Particle Migration Research Articles

Weakening of mechanical parameters of ion-absorbed rare-earth ores subjected to leaching

Ion-adsorbed rare-earth ores are mined using in-situ leaching, and their mechanical properties significantly affect the efficient and safe recovery of rare earth elements. However, the mechanism of the change in the mechanical properties of the ore body due to the physicochemical processes caused by leaching remains unclear. To explore the strength evolution characteristics of the ore body during the leaching process, unconsolidated undrained triaxial tests were conducted to confirm how the stress–strain curve and shear strength of rare-earth samples change during leaching. Magnetic resonance imaging and T2 spectral characterizations were obtained by using nuclear magnetic resonance technology to measure the interior pore structure of samples during leaching. A scanning electron microscope equipped with an energy dispersive spectrometer was used to investigate the morphology evolution and the composition changes of the internal micro-area of the samples, to demonstrate the correlation between the microstructural change and the macroscopic mechanical properties. The results show that when a 2% ammonium sulfate solution is employed for mineral leaching, the effective leaching duration is 0–3 h. During this time, ion exchange occurs along the direction of solution seepage, resulting in the dispersion and migration of fine particles from the top to the bottom of the sample, which further triggers a change in the sample's pore structure and pore size. In addition, the local loss of fine particles resulted in a reduced bond strength between minerals, forming an unstable soil structure with a loose upper part and a dense lower part, which is macroscopically expressed as a declining shear strength parameter of the rare-earth sample.Graphical abstract

Effect of heterojunction order between CaTiO3 and Mn doped SrTiO3 on memristive performance and its mechanism analysis

It is well-known that memristive devices can be applied to brain-like parallel computing, but the interface effects have an indelible impact on memristive performance. In this work, two memristive devices with Ag/CTO/SMTO/Ti and Ag/SMTO/CTO/Ti structures were fabricated based on the heterojunctions between CaTiO3 (CTO) and Mn-doped SrTiO3 (SMTO). By comparison, the Ag/CTO/SMTO/Ti device shows relatively improved memristive characteristics with the optimum sweeping voltage of 1.2 V and the switching ratio of ∼12. That is to say, changing the heterojunction order between CTO and SMTO can regulate the memristive behaviors of the device because the interface effect caused by heterojunction contact has influence on the particle migration, resulting in different rates of conductive filaments formation, so that the memristive characteristics of the device can be regulated. Finally, the resistive switching (RS) characteristics of the device are explained by in-depth mechanism analysis based on fitting experimental data. Therefore, this work not only provides a feasible method for improving the memristive characteristics of the device, but also helps to understand the influence of the interface effect on the particles motion, thereby laying the foundation for the preparation of high-capacity and efficient memristive device in artificial intelligence (AI) applications.

3D CFD-DEM study on fine particle migration in packed proppant layers

Fine particle migration is a crucial issue in hydraulic fracturing-based production from unconventional reservoirs and can significantly affect the effective conductivity of the packed proppant layers. In this study, to investigate the conveying behavior of fine particles, a Eulerian-Lagrangian method, namely the computational fluid dynamics–discrete element method (CFD-DEM), is adopted for 3D simulations. Two types of particles are involved in this process: larger proppant particles and fine particles. Proppant particles form a background skeleton, and a two-phase flow of fine particles and fluid is constrained in the connected pore networks. We adopt a unified CFD-DEM framework to resolve the non-slip boundary condition on the proppant particle surface and track fine particles using a four-way coupling strategy. In particular, a forcing term based on the fictitious domain method is embedded in a regular volume-averaged Navier-Stokes equation to solve the fluid motion. A perfectly packed skeleton of a face-centered cube is designed for subsequent numerical studies. The results show that the fluid and particle flow velocities are higher near the fracture surfaces than inside the interlayers. In addition, it is found that as the fine particle concentration increases, the average horizontal velocity increases, whereas the settling velocity tends to decrease.

CFD-DEM simulation of aggregation and growth behaviors of fluid-flow-driven migrating particle in porous media

Particle migration with liquid flow is a fundamental problem during the highly efficient development of oil and gas. In this study, the aggregation and growth behaviors of fluid-flow-driven fine particles at the pore scale are investigated by considering their successive injection and migration process based on CFD-DEM. The particle aggregation and growth behaviors within porous media are obtained, by which the particle aggregation growth affects hydraulic properties and the risk of pore clogging are explored. Several experiments of particle injection were also conducted to verify the accuracy of the numerical model. Both simulation results and experimental phenomena illustrate that fine particle are prone to aggregate near the inlet and bottom wall with a discontinuous concentration, and the calculated instantaneous velocities of particle are larger at the outlet and near the wall affected by the deviation of pore size and position. The three modes of migration, aggregation and growth of particles due the successive invasion and deposition are captured at pores, and are mainly divided into independent and dependent growth behaviors based on the presence or absence of deposited particles before the arrival of new-injected particles. Results highlight the effect of injected fluid velocity, fluid viscosity and fine particle concentration on the particle migration and aggregation behaviors and its dependency with the risk of pore clogging, so as to provide theoretical guidance to the prevention of pore clogging in engineering applications.

Transport of polystyrene nanoplastics with different functional groups in goethite-coated saturated porous media: Effects of low molecular weight organic acids and physicochemical properties

The influence of low molecular weight organic acids (LMWOAs) and goethite on the migration of nanoplastics in the soil environment remains poorly understood. To elucidate the mechanism of influence, the study investigated the impact of LMWOAs on the migration ability of functionalized polystyrene nanoplastics (PSNPs-NH2/COOH) in quartz sand (QS) and goethite (α-FeOOH)-coated quartz sand (FOS). We investigated the effect of changes in iron valence induced by LMWOAs on the migration of PSNPs. The results revealed that the migration ability of polystyrene nanoplastics (PSNPs) declined as the ionic strength (IS) increased and the pH decreased, primarily due to the compression of the double layer and protonation reactions. The migration of PSNPs is facilitated by LMWOAs through distinct mechanisms in the two media. Specifically, LMWOAs were adsorbed on the FOS and QS surfaces through complexation and hydrogen bonding, respectively. At pH 4.0, LMWOAs exhibit redox activity, resulting in the generation of additional Fe(III). This redox process enhances the electrostatic attraction between the media and PSNPs, thereby reducing the competition at specific points and spatial resistance associated with LMWOAs. In contrast to FOS, LMWOAs at pH 4.0 reduced the migration ability of PSNPs in QS, following the trend of MA > TA > CA. This difference was attributed to the pKa of LMWOAs and the weak hydrogen bonding on the QS surface. The relevant mathematical models effectively validate the migration results. The above conclusions suggest that LMWOAs can alter the valence state of iron on the surface of goethite, thereby influencing the migration of plastic particles in environmental media.

Ni‐phase erosion losses at a Ni‐pore interface of microtubular solid oxide fuel cells during 5218 h of operation

AbstractA microtubular solid oxide fuel cell (μT‐SOFC) with an anode buffer layer in the inner wall of the anode is successfully fabricated. The anode buffer layer comprises nickel (Ni), gadolinium‐doped ceria, and ruthenium (Ru), of which Ru exhibits excellent stability in mixed impurity gases. At 700°C, the maximum power density of the μT‐SOFC reaches around 0.76 W/cm2 under a hydrogen flow rate of 10 sccm. After 5218 h of operation at a constant current density of 0.2 A/cm2 at 650°C, there is almost no degradation (0.093%/kh) in area‐specific resistance after 2000 h of operation under the protection of the anode buffer layer. The aggregation and migration of nanoscale Ni particles are observed at the Ni particle boundary near the triple‐phase boundary (TPB), with gradual corrosion of the Ni particles. Ni migration and loss in the TPB are determined as reasons for the decay of the fuel cells during their long‐term operation. Moreover, Ni in an amorphous state is also observed on the periphery of the Ni particles, indicating that during the corrosion process of Ni, transformation to the amorphous state occurred, which impairs the catalytic activity and electronic conductivity of Ni, resulting in a deterioration in its performance.

Microchannel structure affects fine particulate pollutant interception characteristics

Deep filtration is an important technology for the separation of fine particle pollutants in industrial wastewater. The theoretical core of microchannel separation is the interaction between channel interface and pollutants. Here, microchannels with different shapes and sizes were designed by microfluidic technology and the particle migration process in microchannels was visualized by high-speed camera. This paper studied the effects of apparent depth, inlet structure and channel angle of the interception channel on fluid flow and particle interception. When apparent depth of the interception channel was equidistantly distributed, the flow rate in each channel was uniform, but interception efficiency gradually decreased with the increase of apparent depth. Interception efficiency of the first layer was 16.7 %, while for the 14th layer was only 0.4 %. Interception performance of the upper wall of the inlet of the interception channel was substantially higher than that of the lower wall. The smoother the angle between the interception channel and the mainstream channel was, the better the interception performance. When the angle between the upper and lower walls of the inlet and the mainstream channel was 30° and 60°, respectively, interception efficiency was the optimal, and cumulative interception efficiency of the three filtrations was 92.4 %. Additionally, flow rate and interception efficiency of the interception channel increased with decreasing channel angle. The total interception performance of the 30°–45° group was the optimal, and the total split ratio and total interception efficiency were 65.5 % and 85.5 %, respectively. This provides reference for the change of microchannel structure and the mechanism of particle migration and deposition.

A multiscale computational framework using active learning to model complex suspension flows

We devise a novel computational framework to achieve efficient multiscale modeling for suspension flows. The modeling framework comprises a particle-resolving direct numerical simulation for microscopic computation, a single-fluid continuum model to capture the bulk flow behavior at the macroscale, and a Gaussian process regression that connects the two modeling components. The microscopic calculation simulates detailed flow fields around individual suspended particles. Along with a coarse-grained method, we are able to obtain the mean volume fractions and stresses for discrete particles, which are then further used to calculate the rheological properties of the bulk flow fields at the macroscale under specific forcing conditions. Using Gaussian process regression, a few data points from the microscopic calculations can be further interpolated/extrapolated to form complete constitutive relationships that cover the whole forcing range in macroscopic calculations (i.e., continuum modeling). Moreover, via the uncertainties returned from the Gaussian process regression, the resulting active learning strategy using the Gaussian process regression automatically decides on the required the microscopic calculations given by a specified tolerance. As a result, complex suspension problems can be efficiently solved by the macroscopic continuum model to the desired accuracy with the minimum computational effort (i.e., minimum number of microscopic runs). Four examples are demonstrated, among which two examples involve particle migration as a function of the particle stress, and the resulting nonuniform distribution of particles. This demands a multivariate Gaussian process regression, which is found to be particularly effective in achieving high computational efficiency compared with the off-line cases, i.e., a large number of off-line microscopic calculations to obtain the prescribed constitutive relationships.

Interface interaction between SiO2 and magnetite under high temperature: particle migration and inhibition mechanism

Interface interaction between SiO2 and magnetite under high temperature: particle migration and inhibition mechanism

High activity and sinter-resistance of Ni@silicalite-1 catalyst for dry reforming of methane

This paper reports a highly active and stable Ni@silicalite-1 catalyst for dry reforming of methane. In the reaction conducted at 800[Formula: see text]C, CH4/CO2=1/1 and GHSV=100,000 mL⋅g[Formula: see text]⋅h[Formula: see text] for 100 h, the Ni@silicalite-1 delivered CH4 conversions of 93% without deactivation. This superior property of the catalyst comes from the special structure that large amounts of Ni (13.7 wt.%) are embedded in the silicalite-1 zeolite crystals as small particles ([Formula: see text]4.7 nm), with the zeolite channels transferring molecules while with the zeolite framework resisting the Ni particle migration.

Sediment resuspension causes horizontal variations in the distributions of phosphorus (P) and P-inactivating materials with differing P immobilization in different sediment planes.

The importance of controlling internal phosphorus (P) pollution in lakes has been recognized by scientists, and the application of P-inactivating materials to immobilize sediment P is often considered. However, sediment resuspension, a typical physical process occurring in lakes, has been demonstrated to increase the uncertainty of immobilization. In this study, we explored the characteristics of P immobilization in the horizontal direction under the effects of resuspension using annular flume tests based on drinking water treatment residuals (DWTR). The results showed that resuspension caused the mobile P and bioavailable P to be heterogeneously distributed in sediment planes after DWTR addition, resulting in varying P immobilization efficiencies at different depths. In particular, the coefficient of variation was 14.2-24.5% for mobile P horizontally distributed in the planes, resulting in a range of mobile P decreasing efficiencies at 24.0-47.8%. Further analysis indicated that variations in horizontal distribution were typically due to the varied migration of particles of different sizes. Specifically, P immobilization in sediment planes at different depths was regulated by promoting the migration of <8μm DWTR after relatively low-intensity disturbance (in surface 0-1cm sediment). After relatively high-intensity disturbance (in the whole 0-3cm sediment), immobilization in the horizontal direction was regulated by coupling the migration of >63μm DWTR (to the bottom) with the mixing of <8μm DWTR in the sediment plane at different depths. The varying horizontal distributions of total P, resulting from the migration of 16-32μm sediment, could enhance the heterogeneities of the P immobilization. Thus, the particle size of materials and lake background conditions, for example, the hydrodynamic characteristics and P distributions in differently sized sediments, should be used as key bases to select or develop P-inactivating materials to design proper remediation strategies for controlling internal P pollution in lakes.

New explicit formula for inertial lift in confined flows

Inertial migration has proven effective for high-throughput manipulation of tiny particles in confined flows. However, complex and even controversial relationships between hydrodynamic forces and flow conditions hinder the development of an explicit formula for inertial lift acting on finite-sized particles at Reynolds numbers in the hundreds. Herein, we reveal the different scaling laws for shear gradient-induced inertial lift and wall-induced inertial lift by separating the contributions of pressure stress and viscous stress. A direct result of this treatment is a new explicit formula for the total inertial lift, valid for Reynolds numbers up to 400 and particle blockage ratios up to 0.25. This study provides an alternative way to understand inertial lift, thereby greatly facilitating the prediction of particle migration in practical microfluidic applications.

Shear-induced particle migration in a cement slurry under oscillatory pipe flow

Shear-induced particle migration in a cement slurry under oscillatory pipe flow

Response analysis and effect evaluation of dynamic stabilization for ballasted track

Dynamic stabilization is a process of the railway lines that are newly built must go through before it is put into operation. This study investigates the characteristics of stabilizing operation on ballasted track and proposes methods for detecting and evaluating the state of the ballast bed in an attempt to improve the operation quality. Firstly, the influence mechanism of stabilization on the migration of ballast particles and the settlement of track panel from the macro and micro perspectives through the method of visual non-contact measurement by video images were demonstrated. A three-dimensional simulation model which the first wheelset of the tamping and stabilizing vehicle - ballasted track was established using the discrete element method and multi-flexible body dynamics coupling algorithm, and the logistic mathematical model for sleeper displacement and support stiffness of ballast bed was constructed. Furthermore, an innovative non-contact method for measuring the support stiffness of ballast bed under complex scenes was proposed. Results show that the ballast particles on the slope of the ballast bed are rearranged and gradually compacted by a movement mode of “vertical as the main direction and horizontal as the auxiliary” during the stabilizing operation. As the number of stabilizing operations increases, the settlement of the track panel gradually decreases, with a maximum attenuation amplitude of 44.9%. Comprehensively, the track panel achieves reasonable load distribution after the second stabilizing operation, and achieves the best effect with the relative growth rate of the two-point stiffness being 49.5%. Significantly, this study provides key theoretical basis for the scientific stabilizing operation and evaluation of ballast bed status of railway lines that are newly built.

Characteristics and mechanism of the particle migration subject to the shear flow of concrete flow under pressure

The concrete flow under pressure highly depends on the formation of a lubrication layer between the concrete mass and the pipe wall. Though it is widely believed that particle migration under shear flow influences the formation of this layer, the exact mechanism remains unclear. Therefore, a fully coupled computational fluid dynamic - discrete element method (CFD-DEM) model is developed to investigate the dynamic behaviour of the aggregates within the pipe flow and validated via computed tomography (CT) study and aggregate phase segmentation. The characteristics and influencing factors of the particle migration process is examined by the validated model. The sub-spheres method is employed to accurately model the aggregate shapes in the model. It reveals that the uneven distribution of particle collision frequency contributes to radial particle migration and the subsequent formation of the lubrication layer. The extent of particle migration is found to depend on the pipe size, but independent on the flow rate. The methodology and findings of this paper will also be useful for three dimensional (3D) printing concrete.

An improved dissipative particle dynamics method for the liquid-particle two-phase flow in microchannels

Liquid-particle two-phase flow in microchannel widely exists in the fields of biomedical and environmental monitoring, such as the lab-chip device for disease diagnosis. The standard dissipative particle dynamics (DPD) method has been previously employed to study the liquid-particle two-phase flow in microchannel, but it cannot accurately simulate the real process because of the unsuitable DPD parameters. In the present study, an improved DPD method was developed by changing the system energy and fitting the characteristic curve between the random force coefficient and the Schmidt number. In addition, a new logarithmic relationship between the conservative force coefficient and the particle size was found. The result demonstrated that the improved DPD method enabled more accurate simulation on the liquid-particle two-phase flow in microchannels than the standard DPD method. For instance, in the simulation of particle sedimentation, the relative deviation between the value obtained by the improved DPD method and the theoretical value was less than 6% while the relative deviation was more than 20% for the standard DPD method. The simulated result of the particle migration in microchannel was in good agreement with the result obtained by Matas et al, and the relative deviation was less than 1.5%. Therefore, the improved DPD method would have great potentials in the study on the liquid-particle two-phase flow in microchannels.

Experimental and DEM-CFD Coupling Investigations on the Characteristics and Mechanism of Seepage Erosion for Cohesionless Soil

Seepage erosion is one of the main reasons for the local collapse or instability of embankments. To investigate the characteristics and mechanism of seepage erosion for cohesionless soils, model tests using an independently developed seepage erosion device and numerical simulations based on a discrete element method-computational fluid dynamics (DEM-CFD) coupling model were carried out. The results show that the seepage erosion process of cohesionless soil could be characterized by four stages: stable seepage, upward migration of fine particles, boiling of sand samples, and erosion damage. The skeleton structure of a soil sample under seepage flow was continually changed due to the loss of fine soil particles, which resulted in a significant decrease in the sample strength and could, ultimately, lead to the failure of the sample. The results of this study can provide references and bases for the design, construction, and long-term service of embankments or earth dams under complex seepage conditions, reducing the risk of seepage erosion.

Continuous On-Chip Cell Washing Using Viscoelastic Microfluidics.

Medium exchange of particles/cells to a clean buffer with a low background is essential for biological, chemical, and clinical research, which has been conventionally conducted using centrifugation. However, owing to critical limitations, such as possible cell loss and physical stimulation of cells, microfluidic techniques have been adopted for medium exchange. This study demonstrates a continuous on-chip washing process in a co-flow system using viscoelastic and Newtonian fluids. The co-flow system was constructed by adding a small amount of biocompatible polymer (xanthan gum, XG) to a sample containing particles or cells and introducing Newtonian fluids as sheath flows. Polymer concentration-dependent and particle size-dependent lateral migration of particles in the co-flow system were examined, and then the optimal concentration and the critical particle size for medium exchange were determined at the fixed total flow rate of 100 μL/min. For clinical applications, the continuous on-chip washing of white blood cells (WBCs) in lysed blood samples was demonstrated, and the washing performance was evaluated using a scanning spectrophotometer.

Study on the transport and deposition of micron-sized particles with different densities in the coal mine environment in miners

To address the health problems caused by the long-term intrusion of high-concentration dust generated in coal mining operations into the human respiratory tract, this study reconstructed a 3D digital model of the upper airway based on fine industrial CT scanning images, and established the micron particle migration calculation model by Euler-Lagrange method. The transport and deposition mechanisms of dust with different densities under different inspiratory intensities were simulated and analyzed. The simulated results were compared with the results of dust exposure experiments in the upper airway combined with 3D printing technology, and the relative errors obtained were in the range of 2.9% to 6.3%. The results showed that the airflow volume peaked in the nasopharynx, oropharynx, epiglottis, and larynx, leading to the airflow separation and formation of circulation that tend to cause dust deposition in these regions. As the dust density and inspiratory airflow volume increased, the deposition fraction of small dust particles (<1μm) in the upper airway slightly changed, and most of them escaped to the lung. For 7.07 μm dust particles, as the dust density increased from 1210 kg/m3 to 2750 kg/m3, the escape rate decreased from 31.43% to 0.48%, and this trend can be seen in the fitting function of escape rate and inspiratory airflow volume Y7.07μm=(80–1.37ν + 0.0064ν2) × 100%. When the dust particle diameter exceeded 20 μm, the impact of dust density was weakened by the large size of the dust, and the escape rate was close to 0% when the inspiratory airflow volume was 50–110 L/min. This study can provide theoretical support for the research and development of respiratory dust sensors and targeted dust prevention technologies.

Full-scale mud pumping test of ballastless trackbed under train loading

This paper presents a full-scale physical modeling investigation on the mud pumping in ballastless trackbed. The physical model was equipped with a rainfall device to simulate the in situ precipitation and various testing sensors were installed to monitor the dynamic response of the track-subgrade system. Results showed that the whipping of the track structure induced by high-speed train passages caused a separation between the track structure and the underlying roadbed, which allowed rainwater to intrude into the roadbed. The intruded rainwater infiltrated vertically and then permeated laterally from the bottom of the roadbed, eventually saturating the entire roadbed. Train moving loads partially promoted this rainwater infiltration. The excess pore water pressure (EPWP) was formed in the saturated roadbed caused by train moving loads, and it increased with train speeds and loading carriages. A noticeable upward EPWP gradient and a slight longitudinal EPWP gradient towards the expansion gap were also created in the saturated roadbed. Accordingly, both the vertical hydraulic gradient (HG) and longitudinal HG increased with train speeds and loading carriages, most of which, especially the vertical HG, exceeded the critical HG for particle migration with a diameter smaller than 7.1 mm. It caused severe upward particle migration and slight longitudinal particle migration towards the expansion gap. This spatial fine particle migration process, known as mud pumping, was driven by the HG-induced seepage force and further promoted by the EPWP amplification induced by high-speed train passages. Mud pumping resulted in a significant decrease in stiffness of the trackbed and a noticeable increase in vibration velocity of the track slab.