Virtual Particles Research Articles

Aggregation in experimental studies with microparticles: Experimental settings change particle size distribution during exposure

The ubiquitous occurrence of microplastics is raising broad concerns and motivating effect studies. In these studies, however, particle behaviour in the water and aggregation are rarely considered leading to contradictory results reported by different studies. Using an environmentally relevant experimental setup with Daphnia magna as a test organism, we investigated how experimental conditions affect particle aggregation and the aggregate heterogeneity in terms of the particle size distribution. The experimental factors considered were (1) exposure duration (48 h vs 120 h), (2) the total mass of suspended solids (0–10 mg/l) composed of natural mineral particles (kaolin) and microplastics, (3) the proportion of the microplastics in the particle suspension (0–10% by mass), (4) dissolved organic matter (DOM; 0 vs 20 mg agarose/l), and (5) presence of the test organism (0 and 5 daphnids/vial).We found that particle aggregation occurs within the first 48 h of incubation in all treatments, no substantial change in the aggregate heterogeneity is observed afterwards. The median aggregate size was ∼2-fold higher than the nominal average particle size of clay and microplastics in the stock suspensions used to prepare the experimental mixtures. The strongest positive driver of the aggregate size and heterogeneity was DOM, followed by the presence of daphnids and the concentration of the suspended solids in the system. Also, microplastics were found to facilitate aggregation, albeit they were the weakest contributor. Moreover, besides directly increasing the aggregation, DOM relaxed the effects of the total solids and daphnids on the aggregate size. Thus, the particle size distribution was established early during the exposure and shaped by all experimental factors and their interactions. These findings improve our understanding of the processes occurring in the exposure systems when conducting effect studies with microplastics and other particulates and demonstrate the necessity to access the particle size distribution to characterise the exposure. Aslo, relevant experimental designs with microplastics must include relevant natural particulates and DOM to ensure environmentally realistic particle behaviour and adequate particle-biota interactions.

Operando Modeling of Zeolite-Catalyzed Reactions Using First-Principles Molecular Dynamics Simulations.

Within this Perspective, we critically reflect on the role of first-principles molecular dynamics (MD) simulations in unraveling the catalytic function within zeolites under operating conditions. First-principles MD simulations refer to methods where the dynamics of the nuclei is followed in time by integrating the Newtonian equations of motion on a potential energy surface that is determined by solving the quantum-mechanical many-body problem for the electrons. Catalytic solids used in industrial applications show an intriguing high degree of complexity, with phenomena taking place at a broad range of length and time scales. Additionally, the state and function of a catalyst critically depend on the operating conditions, such as temperature, moisture, presence of water, etc. Herein we show by means of a series of exemplary cases how first-principles MD simulations are instrumental to unravel the catalyst complexity at the molecular scale. Examples show how the nature of reactive species at higher catalytic temperatures may drastically change compared to species at lower temperatures and how the nature of active sites may dynamically change upon exposure to water. To simulate rare events, first-principles MD simulations need to be used in combination with enhanced sampling techniques to efficiently sample low-probability regions of phase space. Using these techniques, it is shown how competitive pathways at operating conditions can be discovered and how broad transition state regions can be explored. Interestingly, such simulations can also be used to study hindered diffusion under operating conditions. The cases shown clearly illustrate how first-principles MD simulations reveal insights into the catalytic function at operating conditions, which could not be discovered using static or local approaches where only a few points are considered on the potential energy surface (PES). Despite these advantages, some major hurdles still exist to fully integrate first-principles MD methods in a standard computational catalytic workflow or to use the output of MD simulations as input for multiple length/time scale methods that aim to bridge to the reactor scale. First of all, methods are needed that allow us to evaluate the interatomic forces with quantum-mechanical accuracy, albeit at a much lower computational cost compared to currently used density functional theory (DFT) methods. The use of DFT limits the currently attainable length/time scales to hundreds of picoseconds and a few nanometers, which are much smaller than realistic catalyst particle dimensions and time scales encountered in the catalysis process. One solution could be to construct machine learning potentials (MLPs), where a numerical potential is derived from underlying quantum-mechanical data, which could be used in subsequent MD simulations. As such, much longer length and time scales could be reached; however, quite some research is still necessary to construct MLPs for the complex systems encountered in industrially used catalysts. Second, most currently used enhanced sampling techniques in catalysis make use of collective variables (CVs), which are mostly determined based on chemical intuition. To explore complex reactive networks with MD simulations, methods are needed that allow the automatic discovery of CVs or methods that do not rely on a priori definition of CVs. Recently, various data-driven methods have been proposed, which could be explored for complex catalytic systems. Lastly, first-principles MD methods are currently mostly used to investigate local reactive events. We hope that with the rise of data-driven methods and more efficient methods to describe the PES, first-principles MD methods will in the future also be able to describe longer length/time scale processes in catalysis. This might lead to a consistent dynamic description of all steps-diffusion, adsorption, and reaction-as they take place at the catalyst particle level.

The micro-mechanical behaviour of sand–rubber mixtures under shear: A numerical study based on X-ray micro-tomography

A CT image-based discrete element model (DEM) incorporating the realistic particle morphology of sand grains was developed to investigate the micro-mechanical behaviours of sand–rubber mixtures (SRMs) under triaxial compression. The model was validated by comparing the numerically simulated macro- and microscale mechanical behaviour of SRMs with the experimental results acquired in triaxial testing with in-situ μCT scanning. SRMs with irregularly shaped grains have a higher degree of heterogeneous distribution of contact force chains than those with spherical grains. Moreover, rubber content and particle shape have a significant influence on the ‘most active phase’, namely, the time-period during which the maximum number of contacts emerged to participate in the stress-transmission, of different types of contacts. The ‘most active phase’ of sand–sand contacts appeared earlier than that of sand–rubber contacts. SRMs with a lower rubber content were more likely to have an earlier ‘most active phase’ for each type of contact. The sequence of the ‘most active phase’ for the sand–rubber contacts and rubber–rubber contacts appeared during shear was affected by the particle shape of the SRM specimens. The results highlight the important role of rubber content and particle shape in the stress-transmission behaviour of SRMs.

Investigating real-time monitoring indices of compaction quality from particle movement characteristics of distinctly-graded unbound aggregate materials subjected to vibratory compaction

The compaction quality greatly affects the in-service performance of unbound aggregate materials (UAMs); however, there still lack real-time, non-destructive, and effective indices for compaction quality assessment. This paper aimed to reveal meso-scale mechanisms of macroscopic vibratory compaction behavior and evaluate compaction quality from particle movement characteristics. A series of newly-developed laboratory vibratory plate compaction tests were conducted on UAM specimens of different gradations. The innovative self-powered, wireless sensors resembling realistic particles (termed as “SR sensors”) were placed inside UAM specimens to monitor three-dimensional (3D) particle accelerations and Euler angles during vibratory compaction. The effects of gradation and vibratory frequency on both macroscopic compaction behavior and meso-scale particle motion characteristics were analyzed. The results show that the entire vibratory compaction process can be divided into three distinct stages according to discernibly-staged particle movement patterns. The degree of compaction (DOC) of UAMs is significantly correlated with two novel particle-motion-based compaction quality indicators.

Characterization of a CMOS Pixel Sensor prototype for the CEPC vertex detector

The proposed Circular Electron Positron Collider poses significant challenges for the vertex detector in terms of material budget, spatial resolution, readout speed, and power consumption. To address these challenges, a Monolithic Active Pixel Sensor prototype, named TaichuPix, has been developed based on a column drain readout architecture. The performance of the second version of this prototype, TaichuPix2, has been evaluated using a radioactive source, which exhibits a time walk of less than 100ns corresponding to the threshold of 380 e− and amplitude variation of analog output from 300mV to 500mV. An infrared laser setup is utilized to estimate the single-point spatial resolution of this sensor for minimum-ionizing particles. The setup includes a high-precision three-dimensional translation stage and an optical system with a 1064nm laser diode. With this setup, the spatial resolution is estimated to be about 4µm. The performance for real particles is to be confirmed in a test beam.

A review of the Discrete Element Method/Modelling (DEM) in agricultural engineering

With the development of high-performance computing technology, the number of scientific publications regarding computational modelling of applications with the Discrete Element Method/Modelling (DEM) approaches in agricultural engineering has risen in the past decades. Many granular materials, e.g. grains, fruits and soils in agricultural engineering are processed, and thus a better understanding of these granular media with DEM is of great significance in design and optimization of tools and process in agricultural engineering. In this review, the theory and background of DEM have been introduced. Some improved contact models discussed in the literature for accurately predicting the contact force between two interacting particles have been compared. Accurate approximation of irregular particle shapes is of great importance in DEM simulations to model real particles in agricultural engineering. New algorithms to approximate irregular particle shapes, e.g. overlapping multi-sphere approach, ellipsoid, etc. have been summarized. Some remarkable engineering applications of the improved numerical models developed and implemented in DEM are discussed. Finally, potential applications of DEM and some suggested further work are addressed in the last section of this review.

Modelling marine microplastics accumulation zones through freshwater discharge sources: Simulated release from the Onkaparinga River and Torrens River, South Australia

Plastic pollution is fast becoming one of the most prominent contamination issues facing the marine environment. Microplastics are a major subset of plastic waste now present in all global oceans, with their numbers standing only to increase. This study applies a coupled hydrodynamic model and Lagrangian particle-tracking model to predict and simulate microplastic transport in South Australian waters. Virtual particles representing microplastics were released daily for 365 days from two major freshwater input sources along the coastline of Adelaide, Australia. These particles entered the Gulf St Vincent and were tracked over two model years using LTRANS software. The model identified general gulf circulation as circular, clockwise, with net southward particle transport from particle release sites. A potential accumulation zone associated with a local eddy was identified. Concentrations of particles that passed through local marine parks were also calculated in response to the potential concerns they pose in vulnerable protected areas.

Study on the Erosion Characteristics of Non-spherical Particles in Liquid-solid Two-phase Flow

Elbow erosion, defined as wall thinning due to the continuous interactions between solid particles and surface, is a common phenomenon in catalyst addition/withdrawal pipeline systems used in residual oil hydrogenation units. This form of erosion can seriously affect the reliable pipeline operation. The present paper describes the construction of realistic cylindrical catalyst particles using the multi-sphere clump method and computational fluid dynamics/discrete element model simulations to study the erosion of pipe walls under different inlet velocities and particle aspect ratios. An optical shooting experiment is carried out to ensure the accuracy of the calculation method, and the model performance is compared using several existing drag models. The results show that the drag model of Haider & Levenspiel is more accurate than the others in revealing the actual cylindrical particle flow. A higher inlet velocity is observed to increase the kinetic energy of the particles and affect their spatial distribution. Specifically, when the Stokes number is greater than 113.7, the position of the maximum erosion rate shifts from the elbow’s outer wall to the inner wall. Cumulative contact energy is introduced to quantify two different types of particle-wall contacts. With a growing particle aspect ratio, the proportion of tangential energy gradually increases, which indicates that sliding is the main contact mode. The results presented in this paper provide a reference for engineering erosion calculations.

Multidimensional Separation by Magnetic Seeded Filtration: Experimental Studies

The current state of separation technology often neglects the multidimensional nature of real particle systems, which are distributed not only in terms of size, but also in terms of other properties, such as surface charge. Therefore, the aim of this study is to experimentally investigate the applicability of magnetic seeded filtration as a multidimensional separation process. Magnetic seed particles are added to a multisubstance suspension, and a selective heteroagglomeration with the nonmagnetic target particles is induced, allowing for an easy subsequent magnetic separation. The results show that high separation efficiencies can be achieved and that the parameters pH and ionic strength govern the agglomeration process. Selective separation based on surface charge was observed, but undesirable heteroagglomeration processes between the target particles lead to a loss of selectivity. Particle size was clearly identified as a second relevant separation feature, and its partially opposite influence on collision frequency and collision efficiency was discussed. Finally, experimental data of multidimensional separation are presented, in which a size-distributed two-substance suspension is separated into defined size and material fractions in a single process step. This study highlights the need for multidimensional evaluation in general and the potential of magnetic seeded filtration as a promising separation technique.

Quantum field theory of physical and purely virtual particles in a finite interval of time on a compact space manifold: diagrams, amplitudes and unitarity

We provide a diagrammatic formulation of perturbative quantum field theory in a finite interval of time τ, on a compact space manifold Ω. We explain how to compute the evolution operator U(tf, ti) between the initial time ti and the final time tf = ti + τ, study unitarity and renormalizability, and show how to include purely virtual particles, by rendering some physical particles (and all the ghosts, if present) purely virtual. The details about the restriction to finite τ and compact Ω are moved away from the internal sectors of the diagrams (apart from the discretization of the three-momenta), and coded into external sources. Unitarity is studied by means of the spectral optical identities, and the diagrammatic version of the identity U†(tf, ti)U(tf, ti) = 1. The dimensional regularization is extended to finite τ and compact Ω, and used to prove, under general assumptions, that renormalizability holds whenever it holds at τ = ∞, Ω = ℝ3. Purely virtual particles are introduced by removing the on-shell contributions of some physical particles, and the ghosts, from the core diagrams, and trivializing their initial and final conditions. The resulting evolution operator Uph(tf, ti) is unitary, but does not satisfy the more general identity Uph(t3, t2)Uph(t2, t1) = Uph(t3, t1). As a consequence, Uph(tf, ti) cannot be derived from a Hamiltonian in a standard way, in the presence of purely virtual particles.

Qubit picture of virtual particles

Qubit picture of virtual particles

An efficient three-dimensional numerical simulation of particle acoustic agglomeration with fine-grained parallelization on graphical processing unit

To overcome the challenge of large particle number concentration in the simulation of realistic particle acoustic agglomeration process, the implementation and validation of an efficient three-dimensional simulation method with a fine-grained Graphical Processing Unit (GPU) based parallel strategy is proposed. In detail, the motion of the simulated particles is solved with the Discrete Element Method (DEM) that includes three major particle acoustic agglomeration mechanisms, two particle collision processes and varying agglomerate porosity. Under the framework of the spatial decomposition method, the fine-grained parallel algorithm allocates the computation workload of each simulated particle one-by-one to one GPU thread. Speed test shows that the developed algorithm could achieve relatively elevated efficiencies and high speedup ratios. For method validation, the predicted particle agglomeration rate is compared with experimental measurements in the literature. The agreement of the results demonstrates that the developed method could reproduce realistic particle agglomeration rate as in the experiment.

Study and application of pollutant diffusion based on SPH method

In this paper, the meshless smooth particle hydrodynamics (SPH) method is used to simulate the transport process of pollutants in the environment by solving the convection-diffusion equation of pollutants under the Lagrangian framework. This method avoids various numerical problems caused by mesh distortion or mesh movement and the calculation of the derivative of the kernel function when approximating the second derivative kernel. In order to reduce the original virtual particle method of SPH to solve the boundary problem due to the large amount of particle searches will increase the amount of redundant calculation and other factors, selected with the finite difference method to solve the boundary. To verify the proposed Lagrangian particle model, numerical experiments were carried out. The one-dimensional calculation results are compared in detail with the results of different schemes in the finite difference method, and the L2, L∞ and relative errors are analyzed. The simulation results show that the SPH-based model requires fewer particles. In addition, the model is numerically analyzed under the condition of ensuring accuracy. The numerical solution is in good agreement with the analytical solution. The research results of this paper can provide a new solution and idea for environmental pollution early warning.

Propagators and widths of physical and purely virtual particles in a finite interval of time

We study the free and dressed propagators of physical and purely virtual particles in a finite interval of time τ and on a compact space manifold Ω, using coherent states. In the free-field limit, the propagators are described by the entire function (ez– 1 – z)/z2, whose shape on the real axis is similar to the one of a Breit-Wigner function, with an effective width around 1/τ. The real part is positive, in agreement with unitarity, and remains so after including the radiative corrections, which shift the function into the physical half plane. We investigate the effects of the restriction to finite τ on the problem of unstable particles vs resonances, and show that the muon observation emerges from the right physical process, differently from what happens at τ = ∞. We also study the case of purely virtual particles, and show that, if τ is small enough, there exists a situation where the geometric series of the self-energies is always convergent. The plots of the dressed propagators show testable differences: while physical particles are characterized by the usual, single peak, purely virtual particles are characterized by twin peaks.

Insights on using the boundary integral SPH formulations to calculate Laplacians with Dirichlet boundaries

Handling boundary conditions in Smooth Particle Hydrodynamics (SPH) has been known as a challenging problem for a long time, especially when particle approximation is implemented for truncated kernels on the boundary. This work develops a novel particle consistent gradient formulation to impose the Dirichlet boundary condition on a Laplacian with boundary integrals. Boundary integral formulation allows the accurate imposition of boundary conditions without creating virtual particles to extend boundaries, as used in conventional SPH modeling practices. Here, boundary integral formulations are presented for Laplacian operators and the imposition of Dirichlet boundary conditions on arbitrary boundary shapes. We also explore using this particle consistent first-order derivative twice to evaluate the entire Laplacian. Further, the convergence characteristics of different Laplacian formulations with boundary integrals are discussed. Finally, numerical experiments solving explicit thermal and fluid problems are presented to demonstrate the robustness of various Laplacian operators for practical use.

Numerical investigation of the three-dimensional paths of plastic polymers in the Gulf of Naples

The high-resolution Campania Regional Ocean Model (CROM), coupled with an online Lagrangian particle tracking algorithm, is used to investigate the horizontal and vertical behavior of different (in terms of size and density) plastic polymer types during February and August 2016 in the Gulf of Naples. The transport of passive particles is evaluated based on the three-dimensional Eulerian velocity fields provided by the ocean model. The virtual particles are released in several hot spot areas in the Gulf of Naples where most of the marine debris is supposed to come from. A sensitivity analysis on the vertical sinking for negatively buoyant particles is carried out. The sinking behavior is determined by the settling velocity, which depends on the physical properties of the individual litter item as well as on the hydrodynamical features of the marine environment. Different numerical experiments are carried out to evaluate the effect of marine dynamics on three-dimensional transport.

An improved continuum surface tension model in SPH for simulating free-surface flows and heat transfer problems

The continuum surface force (CSF) model in conjunction with the smoothed particle hydrodynamics (SPH) method is commonly used in the simulation of multi-phase flows with surface tension effect. However, it cannot be directly applied for modeling free-surface flows since the normalization requirement, which states that the integral of the surface delta function through the interface should be equal to one, is violated due to the particle deficiency issue. In this work, we propose an improved CSF model based on the SPH method with the aim of recovering the normalization requirement. The particle deficiency issue at the free surface is tackled by introducing the jump in color function into the calculation of the surface delta function. As a result, the accuracy of the surface tension measurement is significantly improved without using the virtual particles. Besides, this improved CSF model also allows an accurate and efficient implementation of the Robin boundary condition which is frequently encountered in heat transfer problems. Several benchmark problems are investigated to validate the performance of the proposed method. The numerical results verify that the improved CSF model in SPH method is well capable of simulating free-surface flows and heat transfer problems.

Estimate of three-dimensional Wadell roundness of irregular particles using image processing and topographic analysis

Wadell roundness is a general particle shape descriptor used in many fields, such as mining engineering, civil engineering, and geotechnical engineering. Initially, Wadell roundness was defined as two-dimensional, and now it has been generalized to three-dimensional. This study proposes a novel image-based method using topographic analysis to estimate the three-dimensional Wadell roundness of irregular particles. And an alternative strategy to estimate 3D Wadell roundness from other shape descriptors is further proposed. The effect of mesh density on 3D Wadell roundness is quantitatively analyzed. In addition, the relationships between 3D Wadell roundness and eleven shape descriptors, including 2D Wadell roundness and the other ten 1D, 2D, and 3D shape descriptors, are discussed. Finally, the computation time of the 3D Wadell roundness estimation algorithm proposed in this study is recorded to evaluate the algorithm's efficiency. The results show that the 3D Wadell roundness estimation algorithm is robust and can be used for idealized geometries and realistic particles of various shapes. The mesh density of particles significantly influences the 3D Wadell roundness. It proved difficult to predict the 3D Wadell roundness from the eleven shape descriptors mentioned in this study. The 3D Wadell roundness estimation algorithm can be finished in a few minutes, potentially using the algorithm in rock shape analysis and mechanics responses investigation.

Effect of artificial speed of sound in ACM, EDACM, and EMV for two-dimensional cavity flow

This study compared various numerical schemes that have been reported with the assumption that artificial compressibility aids in the realization of explicit fluid dynamics simulations for a two-dimensional cavity flow. Four artificial compressible schemes, the artificial compressible method (ACM), entropically dumped ACM (EDACM), explicit method with virtual particles (EMV), were compared with a conventional incompressible scheme (simplified marker and cell, SMAC). The purpose of the comparisons is clarifying the validity of EMV in which the artificial speed of sound can be theoretically determined. In addition, the effect of the artificial speed of sound was investigated for ACM and compared with EMV. Moreover, three conditions of Reynolds numbers, Re=100, 1000, and 5000, were employed. Under steady-state conditions, the velocity and pressure distributions were found to be consistent among the EMV, ACM, EDACM, and SMAC. Further, the comparisons of the horizontal and vertical profiles indicated that these artificial compressible schemes reproduced the velocities using the incompressible scheme at three Reynolds numbers. In contrast, the temporal development of the velocity fields in the three artificial compressible schemes clearly exhibited small fluctuations in the velocity and pressure around the overall trends determined by SMAC. Further, the power spectral densities demonstrated that such fluctuations were due to compression-wave propagations with the artificial speed of sound. Although no clear differences were observed among EMV, ACM, and EDACM at these Reynolds numbers, EMV was determined to be advantageous in terms of theoretically determining the unique value of the artificial speed of sound once the lattice-grid system was fixed, whereas ACM and EDACM require sensitivity studies to select suitable values.

Numerical Simulation of Selective Laser Melting of 304 L Stainless Steel

This paper used the smoothed particle hydrodynamics (SPH) method to construct a three-dimensional mathematical model of the selective laser melting (SLM) process of 304 L austenitic stainless steel. Important driving force models for the melt pool in the SLM process were developed, including a surface tension model, a boundary normal-modified wetting effect model, a Marangoni shear force model, and a recoil pressure model. Meanwhile, the virtual particle boundary method prevented particles from flying over the solid boundary. Artificial viscosity, artificial stress, and artificial heat were added to correct the SPH equation, which provided a guarantee for the accuracy and speed of the numerical simulation of the SLM process. Finally, the temperature field and velocity field in the SLM process were explored according to the constructed mathematical model. The evolution mechanism in the melting process was analyzed, and the influence of different laser powers on the shape of the molten pool was mainly analyzed, which provided a reference for optimizing the laser parameters to reduce the surface roughness of the formed specimen.