Ellipsoidal Particles Research Articles

Multi-level DEM study on silo discharge behaviors of non-spherical particles

The silo discharge of non-spherical particles has been widely practiced in engineering processes, yet the understanding of multi-level mechanisms during solid transportation is still lacking. In this study, a high-fidelity super-ellipsoid Discrete Element Method (DEM) model is established to investigate the discharge behaviors of non-spherical particles with different size distributions. After the comprehensive model validations, we investigated the effects of particle shape (aspect ratio and particle sharpness) on the particle level discharge behaviors. The discharge rates of the ellipsoid particles used in the current work are larger than the spherical particles due to the larger solid fraction. The discharge rates of the cuboid-like particles are determined by the combined effect of the solid fraction and the contact force. Parcel level data show that the translational movements of the ellipsoid particles are more ordered, which is supported by the global level data. Strong correlations exist between the particle level and parcel level data, especially the ellipsoid particles and the large particles in the polydispersed cases.

Metaball-Imaging discrete element lattice Boltzmann method for fluid–particle system of complex morphologies with case studies

Fluid–particle systems are highly sensitive to particle morphologies. While many attempts have been made on shape descriptors and coupling schemes, how to simulate particle–particle and particle–fluid interactions with a balance between accuracy and efficiency is still a challenge, especially when complex-shaped particles are considered. This study presents a Metaball-Imaging (MI) based Discrete Element Lattice Boltzmann Method (DELBM) for fluid simulations with irregular shaped particles. The major innovation is the MI algorithm to capture the real grain shape for DELBM simulations, where the Metaball function is utilized as the mathematical representation due to its versatile and efficient expressiveness of complex shapes. The contact detection is tackled robustly by gradient calculation of the closest point with a Newton–Raphson based scheme. The coupling with LBM is accomplished by a classic sharp-interface scheme. As for refiling, a local refiling algorithm based on the bounce back rule is implemented. Validations on the Jeffery orbit of ellipsoidal particles and three settling experiments of irregular-shaped natural cobblestones indicate the proposed model to be effective and powerful in probing micromechanics of irregular-shaped granular media immersed in fluid systems. The potential of this model on studies of shape-induced physical processes is further investigated with numerical examples that consider the drag and lift forces experienced by realistic particles, as well as the “drafting, kissing and tumbling” process of pairs of non-spherical particles.

A Mori–Zwanzig Dissipative Particle Dynamics Approach for Anisotropic Coarse Grained Molecular Dynamics

Coarse grained (CG) molecular dynamics simulations are widely used to accelerate atomistic simulations but generally lack a formalism to preserve the dynamics of the system. For spherical particles, the Mori-Zwanzig approach, while computationally complex, has ameliorated this problem. Here we present an anisotropic dissipative particle dynamics (ADPD) model as an extension of this approach, which accounts for the anisotropy for both conservative and nonconservative interactions. For a simple anisotropic system we parametrize the coarse grained force field representing ellipsoidal CG particles from the full-atomistic simulation. To represent the anisotropy of the system, both the conservative and dissipative terms are approximated using the Gay-Berne (GB) functional forms via a force-matching approach. We compare our model with other CG models and demonstrate that it yields better results in both static and dynamical properties. The inclusion of the anisotropic nonconservative force preserves the microscopic dynamical details, and hence the dynamical properties, such as diffusivity, can be better reproduced by the aspherical model. By generalizing the isotropic DPD model, this framework is effective and promising for the development of the CG model for polymers, macromolecules, and biological systems.

Effective thermal conductivity simulations of suspensions containing non-spherical particles in shear flow

Suspensions of oblate spheroid particles can have different thermal characteristics from suspensions of spherical particles based on its orthotropic geometry. However, the effective thermal conductivity of non-spherical particle suspensions under shear flow conditions has not been reported yet. In a first step dilute suspensions are considered ignoring particle-particle interactions, and a representative volume is observed containing one single oblate spheroid rotating at the center coupled to the shear flow. The fluid-side convective heat and mass transfer is computed by the Lattice Boltzmann Method (LBM). And the Discrete Element Method (DEM) accounts for particle dynamics, while the Finite Element Method (FEM) for heat conduction inside particles. Three codes are coupled together. As predicted by Maxwell’s model and the model of Nan et al., the effective thermal conductivity of a water suspension would saturate if the conductivity of the particle exceeds 20∼30W/(mK) at stagnant state, because the thermal resistance of the particles becomes negligible in comparison to the fluid thermal resistance. The simulation results in this study confirm such saturation also to a shear flow. Besides, another saturation mechanism exists for spherical particles if the shear rate is increased to a point where particle rotation induces convective transport that supersedes conductive transport within the particle. However, this is not the case for the oblate ellipsoidal particles due to different rotating states. And comparing different rotating states, if the particle’s thermal conductivity is higher than the base fluid, the shear-induced rotation around the oblate minor axis (log-rolling) results in a higher effective thermal conductivity than around the oblate major axis (tumbling) or the rotation of a spherical particle of equivalent volume fraction. The results provide a design guideline for particle in suspensions with better heat transfer behaviors.

The Influence of Different Hematite (α-Fe2O3) Particles on the Thermal, Optical, Mechanical, and Barrier Properties of LDPE/Hematite Composites.

There is an increasing need to develop new polymer composites with improved properties compared to conventional pure polymer materials. This work aims to develop composites of low-density polyethylene (LDPE) and iron oxide hematite particles. For this purpose, different types of hematite particles with well-defined shapes and narrow size distributions were synthesized: HC2 sample with pseudocubic hematite particles of an average diameter of 1020 nm, HE1 sample with ellipsoidal hematite particles of an average diameter of 533 nm, and HS1 sample with spherical hematite particles of an average diameter of 168 nm. The mass fractions of hematite in the composites were 0.25%, 0.5%, and 1%. Prepared LDPE/hematite composites were characterized by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and diffuse reflectance ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. The mechanical and barrier properties were also studied. The obtained results showed that all prepared composites have improved properties compared to the pure LDPE, especially the composites with pseudocubic hematite particles of well-defined shapes. The results of this study indicate that LDPE/hematite composites can be promising materials for a wide range of applications, especially as packaging materials where improved thermal and mechanical properties as well as resistance to ultraviolet (UV) irradiation are required.

Self Assembly of Patchy Anisotropic Particle Forming Free Standing Monolayer Film

AbstractSynthesizing thin films using self assembly is one of the important challenges which needs to be addressed for making engineered material using colloidal particle, necessary for creating novel material for photonic, filtration application, etc. Here, a simple model of prolate ellipsoidal particle with two directional bonds is presented and the phase behavior is studied using NVT and NpT Monte Carlo simulation. It is observed that for anisotropic particles with patches two different type of 3D ordered structures are formed along with thermodynamically stable monolayer which is confirmed using thermodynamic integration. The phase diagram shows that the desired 2D super structure can be created using simple self assembly route.

(Cd,Mn)S in the Composite Photocatalyst: Zinc Blende and Wurtzite Particles or Integrowth of These Two Modifications?

In this study, the crystalline structure and particle shape of Cd1-xMnxS (x~0.3) in the composite photocatalysts prepared by hydrothermal synthesis at different temperatures (T = 80, 100, 120, and 140 °C) were analyzed. Along with mixed Cd-Mn sulfide, the catalysts contain a small amount of β-Mn3O4. XRD patterns of (Cd,Mn)S have features inherent to both cubic zinc blende and hexagonal wurtzite structure. Moreover, XRD peaks are anisotropically broadened. First, the heterogeneous (or two-phased) model was considered by the commonly used Rietveld method. Phase ratio, average crystallite sizes, and strains for both phases were formally determined. However, it was shown that this model is not correct because relatively narrow and broad peaks cannot be fitted simultaneously. Then, the homogeneous model was tested by Debye Function Analysis. This model assumes that particles are statistically homogeneous, but each particle contains lamellar intergrowth of zinc blende and wurtzite modifications. The probability of stacking faults, as well as the average radii of spherical and ellipsoidal particles, were varied. It was shown that nanocrystalline Cd0.7Mn0.3S particles have an ellipsoidal shape. Ellipsoids are elongated along the direction normal to the plane of defects. An increase in the hydrothermal synthesis temperature from 80 °C to 140 °C leads to an enlargement of particles and a gradual decrease in the probability of stacking faults in the wurtzite structure from 0.47 to 0.36. Therefore, with increasing temperature, the structure of (Cd,Mn)S nanoparticles transforms from almost random polytype cubic/hexagonal (ZB:WZ = 47:53) to a preferably hexagonal structure (ZB:WZ = 36:64). Mn2+ ions facilitate CdS phase transformation from zinc blende to wurtzite structure. There is no direct correlation between the structure and photocatalytic activity.

Multiple scattering transmission characteristic of polarized light in ellipsoidal fine particles

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

Dynamics of prolate spheroids in the vicinity of an air-water interface.

In this article, we present the mobilities of prolate ellipsoidal micrometric particles close to an air-water interface measured by dual wave reflection interference microscopy. Particle's position and orientation with respect to the interface are simultaneously measured as a function of time. From the measured mean square displacement, five particle mobilities (3 translational and 2 rotational) and two translational-rotational cross-correlations are extracted. The fluid dynamics governing equations are solved by the finite element method to numerically evaluate the same mobilities, imposing either slip and no-slip boundary conditions to the flow at the air-water interface. The comparison between experiments and simulations reveals an agreement with no-slip boundary conditions prediction for the translation normal to the interface and the out-of-plane rotation, and with slip ones for parallel translations and in-plane rotation. We rationalize these evidences in the framework of surface incompressibility at the interface.

A tensorial form to describe the rheological properties of a rigid triaxial ellipsoid suspended in a Newtonian fluid

In 1922, Jeffery derived the motion of a triaxial–ellipsoidal particle immersed in Stokes flow as well as its associated stress tensor, which laid the foundation to describe the rheological behavior of non-spherical suspensions and has been used in many applications. In this work, we propose an original derivation of Jeffery equations for the particle motion by discretizing the rigid body with isovolume elements. The conservation of linear and angular momentum leads to introduction of a conformation tensor that describes the microscopic structure at each point in the Newtonian fluid. A structural-continuum approach is then followed to derive a material constitutive equation for a triaxial ellipsoid suspended in a Newtonian fluid based on Jeffery’s results. Some representative rotational motions of a triaxial ellipsoid in shear flow are reported and it is shown that the particle exhibits stable and periodic rotations depending on its initial orientation. Some details are also discussed on how to consider a suspension made of triaxial ellipsoids and on the extension of the approach for particles with arbitrary shapes.

Faster and More Accurate Geometrical-Optics Optical Force Calculation Using Neural Networks.

Optical forces are often calculated by discretizing the trapping light beam into a set of rays and using geometrical optics to compute the exchange of momentum. However, the number of rays sets a trade-off between calculation speed and accuracy. Here, we show that using neural networks permits overcoming this limitation, obtaining not only faster but also more accurate simulations. We demonstrate this using an optically trapped spherical particle for which we obtain an analytical solution to use as ground truth. Then, we take advantage of the acceleration provided by neural networks to study the dynamics of ellipsoidal particles in a double trap, which would be computationally impossible otherwise.

SYNTHESIS AND PHOTOLUMINESCENCE PROPERTIES OF ELLIPSOIDAL La2O2SO4:Eu3+ PHOSPHORS

This research was focused on the homogeneous precipitation synthesis of a series of Eu3+ ion-activated ellipsoidal La2O2SO4 phosphors based on the La2(SO4)3-CO(NH2)2 reaction system. The structural identification, thermal analysis, morphology and luminescence properties of the as-prepared products were characterized with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry (TG/DSC), field emission scanning electron microscopy (FESEM) and photoluminescence (PL) spectra. Pure La2O2SO4 ellipsoidal particles with a long radius of about 2 µm and a short radius of about 1 µm were successfully prepared by calcining a rhombus-like precursor at 800 °C for 2 h in air. The formation mechanism of the precursor and its corresponding calcination product were also proposed. Photoluminescence results revealed that the strongest red emission peak was centered at 619 nm upon 249-nm ultraviolet (UV) light excitation in La2O2SO4:x%Eu3+ (x = 3, 6, 9, 12 and 15) phosphors. The exchange interaction was responsible for the concentration quenching mechanism of the 5D0→7F2 transition of Eu3+ ions in the La2O2SO4 host lattice. The optimal x value was 15 and the corresponding decay process showed a single exponential decay behavior whose lifetime t and color correlation temperature (CCT) were calculated to be 2.112 ms and 2752 K, respectively.

An Investigation of Instability on Constant Shear Drained (CSD) Path under the CSSM Framework: A DEM Study

Soil liquefaction or instability, one of the most catastrophic phenomena, has attracted significant research attention in recent years. The main cause of soil liquefaction or instability is the reduction in the effective stress in the soil due to the build-up of pore water pressure. Such a phenomenon has often been thought to be related to the undrained shearing of saturated or nearly saturated sandy soils. Notwithstanding, many researchers also reported soil instability under a drained condition due to the reduction in lateral stress. This condition is often referred to as the constant shear drained (CSD) condition, and it is not uncommon in nature, especially in a soil slope. Even though several catastrophic dam failures have been attributed to CSD failure, the failure mechanisms in CSD conditions are not well understood, e.g., how the volumetric strain or effective stress changes at the triggering of flow deformation. Researchers often consider the soil fabric to be one of the contributors to soil behaviour and use this parameter to explain the failure mechanism of soil. However, the soil fabric is difficult to measure in conventional laboratory tests. Due to that reason, a numerical approach capable of capturing the soil fabric, the discrete element method (DEM), is used to investigate the CSD shearing mechanism. A series of simulations on 3D assemblies of ellipsoid particles was conducted. The DEM specimens exhibited instability behaviour when the effective stress paths nearly reached the critical state line. It can be clearly observed that the axial and volumetric strains changed suddenly when the stress states were close to the critical state line. Alongside these micromechanical observations, the study also presents deeper insights into soil behaviour by relating the macro-observations to the micromechanical aspect of the soil.

Numerical simulations on the settling dynamics of an ellipsoidal particle in a viscoelastic fluid

The settling dynamics of a rigid ellipsoid in an unbounded viscoelastic fluid under inertialess conditions is studied through direct numerical simulations. The governing equations are solved by the finite element method with an Arbitrary Lagrangian–Eulerian formulation to handle the particle motion. The viscoelastic fluid is modeled through the Giesekus constitutive equation. Simulations are carried out up to a value of Deborah number of 5. The settling of prolate and oblate spheroidal particles is first addressed. The sedimentation, lift, and angular velocities are computed as a function of the orientation angle and for aspect ratios from 1/8 to 8. Regardless of the particle shape, initial orientation, and Deborah number, the particle rotates to align its longest axis along the force direction. A high extensional stress region behind the particle is observed at high aspect ratios due to the large curvature of the tip, leading to a fast decay of the axial fluid velocity downstream and to the appearance of a negative wake. Similarly, a triaxial ellipsoid reaches a final orientation with major axis parallel to the falling direction. The particle shape affects the orientational dynamics, both in terms of the orbits followed by the orientation vectors and the time needed to reach the equilibrium orientation, and the steady-state settling velocity. The fastest sedimentation rate is observed for a prolate spheroid with aspect ratio of about 2 whereas the slowest one for a high aspect ratio oblate spheroid. Triaxial ellipsoids settle with rates in between these two limiting behaviors.

Lipid nanoparticles with erythrocyte cell-membrane proteins

When the separated erythrocyte membranes (known as ghosts) are ultrasonicated, a significant part of the membrane proteins are released in the aqueous solvent, instead of being incorporated into the membranes of the formed nanoerythrosomes. In contrast to their membrane-bound counterparts, where helices and β-strands dominate, the released proteins show perturbed secondary structures with an increased ratio of helices, presumably participating in molten globules, as it has been revealed by circular dichroism (CD) and infra-red spectroscopy (IR). The shape and size of these proteins is diverse, and even their aggregates appear. When excess lipid (palmitoyl-lysophosphatidylcholine, LPC) is added to different ghost-derivatives (the full nanoerythrosome system, and its ultracentrifugation pellet and supernatant) in 2 × and 5 × lipid-to-protein mass ratio, various lipid nanoparticles are produced. The core–shell and nanodisc structural models obtained by small-angle X-ray scattering (SAXS) indicate that the choice of the precursor system has a more prominent effect on the resulting shape than the amount of lipid added: when starting from the protein-rich supernatant fraction, small (approx. 5 nm high and 7 nm wide) nanodisks are created. When lipid membranes are already present (in the pellet and the full nanoerythrosome fraction), similar LPC addition results in prolate ellipsoidal particles, with an aspect ratio between 3 and 5, and decreasing overall size when the amount of added lipid is increased. The ellipsoids formed from the total nanoerythrosome fraction are smaller than those from the ultracentrifugation pellet (longest axis around 15 vs 26 nm), whereas for higher LPC-to-protein ratio, the size in both cases reduce to nearly the same (13–14 nm) in both cases.

Multifield Coupling of Local Surface Plasmon Resonance‐Assisted Au–TiO2 Photocatalysis Considering Bimodal Resonance and Superposition Effect

This study concerns the improvement of the synergetic catalytic efficiency of Au–TiO2 nanorods considering the local surface plasmon resonance (LSPR) effect of different Au nanoparticles (NPs). The absorption spectrum and selective absorption efficiency for visible light, the local electric field, and the generated thermal effect are specifically analyzed based on the multifield decoupling of LSPR‐assisted Au–TiO2 catalysts. The simulation results show that the extinction spectra of both spherical and ellipsoidal Au particles are consistent with the experimental data. Interestingly, the latter is characterized by significant bimodal resonance modes. Comparatively, the simulation results show that the longitudinal mode, which is sensitive to the aspect ratio, is more favorable for the improvement of the photocatalytic activities. It is found that the resonance peaks are highly controllable, and are linear to the particle size and aspect ratio. Meanwhile, the electric field mode of TiO2 is significantly increased under the resonance wavelength. It is worth mentioning that the superposition effect makes a non‐negligible impact on the actual catalysts, leading to a relative shift of resonance wavelength. The consideration of the hot spots caused by the superposition effect influence the photocatalytic results significantly, providing values in diminishing the inadaptability of the theory in near‐touching regions of plasma particles.

Drag force acting on ellipsoidal particles with different shape characteristics

Non-spherical particles and their interaction with the surrounding fluids are omnipresent in nature and industry. While moving through fluids, particles are subjected to drag forces which is key in understating their dynamic behaviour and is highly dependent on their shape. In this research, to investigate the drag force acting on ellipsoidal particles, numerical simulations and twelve drag models are utilised to predict the particles' drag coefficients. The results are compared in a unified framework using Zingg charts. Most models compare well with the simulation results in both drag coefficient values and trends. The maximum error of these models ranges from 40% to 150%. Some models such as Haider and Levenspiel, Ganser, and Leith are capable in estimating the ellipsoids' drag coefficient with high accuracy while others either overestimate or underestimate the values. Each drag model is suitable for particles in a specific shape category including compact, bladed, elongated, or flat. • The drag coefficient of ellipsoids with various shape characteristics was studied. • The drag coefficient of CFD was compared to data of 12 known empirical drag models. • Each model is ideal for specific particle shapes: compact, bladed, elongated, flat. • The models by Haider and Ganser showed the highest accuracy for all the ellipsoids.

Decomposition and phase transformation mechanisms of α2 lamellae in β-solidified γ-TiAl alloys

The decomposition of (α2/γ) lamellae during service is an important microstructure degradation phenomenon that is worth studying. In this study, the decomposition mechanism of (α2/γ) lamellae was systematically investigated by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. The results showed facilitated α2 →βo phase transformation by [112¯0]α2/[01¯1]γ semi-coherent α2 /γ interface due to its larger misfit than that of [112¯0]α2/[1¯10]γ interface. Ellipsoidal ωo particles were formed at βo /α2 interface and nucleated at the interface dislocations. The energy-dispersive X-ray spectroscopy (EDS) results demonstrated feasible depletion of Mo and enrichment of Ti at α2 /βo interface to form ωo phase. In addition, lath-shaped ωo precipitates were observed between two precipitated large βo grains formed by direct α2 →ωo phase transformation. The formation mechanism of the lath ωo phase was attributed to both element segregation and strain accommodation caused by βo precipitate in α2 lamellae. The α2 /βo interface served as preferential nucleation sites for ωo phase due to the atomic structure of the interface, which crystallographically was closely associated with the structure of ωo phase. Moreover, βo precipitates at low stress introduced stacking faults in α2 lamellae, while delaying the parallel decomposition of α2 lamellae at high stress. In sum, these findings provided profound insights into the decomposition mechanism of (α2/γ) lamellae, relevant as theoretical basis for controlling the microstructures and improving the performance of β-solidified γ-TiAl alloys.

Mathematical Modeling of Nucleation and Growth Processes of Ellipsoidal Crystals in Binary Melts

The transient behavior of an ensemble of ellipsoidal particles in a supercooled binary melt is considered. The model laws, based on the Fokker-Planck type kinetic equation for the particle-volume distribution function, the thermal and mass integral balances for the binary melt temperature and solute concentration, as well as the corresponding boundary and initial conditions, are formulated and solved analytically. We show that the temperature and concentration increase with time due to the effects of impurity displacement and latent heat emission by the growing ellipsoidal particles. These effects are also responsible for metastability reduction. As this takes place, increasing the initial solute concentration in a metastable binary melt increases the intensity of its desupercooling. The theory is developed for arbitrary nucleation frequency with special consideration of two important nucleation kinetics according to the Meirs and Weber-Volmer-Frenkel Zel’dovich mechanisms. An analytical solution to the integrodifferential model equations is found in a parametric form. The theory contains all limiting transitions to previously developed analytical approaches. Namely, it contains the growth of spherical crystals in binary melts and ellipsoidal crystals in single-component melts.

Collective diffusion in a two-dimensional liquid composed of Janus particles

The collective diffusion of anisotropic particles in liquids plays a crucial role in many processes, such as self-assembly. The patchy particle, which is usually nearly spherical in shape, is an important anisotropic particle with different properties from other anisotropic particles like the ellipsoid liquid crystal particles. In the present study, molecular dynamics simulations are performed to study the collective diffusion of a two-dimensional anisotropic liquid system composed of Janus particles. The static structures and diffusion behaviours of anisotropic and isotropic Lennard-Jones liquids are compared. The long-time diffusion behaviour of an anisotropic liquid of nearly spherical Janus particles is found to be similar to that of an isotropic liquid because the orientation of the particles disappears over long-term averaging. The anisotropic properties of the Janus particles are mainly reflected in the spatial correlation of particle orientations and mid-time diffusion behaviour. The difference between nearly spherical anisotropic particles and rod-like particles is also discussed in this paper.