long as particle–cluster interaction is the dominant mode, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size, and concentration are also reported.     

Study of subsonic�Csupersonic gas flow through micro/nanoscale nozzles using unstructured DSMC solver

We use an extended direct simulation Monte Carlo (DSMC) method, applicable to unstructured meshes, to numerically simulate a wide range of rarefaction regimes from subsonic to supersonic flows through micro/nanoscale converging–diverging nozzles. Our unstructured DSMC method considers a uniform distribution of particles, employs proper subcell geometry, and follows an appropriate particle tracking algorithm. Using the unstructured DSMC, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number on the flow field in micro/nanoscale nozzles. If we apply the back pressure at the nozzle outlet, a boundary layer separation occurs before the outlet and a region with reverse flow appears inside the boundary layer. Meanwhile, the core region of inviscid flow experiences multiple shock-expansion waves. In order to accurately simulate the outflow, we extend a buffer zone at the nozzle outlet. We show that a high viscous force creation in the wall boundary layer prevents any supersonic flow formation in the divergent part of the nozzle if the Knudsen number exceeds a moderate magnitude. We also show that the wall boundary layer prevents forming any normal shock in the divergent part. In reality, Mach cores would appear at the nozzle center followed by bow shocks and expansion region. We compare the current DSMC results with the solution of the Navier–Stokes equations subject to the velocity slip and temperature jump boundary conditions. We use OpenFOAM as a compressible flow solver to treat the Navier–Stokes equations.     

Development of Wall Climbing Robot System by Using Impeller Type Adhesion Mechanism

In this paper, we present a wall climbing robot system, called “LARVA”, developed for visual inspection of structures with flat surfaces. The robot has two differential driving wheels with a suspension and an adhesion mechanism. The adhesion mechanism is composed of an impeller and two–layered suction seals. It is designed to provide sufficient adhesion force and be controlled so that the robot can move freely on various wall surfaces. The static and aerodynamic modeling of the adhesion mechanism is given and the analysis of the adhesion mechanism, air leakage, and inner flow are carried out to be useful for the design as well as the control. Finally, the performances of the robot are experimentally verified on several kinds of walls and its feasibility is validated.     

Lift and multiple equilibrium positions of a single particle in Newtonian and Oldroyd-B fluids

A heavy particle is lifted from the bottom of a channel in a plane Poiseuille flow when the Reynolds number is larger than a critical value. In this paper we obtain correlations for lift-off of particles in Oldroyd-B fluids. The fluid elasticity reduces the critical shear Reynolds number for lift-off. The effect of the gap size between the particle and the wall, on the lift force, is also studied. A particle lifted from the channel wall attains an equilibrium height at which its buoyant weight is balanced by the hydrodynamic lift force. Choi and Joseph [Choi HG, Joseph DD. Fluidization by lift of 300 circular particles in plane Poiseuille flow by direct numerical simulation. J Fluid Mech 2001;438:101-128] first observed multiple equilibrium positions for a particle in Newtonian fluids. We report several new results for the Newtonian fluid case based on a detailed study of the multiple equilibrium solutions, e.g. we find that at a given Reynolds number there are regions inside the channel where no particle, irrespective of its weight, can attain a stable equilibrium position. This would result in particle-depleted zones in channels with Poiseuille flows of a dilute suspension of particles of varying densities. Multiple equilibrium positions of particles are also found in Oldroyd-B fluids. All the results in this paper are based on 2D direct numerical simulations.     

柴油机碳烟颗粒物的分形生长分析与凝聚控制

本文针对柴油机械的主要排放颗粒物–碳烟颗粒物,基于其分形生长物理数学模型,模拟了单微粒与单微粒、单微粒与团簇、团簇与团簇凝聚体碰撞的形态结构,结合碰撞频率,利用主要环境因素干扰对柴油机碳烟颗粒物的分形生长进行凝聚控制,使其凝聚成规则的几何体或密度较大的颗粒物,减少表面积和复杂度,减小粘滞阻力和对有毒颗粒物的吸附,以便于捕捉器的捕捉或沉降,实现了环境污染的防控.数值仿真结果表明此凝聚控制分析是可行、有效的,这将有助于理解和分析实际碳烟颗粒物非平衡凝聚生长的物理机制和动力学行为,为进一步降低可吸入性柴油机颗粒物的排放提供思路.     

Microfluidic continuous magnetophoretic protein separation using nanoparticle aggregates

We demonstrate a microfluidic continuous-flow protein separation process in which silica-coated superparamagnetic nanoparticles interact preferentially with hemoglobin in a mixture with bovine serum albumin, and the resulting hemoglobin-nanoparticle aggregates are recovered online using magnetophoresis. We present detailed modeling and analysis of this process yielding quantitative estimates of the recovery of both proteins, validated by experiments. While several previous studies utilize an average particle size in modeling magnetophoretic particle trajectories or process design, in this study we emphasize the importance of accounting for particle size distributions in calculating particle recovery, and therefore in estimating separation efficiency. We combine experimentally measured size distributions of protein-nanoparticle aggregates with simulations of particle trajectories and provide a simple analytical method to calculate the efficiency of separation at various flow speeds, which fully accounts for heterogeneity in particle sizes. Our method can potentially be used for affinity based biomolecular separations at both analytical and preparative scales by exploiting well-established techniques to functionalize nanoparticle surfaces with selective ligands. Further, the modeling methodology presented here may be applied to provide better estimates of particle recovery in a broad range of magnetophoretic separation processes involving heterogeneity in particle sizes.     

Electrocasting - formation and structuring of suspended microbodies using a.c. generated field cages

Hybrid microelectrode chip structures were used to generate three dimensional field cages. The a.c. field generated negative dielectrophoretic forces and caused suspended particles to aggregate. The shape and size of the aggregate depends on the material, suspending liquid, electrode arrangement, electrode driven and size. Particle-aggregates can be stabilised by chemical or physical means. Photopolymerization was used to solidify latexes into artificial microbodies in the micrometer range and antibody-mediated agglutination was also used. Single particles or aggregates can be covered by one or more layers of different materials and geometries. We show a glass sphere covered by a structured multivesicular coating and a gas bubble covered by a latex particle layer. The microbody casting technology can find application in materials science, pharmaceutical formulation and biotechnology. This work was supported by grant No. 0310260A and 13 MV03032 of BMFT/VDI (Germany).