Particle Collisions Research Articles

Two-dimensional collisional particle model of the divertor sheath with electron emissive walls

A novel two-dimensional particle-in-cell (PIC) code, named Divertor Edge Simulator of Plasma-wall Interaction with Consistent COllisions (DESPICCO) and developed at CNR-ISTP, is capable of simulating the thin plasma layer of several millimeters, adjacent to the divertor tiles of a Tokamak fusion reactor. Here, kinetic effects and non-neutral plasma physics in the Debye sheath can be self-consistently captured by the PIC approach. The code is firstly benchmarked against literature one-dimensional codes and additional theoretical predictions for a magnetized sheath. Then, it is applied to a realistic divertor scenario featuring an attached plasma with monoblocks (MBs) radial misalignment and gaps, to compute the energy flux amplification factor at the exposed MB edge. A non-ambipolar local current density close to the leading edge and an average sheath heat transmission coefficient larger than the one predicted by classical sheath theory, are found. The effects of electron wall emission and plasma-gas collisions on the ion Mach number and on particle and energy fluxes to the walls are finally estimated to determine future guidelines for simulations. Ion collisions with recycled neutrals and both secondary and thermionic electron emission from the wall are found to have a relevant impact, with the overall effect of reducing by 25% the average ion impact energy, and by 15%–20% the total heavy particles energy flux to the walls, with relevant implications on the divertor wall erosion.

Comprehensive Euler/Lagrange modelling including particle erosion for confined gas-solid flows

The present research aims to assess the capability of a comprehensive Euler/Lagrange approach for predicting gas-solid flows and the associated solid particle erosion. The open-source code OpenFOAM® 4.1 was used to carry out the numerical simulations, where the standard Lagrangian libraries were substantially extended to account for all necessary models. Particles are tracked considering both translational and rotational motion as well as all relevant forces, such as gravity/buoyancy, drag and transverse lift due to shear and particle rotation. The tracking time step was dynamically adapted according to the locally relevant time scales, which drastically reduces computational times. Stochastic approaches are adopted to model particle turbulent dispersion, particle collisions with rough walls and particle-particle interactions. Five solid particle erosion models, available in the literature, were considered to estimate pipe bend erosion. Three study cases are provided to validate the adopted numerical approach and erosion models extensively. The first case intends to evaluate the ability of the extended CFD code to predict the behaviour of gas-solid flows in pneumatic conveying systems. This goal is achieved by comparing the numerical results with the experimental data obtained by Huber (1997) and Huber and Sommerfeld (1994, 1998) in a pneumatic conveying system. Here, the importance of considering inter-particle collisions and surface roughness for predicting particle velocity, mass flux and mean diameter distributions in gas-solid flows is highlighted. The second and third case intend to evaluate the ability of the erosion models in estimating bend erosion in diluted gas-solid flows. The erosion data obtained experimentally by Mazumder et al. (2008) and Solnordal et al. (2015) in very dilut pneumatic conveying systems is used for validating the numerical results, neglecting now inter-particle collisions and two-way coupling. Besides a comprehensive analysis of the different influential properties on erosion, the innovation of the present study is as follows. For the first time also a temporal modification of the surface roughness due to the erosion was considered in the simulations obtained from previous measurements (Novelletto Ricardo & Sommerfeld, 2020). As the surface roughness is increased due to erosion, eventually erosion rate becomes lower. This is the result of diminishing wall collision frequency. Simulations for several degrees of surface roughness showed that larger roughness is coupled with a drastic reduction of erosion. Hence, numerical simulations neglecting wall surface roughness are not realistic. The consideration of a particle size distribution instead of mono-sized computations showed a possible reduction of erosion rate. The detailed analysis of the different single-particle erosion models revealed that the model proposed by Oka et al. (2005) and Oka and Yoshida (2005) yields the best agreement with the measurements, however particle and wall properties are needed.

Mechanism of nanoparticle aggregation in gas-liquid microfluidic mixing

Using gas-liquid segmented micromixers to prepare nanoparticles that have a homogeneous particle size, controllable shape, and monodispersity advantages. Although nanoparticle aggregation within a microfluid has been shown to be affected by the shear effect, the shear effect triggering conditions in gas-liquid two-phase flow is unclear and the aggregation behavior of nanoparticles under the shear effect is difficult to predict, resulting in uncontrollable physical and chemical properties of nanoparticle aggregates. In this study, a numerical simulation of nanoparticle aggregation in gas-liquid two-phase flow under the shear effect is performed using the CFD-DEM method. Then, the effects of total flow rate, gas-liquid two-phase flow ratio, and particle volume fraction on particle aggregation were analyzed to achieve control of particle aggregation shape and size. Meanwhile, the triggering mechanism of the shear effect and the mechanism of the shear effect on the aggregation of nanoparticles were clarified. The results show that increasing the total flow rate or decreasing the gas-liquid two-phase flow rate ratio can induce the shear effect, which reduces the particle aggregation size and makes the morphology tend to be spherical. Moreover, increasing the particle volume fraction, and total flow rate or decreasing the gas-liquid two-phase flow rate ratio also increases the number of particle collisions and induce interparticle adhesion. Hence, particle adhesion and the shear effect compete with each other and together affect particle aggregation.

Structure and physicochemical properties of MgB2 nanosheets obtained via sonochemical liquid phase exfoliation

We have performed ultrasonic liquid phase exfoliation (LPE) of MgB2 in ethanol and investigated the resulting nano-assemblies as a function of the ultrasound processing time. TEM morphological and structural analysis, along with STM topographic characterization, showed that for short sonication times the exfoliated grains preserve the MgB2 crystal structure, have a 2D character and produce Moiré patterns corresponding to stacked layers with rotational misalignment. On the other hand, the longest process times result in spheroidal nanoparticles with diameters of the order of 10 nm, which can also coalesce into larger agglomerates. Optical absorbance spectra confirmed that the exfoliated material preserves a metallic nature with a predominant 2D character corresponding to the ab-plane, whereas Raman spectra showed the presence of extra-modes induced by 3D symmetry breaking in the exfoliated products, along with indications of some B-H stretching modes. The detection of a positive zeta potential confirms that an active surface hydrogenation process has taken place during sonication. The mechanochemistry of the exfoliation mechanism has been attributed to particle–particle collisions and particle-shockwave interactions originating from the implosive bubble-collapse. The present study provides important information useful for implementing the LPE process in different contexts for MgB2, like few-layer superconductivity, antibacterial coating and nano-drug preparation.

Optical properties of N2/Ar plasma generated by electron beams with silicon nitride transmission window at high pressures

In this paper, we report the generation of nitrogen/argon electron beam plasmas with a 0.4 μm-thick silicon nitride transmission window. The measured transmittance of the electron beam is about 92% when the incident energy of electron beam is 40 keV. The optical properties of the electron beam plasma are investigated at high pressures. The luminescence intensity distribution of nitrogen and argon electron beam plasmas shows a good similarity with changing the gas pressure, suggesting that electron beam ionization is independent of background gas. At the same gas pressure, the nitrogen plasma with conical structure has a longer length along the incident direction of the electron beam and a smaller divergence cone angle compared with the argon plasma. In the nitrogen discharge, it is found that the intensities I380.4, I399.7, and I391.4 corresponding to characteristic lines at 380.4, 399.7, and 391.4 nm decrease along the incident direction of the electron beam, which is correlated with the reduction of the electron density along the incident direction of the electron beam. There is no change in the ratio of I399.7 to I380.4, indicating the spatial uniformity of the electron temperature. When the pressure increases, the ionization process induced by primary electron is enhanced, generating more plasma electrons, which is responding to the increase in the characteristic line intensity. At the same time, the neutral particle collision processes are enhanced and the electron temperature decreases with the increase in the pressure, which is believed to contribute to the observed reduction of the I399.7/I380.4 ratio.

Establishment of theoretical models for beam structures with particle impact damper

Over the past few decades, research on impact dampers has grown significantly, leading to numerous analytical and experimental investigations in that field. Vibrations can harm industrial equipment and cause process errors. Particle impact damper has a significant impact on lowering vibration and has been broadly implemented in various engineering applications. This work deals with the theoretical modeling for cantilever beam and fixed-fixed beam attached with a cylindrical enclosure filled with particles. Transient and forced excitations are considered to trace the motion of the primary mass with respect to time. Also, theoretical modeling of particle arrangement in enclosure, condition for particles to detach from base or ceiling of enclosure, motion of particle after detachment from enclosure, and mechanism of collision and friction of particles are discussed and presented in detail. Theoretical models proposed in this study can be used to generate theoretical readings for practical engineering applications.

Insights into protective performance of CoCrNi medium entropy alloy coating subjected to supersonic micro-ballistic impact

Medium- and high-entropy alloys (MEAs and HEAs) coatings have attracted increasing attention owing to their exceptional mechanical properties and potential applications in structures protective from supersonic microparticle impact. However, there are no experimental insights into their protective performance. In this study, we fabricated high-quality amorphous CoCrNi MEA coatings with a thickness of 4.5 μm on the rigid and flexible substrates through ion beam-assisted deposition. Based on nanoindentation and laser-induced supersonic microparticle impact experiments, the mechanical properties and protective performance of the CoCrNi MEA coating were characterized and compared with those of the aluminum and copper coating. The results show that the CoCrNi MEA coating exhibits a high hardness of 9.7 GPa and an elastic modulus of 146.8 GPa. Under the supersonic impact, the CoCrNi MEA coating shows high coefficient of restitution, low rebound-to-fracture transition velocity, and small impact-mode ratios. The impact-mode ratios of the CoCrNi MEA coating subjected to microparticles of different materials further indicate that the CoCrNi MEA coating can efficiently withstand the high-velocity impact of most metallic particles except tungsten ones. Our work provides direct experimental insights into the excellent impact resistance of CoCrNi MEA coatings, which holds a great promise for improving the reliability and durability of equipment subjected to high-speed collisions of solid particles entrained within the air such as ice and sand dust.

A novel particle–particle and particle–wall collision model for superellipsoidal particles

AbstractIn the framework of computational studies of particulate multiphase flow systems, either dilute or dense, particle–particle as well as particle–wall collisions need to be considered, which in the case of nonspherical particle shapes still presents a computational challenge. In this study, we present an efficient numerical implementation of a novel superellipsoidal particle collision model that can be used in general fluid flows. The superellipsoid shape formulation can be viewed as an extension of spherical or ellipsoidal shapes and can be used to represent spherical, ellipsoidal, cylindrical, diamond-like and cubic particles by varying solely five shape parameters. In this context, we present a fast, stable Newton–Raphson-based method for modeling frictional collisions of nonspherical superellipsoidal particles, and demonstrate the performance of our algorithms.

Simulation and verification of particle flow with an elastic collision by the immersed edge-based smoothed finite element method

The immersed edge-based smoothed finite element method (IES-FEM) is proposed for the study of elastic collision particulate flow. Particle collision becomes more realistic by using the penalty function and the hyperelastic constitutive model. The effects of grid resolution and Reynolds numbers on particle terminal velocity and drag coefficient are discussed to verify the calculation accuracy and stability. Single-particle collisions with the bottom and side walls are analyzed and experimentally verified. Results show that the calculation error of IES-FEM is less than 0.6% when the fluid grid size is 0.5 times the particle mesh size and the time step is 10–4 s. Particle drag coefficient and flow characteristics agree well with the published models and experiment results. To demonstrate the capabilities of IES-FEM in complex elastic particle systems, the collision and rebound of multiple particles are determined, including the drafting–kissing–tumbling of two circular particles; the chase, collision, and deformation of rectangular particles; and the repeated formation and separation of particle clusters. This work extends the application of IES-FEM in particle-resolved direct numerical simulation methods, which will provide an optional tool for future elastic blood cell flow and collision.

Continuous sonochemical nanotransformation of lignin – Process design and control

As the most abundant renewable aromatic polymer on the planet, lignin is gaining growing interest in replacing petroleum-based chemicals and products. However, only <5 % of industrial lignin waste is revalorized in its macromolecular form as additives, stabilizing agents or dispersant and surfactants. Herein, revalorization of this biomass was achieved by implementing an environmentally-friendly continuous sonochemical nanotransformation to obtain highly concentrated lignin nanoparticles (LigNPs) dispersions for added-value material applications. With the aim to further model and control a large-scale ultrasound-assisted lignin nanotransformation, a two-level factorial design of experiment (DoE) was implemented varying the ultrasound (US) amplitude, flow rate, and lignin concentration. Size and polydispersity measurements together with the UV–Vis spectra of lignin recorded at different time intervals of sonication allowed to monitor and understand the sonochemical process on a molecular level. The light scattering profile of sonicated lignin dispersions showed a significant particle size reduction in the first 20 min, followed by moderate particle size decrease below 700 nm until the end of the 2 h process. The response surface analysis (RSA) of the particle size data revealed that the lignin concentration and sonication time were the most important factors to achieve smaller NPs. From a mechanistic point of view, a strong impact of the particle–particle collisions due to sonication seems to be responsible for the decrease in particle size and homogenization of the particle distribution. Unexpectedly, a strong interaction between the flow rate and US amplitude on the particle size and nanotransformation efficiency was observed, yielding smaller LigNPs at high amplitude and low flow rate or vice versa. The data derived from the DoE were used to model and predict the size and polydispersity of the sonicated lignin. Furthermore, the use of the NPs spectral process trajectories calculated from the UV–Vis spectra showed similar RSA model as the dynamic light scattering (DLS) data and will potentially allow the in-line monitoring of the nanotransformation process.

Charged particles and Penrose process near charged black holes in Einstein–Maxwell-scalar theory

We study the dynamics of charged test particles around an electrically charged black hole in Einstein–Maxwell-scalar (EMS) gravity. The event horizon properties of the spacetime around the black hole are explored and the upper limit for the EMS theory parameters corresponding to extreme charge and minimal value of the event horizon are found. The effective potential for the radial motion of the charged particles at the equatorial plane is investigated. Specific energy and angular momentum of the particles corresponding to circular stable orbits are also studied. We also investigate the effects of the EMS parameter and the black hole charge on innermost stable circular orbits (ISCOs). We also investigate synchrotron radiation of charged particles in the spacetime of the charged black hole in EMS gravity. We also explore electric Penrose and Bañados–Silk–West processes near the black hole horizon, where we analyse in detail the effects of EMS parameters on energy efficiency in the Penrose process and critical angular momentum that allows colliding particles near the horizon, together with the center of mass energy in charged particles collisions.

Microquasar Cyg X-3 – a unique jet-wind neutrino factory?

ABSTRACT The origin of astrophysical neutrinos is one of the most debated topics today. Perhaps the most robust evidence of neutrino counterpart comes from supermassive black holes in active galactic nuclei associated with strongly collimated outflows, or jets, that can accelerate particles to relativistic energies and produce neutrinos through hadronic interactions. Similar outflows can also be found from X-ray binaries, or ‘microquasars’, that consist of a neutron star or a stellar-mass black hole accreting matter from a non-degenerate companion star. In some cases, these systems can accelerate particles up to GeV energies implying an efficient acceleration mechanism in their jets. Neutrino production in microquasar jets can be expected with suitable conditions and a hadronic particle population. Microquasar Cyg X-3 is a unique, short orbital period X-ray binary hosting a Wolf–Rayet companion star with a strong stellar wind. The interaction of the dense stellar wind with a relativistic jet leads to particle collisions followed by high-energy gamma-ray and potentially neutrino emission. Here, using the 10-yr neutrino candidate sample of the IceCube neutrino observatory, we find that the events with the highest spatial association with Cyg X-3 occur during short-lived high-energy gamma-ray flaring periods indicating the possible astrophysical nature of these events.

Microgel Particle Collision with Smooth and Nanofiber Hydrophobic Surfaces during Three-Dimensional Printing of a Biopolymer Layer.

The impact of microgel particles onto a wall represents an elementary process that determines the one-stage production of a biopolymer layer on a nanofiber scaffold in the framework of tissue bioengineering. The formation of a microgel layer is experimentally examined on a hydrophobic uniform surface and a nonwoven polymer membrane made of vinylidene fluoride-tetrafluoroethylene copolymer. In-air microfluidics methods, namely, an external vibration disturbance on the microflow of a cross-linkable biopolymer, make it possible to form the microstructures of "beads-on-thread" with a uniform distance between microgel particles of identical size (340-480 μm, depending on the sample). The successive particle-surface and particle-particle collisions are explored to develop the concept of technology for depositing microgel particles on surfaces for mobile one-stage production of microgel layers with a thickness of one and two particles, respectively. A physical model of successive particle-surface and particle-particle interactions is proposed. Empirical expressions are derived for predicting the diameters of maximum spreading (deformation) and the minimum heights of microgel particles on smooth and nanofiber surfaces, as well as in particle-particle collisions using a dimensionless criterion of gelation degree. The effect of microgel viscosity and fluidity on the maximum particle spreading during successive particle-surface and particle-particle collisions is elucidated. The consistent findings have made it possible to develop a predictive method for determining the growth dynamics of microgel layer area with a thickness of one or two particles on a nanofiber scaffold within a few seconds. The specific behavior of a microgel with a given gelation degree is simulated to produce a layer.

Effect of polarity reversal on floc formation and rheological properties of a sludge formed by the electrocoagulation process

Anode fouling is one of the key limiting factors to the widespread application of electrocoagulation (EC) for treatment of different types of contaminated water. Promising mitigation strategy to fouling is to operate the process under polarity reversal (PR) instead of direct current (DC). However, the PR operation comes at the cost of process complexity due to the alternation of electrochemical and chemical reactions. In this study, we systematically investigated the link between evolving fouling layer during DC and PR close to iron and aluminum electrodes and morphological and rheological properties of the formed sludge. By operando visualization of EC process, we demonstrate that during PR operation, precipitation of the iron and aluminum species occurs close to the anode interface, resulting in flocs with higher porosity and lower density than those formed under DC conditions. However, rheological investigation revealed that the PR conditions resulted in a sludge with more pronounced solid-like signature, but this enhancement in its viscoelastic properties is closely related to a period of the current's polarity reversal. We attribute this unexpected result to higher shear rate and collision of particles during PR conditions.

Theoretical derivation of particle collision kernels and its enhancement at a high concentration from a first-time-passage approach in the diffusive regime

A theoretical explanation for obtaining collision kernels from the first-time passage approach proposed by [Gopalakrishnan, R., & Hogan Jr, C. J. (2011). Aerosol Science and Technology, 45(12), 1499-1509] is developed in this work. This new approach leads to an interesting correction for particle coagulation at high concentrations and enlights the limitations of classical derivations by [Fuchs, N. A. (1964). Dover Publications] and [Friedlander, S. K. (2000). (Vol. 198). New York: Oxford university press] where the transient part of the diffusive collision kernel is typically neglected.

A universal mechanism for the development of self-ordering processes in systems of identical particles

Based on the hypothesis of the existence of wave long-range action and its role in the development of self-ordering processes, expressed by us in previous works, we carried out a methodological analysis of the applicability of wave representations in systems of identical particles. The main attention in the analysis is paid to the practical application of de Broglie waves in systems of interacting particles. For this purpose, Bohr's theory of the hydrogen atom has been revised and inconsistencies that contradict modern ideas have been corrected. Two conclusions are made: wave representations of particles are of a material nature; the de Broglie wavelength should be determined in terms of the relative momentum of the interacting particles . Based on the materiality of wave representations, the features of the long-wave interaction of particles are determined. It is emphasized that in many manifestations this interaction has a resonant character. This interaction is the foundation for the universal mechanism for the development of self-ordering processes in systems of identical particles. The paper presents an algorithm for the emergence and functioning of a universal mechanism. The condition for the emergence is always any withdrawal of the system from the state of isotropic chaos and the formation of an initial subgroup of particles with the same magnitude and direction of momenta. The most common cause leading to the fulfillment of the condition is monoenergization of the spectrum of particles in the system. The causative agent of the mechanism is direct collisions of particles, in which the probability of long-wavelength representations is the highest. The development of the mechanism is limited only by the fulfillment of the percolation condition - overcoming the percolation threshold. The final stage of the mechanism is the formation of a superfluid component - a kind of macroparticle that does not interact with walls and other molecules. A characteristic wave feature of such a macroparticle is its coherence. Several final stages in the development of self-ordering processes for typical special phenomena in systems of identical particles are analyzed.

A multiple-time-step integration algorithm for particle-resolved simulation with physical collision time

In this paper, we present a multiple-time-step integration algorithm (MTSA) for particle collisions in particle-resolved simulations. Since the time step required for resolving a collision process is much smaller than that for a fluid flow, the computational cost of the traditional soft-sphere model by reducing the time step is quite high in particle-resolved simulations. In one state-of-the-art methodology, collision time is stretched to several times the flow solver time step for the fluid to adapt to the sudden change in particle motion. However, the stretched collision time is not physical, the hydrodynamic force may be severely underestimated during a stretched collision, and the simulation of sediment transport may be sensitive to the stretched collision time. The proposed MTSA adopts different time steps to resolve fluid flow, fluid-particle interaction, and particle collision. We assessed the MTSA for particle-wall collisions as well as particle-particle collisions, determined the optimal iteration number in the algorithm, and obtained excellent agreements with experimental measurements and reference simulations. The computational cost of the MTSA can be reduced to about one order of magnitude less than that using the traditional soft-sphere model with almost the same accuracy. The MTSA was then implemented in a particle-resolved simulation of sediment transport with thousands of particles. {By comparing the results obtained using the MTSA and a version of the stretching collision time algorithm similar to Costa et al.(2015), we found that stretching the collision time reduced particle stiffness, weakened particle entrainment, and affected some turbulence and particle statistics.

New Insights into Variations in Enceladus Plume Particle Launch Velocities from Cassini-VIMS Spectral Data

Enceladus’s plume consists mainly of a mixture of water vapor and solid ice particles that may originate from a subsurface ocean. The physical processes underlying Enceladus’s plume particle dynamics are still being debated, and quantifying the particles’ size distribution and launch velocities can help constrain these processes. Cassini’s Visual and Infrared Mapping Spectrometer observed the Enceladus plume over a wavelength range of 0.9–5.0 μm for a significant fraction of Enceladus’s orbital period on three dates in the summer of 2017. We find that the relative brightness of the plume on these different dates varies with wavelength, implying that the particle size distribution in the plume changes over time. These observations also enable us to study how the particles’ launch velocities vary with time and observed wavelength. We find that the typical launch velocity of particles remains between 140 and 148 m s−1 at wavelengths between 1.2 and 3.7 μm. This may not be consistent with prior models where particles are only accelerated by interactions with the vent walls and gas and could imply that mutual particle collisions close to the vent are more important than previously recognized.

Investigating the effects of initial concentration and population distribution on the transport of aggregating nanoparticles in porous media

In order to investigate the migration of nanoparticles in porous media, the model developed by Katzourakis and Chrysikopoulos (2021) is applied to simulate the transport of aggregating nanoparticles under various initial conditions. In the aforementioned model, nanoparticles may collide with each other and form larger particle structures with different mobility and reactivity characteristics. Individual particles as well as aggregates can be found suspended in aqueous phase or attached, reversibly and/or irreversibly, on the solid matrix. The aggregation process modelled after the Smoluchowski population balanced equation (PBE), is coupled with the conventional advection-dispersion-attachment (ADA) equation to form a system of coupled equations that govern the transport of aggregating nanoparticles. Particle collisions are expected to increase exponentially with increasing initial number of injected particles (N0). Therefore, substantially pronounced aggregation is expected when N0 is increased. Similarly, the initial particle diameter distribution of the injected particles is expected to affect the average size of aggregates and in turn influence their mobility in a porous medium. Several model simulations were performed with different N0 and particle diameter distributions. The results indicated the strong importance of taking into account the initial particle concentration and realistic particle diameter population distribution into consideration.

Investigation of carbon steel particle collisions in dry and wet environments

Investigation of carbon steel particle collisions in dry and wet environments