Ellipsoidal Particles Research Articles

Transport of ellipsoidal microplastic particles in a 3D lid-driven cavity under size and aspect ratio variation

The primary goal of this paper is to model and study the behavior of ellipsoidal microplastic particles in a 3D lid-driven cavity. An Eulerian equation for the hydrodynamics is solved toward stationarity under the Reynolds number 1000 and coupled with a Lagrangian system governing the particle dynamics. The key points of departure in the modeling are laminar flow and small particle Reynolds number under which the drag force has extensively been studied. We then address the question, what would be the behavior of prolate and oblate particles under extreme conditions, where the aspect ratio tends to either ∞ (thin needle) or 0 (thin disk). The corresponding tool, Singular Perturbation Theory, not only settles the analysis of the critical manifold but also underlies a quasi-steady state approximation (QSSA) of the particle velocity around the manifold. We show that in a certain range of particle aspect ratio, the QSSA gives quite good approximations of the particle positions yet more computational efficiency. Sedimentation is shown to highly be dependent on the particle’s initial position, aspect ratio, and size. Meanwhile initial position determines to which direction the fluid stream drifts the particles, larger size and aspect ratio closer to 1 increase the likelihood of sedimenting. We also show that neutrally buoyant particles prefer to deposit on the cavity base as well as on vorticity-dominating regions. Generally, buoyant particles with aspect ratios between 1/20 and 20 spin faster than tumble, and they spin even faster as the aspect ratio gets smaller.

Advanced DEM simulation on powder mixing for ellipsoidal particles in an industrial mixer

Tons of products are facilitated through powder mixing processes. The quality of the final products is highly influenced by the mixing state. Hence, detailed information about the mixing state is necessary to improve the quality. Recent remarkable advancements in computer hardware enable the evaluation of the mixing state via numerical simulation. The Discrete Element Method (DEM) is frequently used in numerical simulations, where computational particle shapes are typically modeled by a spherical body. On the other hand, not all particle shapes in industries are spherical, and the structure of industrial mixers is generally complex. Thus, when the DEM is applied to an industrial mixer, modeling of particle shape and the complex structure of the mixers may be taken into consideration. In the present study, an innovative model is newly developed to simulate the powder mixing of non-spherical particles in an industrial mixer. Here, a non-spherical particle is modeled by the ellipsoidal equation, and an industrial mixer is modeled using the Signed Distance Function (SDF), where the wall boundary shape is flexibly modeled by a scalar field. This approach is referred to as the ellipsoidal DEM/SDF. In the current study, the adequacy and applicability of the ellipsoidal DEM/SDF are demonstrated by the following steps. First, the compatibility of the ellipsoidal DEM with the SDF wall boundary model and the applicability of a soft linear spring in the ellipsoidal DEM/SDF are demonstrated through validation tests. Subsequently, the effect of particle shape on mixing progress in a ribbon mixer is examined. Through computations, the effect of the non-spherical shape is shown to be insignificant on powder mixing progress in the ribbon mixer. Consequently, the ellipsoidal DEM/SDF is shown to be applicable to the industrial powder mixing, and therefore, will contribute to optimization of the mixer design and the operational conditions in chemical, food, and pharmaceutical engineering fields.

Parabolic velocity profile causes shape-selective drift of inertial ellipsoids

Understanding particle drift in suspension flows is of the highest importance in numerous engineering applications where particles need to be separated and filtered out from the suspending fluid. Commonly known drift mechanisms such as the Magnus force, Saffman force and Segré–Silberberg effect all arise only due to inertia of the fluid, with similar effects on all non-spherical particle shapes. In this work, we present a new shape-selective lateral drift mechanism, arising from particle inertia rather than fluid inertia, for ellipsoidal particles in a parabolic velocity profile. We show that the new drift is caused by an intermittent tumbling rotational motion in the local shear flow together with translational inertia of the particle, while rotational inertia is negligible. We find that the drift is maximal when particle inertial forces are of approximately the same order of magnitude as viscous forces, and that both extremely light and extremely heavy particles have negligible drift. Furthermore, since tumbling motion is not a stable rotational state for inertial oblate spheroids (nor for spheres), this new drift only applies to prolate spheroids or tri-axial ellipsoids. Finally, the drift is compared with the effect of gravity acting in the directions parallel and normal to the flow. The new drift mechanism is stronger than gravitational effects as long as gravity is less than a critical value. The critical gravity is highest (i.e. the new drift mechanism dominates over gravitationally induced drift mechanisms) when gravity acts parallel to the flow and the particles are small.

Dynamic Analysis and Simulation of an Optically Levitated Rotating Ellipsoid Rotor in Liquid Medium

Optical trap, a circularly polarized laser beam can levitate and control the rotation of microspheres in liquid medium with high stiffness. Trapping force performs as confinement while the trapped particle can be analog to a liquid floated gyroscope with three degree-of-freedom. In this work, we analyzed the feasibility of applying optically levitated rotor in the system. We presented the dynamic analysis and simulation of an ellipsoid micron particle. The precession motion and nutation motion of a rotating ellipsoid probe particle in optical tweezers were performed. We also analyzed the attitude changes of an optically levitated ellipsoid when there was variation of the external torque caused by deviation of the incident light that was provided. Furthermore, the trail path of the rotational axis vertex and the stabilization process of a particle of different ellipticities were simulated. We compared the movement tendencies of particles of different shapes and analyzed the selection criteria of ellipsoid rotor. These analytical formulae and simulation results are applicable to the analysis of the rotational motion of particles in optical tweezers, especially to the future research of the gyroscope effect.

Effects of AP powder topology on microscale combustion properties of AP/HTPB propellant

A three-dimensional model has been developed to capture burning properties of ammonium perchlorate(AP)/hydroxyl–terminated polybutadiene(HTPB) composite propellant. Interfacial coupling between condensed domain and gas phase was conducted to obtain the surface temperature and burning rate. The condensed phase was governed by heat transfer equation, and reacting Navier-Stokes equations with pressure dependent 3-step and 12-species global kinetics were adopted. The current model was verified and validated by comparing with analytical and experimental results. Thereafter, the influences of ellipsoidal AP's orientation and aspect ratio on the burning rate were investigated. The flame structure of a typical propellant with ellipsoidal AP particle was examined, and the characteristic heights of multiple flames were determined. Besides, the flame properties of a typical propellant with spherical/ellipsoidal AP particle and different AP topologies were analyzed. Furthermore, the temperature sensitivity coefficients were calculated for composite propellant with different AP contents and sizes, matching reasonably with the experimental results.

Numerical investigation on effect of particle aspect ratio on the dynamical behaviour of ellipsoidal particle flow

Flow of ellipsoidal particles in a modal shear cell was investigated at the microdynamic level based on discrete element method simulations. In a stress-controlled double-shear condition, the flow was studied by varying the aspect ratio of ellipsoidal particles and comparing with the flow of spherical particle assembly in terms of some key properties, including particle alignment, linear velocity, angular velocity, porosity, contact force and contact energy. It was found that particle elongation impacts the rotational displacement around the axis perpendicular to the shear direction, which causes that the ellipsoidal particles with higher elongation are more aligned with the direction of the shear velocity, with more uniform force network. This then affects other particle properties. The fluctuation of linear velocity and the angular velocity decreases with an increase in particle aspect ratio, although the particle elongation does not significantly affect the flow velocity gradient. There is a reduction in both normal and tangential forces per contact with an increase of particle elongation. Due to the variation of the particle alignment with elongation, the standard deviation of the contact energies increases and then reduces when an increase in particle aspect ratio occurs, and on contrary, the porosity has an opposite variation trend.

Macro- and micro-mechanical relationship of the anisotropic behaviour of a bonded ellipsoidal particle assembly in the elastic stage

Macro- and micro-mechanical relationship of the anisotropic behaviour of a bonded ellipsoidal particle assembly in the elastic stage

Enhanced upconversion emission of Er3+-Yb3+ co-doped Ba5(PO4)3OH powder phosphor for application in photodynamic therapy

Er3+-Yb3+ co-doped Ba5(PO4)3OH nanoparticle powder phosphors were successfully synthesized by urea combustion method. The resulting powder phosphors were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). XRD data confirmed crystallization of pure hexagonal phase of Ba5(PO4)3OH and HRSEM images showed formation of ellipsoidal particles. XPS data combined with EDS analysis confirmed the materials composition that corresponds with identification of all the chemical elements constituting the materials. The in vitro dark cytotoxicity of the particles confirmed lack of cytocidal behaviour in the absence of light, but considerable photodynamic therapy (PDT) activity was observed upon illumination. Upon excitation using a 980 nm laser, multiple emission peaks in the green and red regions corresponding to the optical transitions of Er3+ ion were observed. Upon co-doping with Yb3+, upconverted red emission was detected and this was attributable to non-radiative energy transfer from Yb3+ to Er3+. The proposed mechanism of upconversion photoluminescence is discussed.

Nucleation and growth dynamics of ellipsoidal crystals in metastable liquids.

When describing the growth of crystal ensembles from metastable solutions or melts, a significant deviation from a spherical shape is often observed. Experimental data show that the shape of growing crystals can often be considered ellipsoidal. The new theoretical models describing the transient nucleation of ellipsoidal particles and their growth with and without fluctuating rates at the intermediate stage of bulk phase transitions in metastable systems are considered. The nonlinear transport (diffusivity) of ellipsoidal crystals in the space of their volumes is taken into account in the Fokker-Planck equation allowing for fluctuating growth rates. The complete analytical solutions of integro-differential models of kinetic and balance equations are found and analysed. Our solutions show that the desupercooling dynamics is several times faster for ellipsoidal crystals as compared to spherical particles. In addition, the crystal-volume distribution function is lower and shifted to larger particle volumes when considering the growth of ellipsoidal crystals. What is more, this function is monotonically increasing to the maximum crystal size in the absence of fluctuations and is a bell-shaped curve when such fluctuations are taken into account. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

A study of ellipsoidal and spherical particle flow, clogging and unclogging dynamics

To study ellipsoidal and spherical particle flow, clogging behavior and unclogging dynamics, a series of numerical simulations were carried out using our validated 3-D coupled computational fluid dynamics–discrete element method (CFD–DEM) model. For the first time, the strength of clogging arches is assessed in terms of fluid pressure gradient, which can effectively break up the clog. The main findings are as follows. In a clogged assembly, only when the flushing fluid reaches the unclogging pressure gradient, the ellipsoidal and spherical particles can flow in a continuous state. For the ellipsoids and spheres with the same equivalent volume, the unclogging pressure gradient of an ellipsoidal assembly is 11.8 times higher than that of a spherical assembly. Clogging arches at the outlet show the highest strength when the longitudinal orientations of the ellipsoids are perpendicular to the outlet plane. In contrast, the formed arch has the lowest stability when the longitudinal orientations of the ellipsoids are in the same direction as the long side of the outlet, as shown in the graphical abstract. The unclogging pressure gradient of the former case is 1.9 times greater than that of the latter case. The granular arches observed in our 3-D model are categorized as complex 3-D backbone arches. In the flowing region, particles rotate nearly perpendicular to the outlet plane, especially those approaching the outlet. Moreover, triangular stagnant zones are formed on both sides of the bottom of the rectangular box. The model presented here offers a great promise for future investigation of various granular systems at the particle scale.

The importance of Mie resonances in ultra-black dragonfish skin pigment particles

The ultra-black skin of the deep-sea dragonfish consists of small pigment particles which together provide optimal light absorption to prevent detection from bioluminescent predators or prey. The mechanism of light absorption in these pigment particles resembles the nanophotonic approaches to increase solar cell efficiency via Mie scattering and resonances. In this work, the Mie resonance responses of dragonfish pigment particles were investigated with finite-difference time-domain (FDTD) simulations to elucidate the exact mechanism responsible for the ultra-black skin of the dragonfish. Ellipsoidal pigment particles were found to have superior light absorption over spherical pigment particles. The pigment particles were also shown to exhibit forward scattering, demonstrating an important feature for repeated light absorption in pigment-containing skin layers. Although this work contributes to a deeper understanding of the ultra-back skin of the dragonfish, the nanophotonic mechanisms proposed here are likely more general, and could be applied to photovoltaic light management designs and immunometric detection based on light extinction.

Particle-resolved simulation on viscous flow past random and ordered arrays of hot ellipsoidal particles

Particle-resolved simulation is performed to study the drag force on arrays of ellipsoidal particles with a temperature difference with respect to the surrounding fluid. The effect of particle shape, arrangement, solid volume fraction and particle temperature is examined. We found that the particle shape and arrangement strongly influence the drag coefficient CD when the particle volume fraction ϕ is small. The influence, however, is inhibited as ϕ increases. We also show that the drag coefficient decreases with the increase of the solid volume fraction, which is a reversal of the trend found in previous uniform suspension systems. Moreover, CD increases with the increase of particle temperature while slightly decreases with the fluid temperature. Through a novel decomposition analysis, we show that the increase of fluid viscosity causes the increase of CD when the particle temperature is increased, while a combined effect of the fluid density, inlet velocity and the integral of the velocity gradient around particle on CD was observed when the fluid temperature changes. Finally, new drag correlations, as power functions of Reynolds number, particle temperature and volume fraction, are proposed for clusters with different particle shapes and arrangements.

3D percolation modeling for predicting the thermal conductivity of graphene-polymer composites

Graphene-based polymer composites exhibit a microstructure formed by aggregates within a matrix with enhanced thermal conductivity. We develop a percolation-based computational method, based on the multiple runnings of the shortest path iteratively for ellipsoidal particles to predict the thermal conductivity of such composites across stochastically-developed channels. We analyze the role of the shape and the aspect ratio of the flakes and we predict the onset of percolation based on the density and particle dimensions. Consequently, we complement and verify the conductivity trends via our experiments by inclusion of graphene aggregates and fabrication of graphene-polymer composites. The analytical development and the numerical simulations are successfully verified with the experiments, where the prediction could explain the role of larger set of particle geometry and density. Such percolation-based quantification is very useful for the effective utilization and optimization of the equivalent shape of the graphene flakes and their distribution across the composite during the preparation process and application.

Spherical versus prolate spheroidal particles in biosciences: Does the shape make a difference?

AbstractIn this mini‐review, the main differences between spherical and spheroidal particles in terms of their size, surface area, morphology, and ability to penetrate biological membranes are described. The main routes of manufacturing of prolate spheroidal particles with controlled aspect ratio are reported. There are also shown exemplary results of computational and experimental studies of spheroids useful in biomedical applications, such as controlled drug delivery and numerical studies of the passage of spheroids through biological membranes. The relations of shape, size, aspect ratio of spheroidal particles and particle–biological membrane interactions, bioavailability, and flow in blood vessels are presented. The perspectives for future studies leading to resolve crucial problems and to explain basic issues related to acting of ellipsoidal particles in biological environment are also discussed.

Are Langmuir Trough Studies Useful? Unexpected Emulsification Behavior Using Colloidal Rods.

While studies carried out in a Langmuir trough have rigorously demonstrated that, at high surface pressure, ellipsoidal particles do flip and spherocylinders (rods) can flip, much less is known about the practical situation on the surface of a droplet or bubble. We present emulsification studies using colloidal rods and find that the droplets are bridged by the rods independent of shear rate and particle concentration and are only weakly dependent on the pH of the continuous phase. In a trough, it is the low aspect ratio rods which flip and the high aspect ratio rods which form bilayers; on the surface of a droplet we found that the high aspect ratio rods always bridge whereas the shorter rods show random bridging behavior. Hence, the behavior of anisotropic particles "in action" is essentially opposite to expectations from trough studies.

Unresolved CFD-DEM simulation of spherical and ellipsoidal particles in conical and prismatic spouted beds

Various drag models were implemented in a superquadric CFD-DEM code and validated for the simulation of spherical and ellipsoidal particles in spouted beds. The most suitable drag models were identified by comparing the predicted local particle velocity with those measured by particle tracking velocimetry (PTV). The model by Beetstra et al. is the one that best reproduces the experimental results for the spouting of spherical particles, whereas the one by Sanjeevi et al. is the most suitable for ellipsoidal irregular ones. Their capability for the prediction of key operating parameters was demonstrated in both conical and prismatic spouted beds, as they correctly predict the minimum spouting velocity (ums) and fountain height in different configurations (without draft tube and different draft tubes) for particles of different size and shape. The CFD-DEM model predicts the preferential orientation of ellipsoidal particles at each location in the bed and the influence of internal devices on this parameter, which is of the utmost importance in the design of reliable coating apparatuses in the pharmaceutical industry.

Fabrication and Electric Field-Driven Active Propulsion of Patchy Microellipsoids.

Active colloids are a synthetic analogue of biological microorganisms that consume external energy to swim through viscous fluids. Such motion requires breaking the symmetry of the fluid flow in the vicinity of a particle; however, it is challenging to understand how surface and shape anisotropies of the colloid lead to a particular trajectory. Here, we attempt to deconvolute the effects of particle shape and surface anisotropy on the propulsion of model ellipsoids in alternating current (AC) electric fields. We first introduce a simple process for depositing metal patches of various shapes on the surfaces of ellipsoidal particles. We show that the shape of the metal patch is governed by the assembled structure of the ellipsoids on the substrate used for physical vapor deposition. Under high-frequency AC electric field, ellipsoids dispersed in water show linear, circular, and helical trajectories which depend on the shapes of the surface patches. We demonstrate that features of the helical trajectories such as the pitch and diameter can be tuned by varying the degree of patch asymmetry along the two primary axes of the ellipsoids, namely longitudinal and transverse. Our study reveals the role of patch shape on the trajectory of ellipsoidal particles propelled by induced charge electrophoresis. We develop heuristics based on patch asymmetries that can be used to design patchy particles with specified nonlinear trajectories.

Modeling the Influence of Particle Shape on Mechanical Compression and Effective Transport Properties in Granular Lithium‐Ion Battery Electrodes

The calendering step during manufacturing of lithium‐ion batteries is an essential process in the production, as it significantly influences the microstructure of electrodes and, therefore, the performance of the battery. Within this context, this article investigates the influence of particle shapes on the micromechanical responses during calendering and, in turn, their impact on the effective transport properties of battery electrodes. The electrodes are modeled using discrete elements. For this reason, a novel algorithm for the generation of random stress‐free particle assemblies consisting of superellipsoids is presented. The effective conductivities of solid and pore phase are calculated with a resistor network approach. In this context, a new analytical formula for calculating the individual resistance between two mechanically deformed ellipsoidal particles is presented. Furthermore, a geometrical approach is chosen for the pore phase for calculating individual resistances of pore throats in superellipsoidal particle assemblies. With the theoretical fundamentals, the effective conductivities of solid and pore phases of uniaxially compressed ellipsoidal particle assemblies are investigated. The mechanical response and its influence on the evolution of the effective conductivities are discussed. The deeper insight into the interplay between the calendering process and electrode microstructure can be a helpful information regarding a specific electrode design.

Correlations for aerodynamic coefficients for prolate spheroids in the free molecular regime

A new set of aerodynamic correlations are proposed for an ellipsoidal particle in the free molecular regime by carrying out flow simulations using the Direct Simulation Monte Carlo (DSMC) method. The in-house DSMC solver is modified to be consistent with free molecular flow assumptions as well as to reduce computational cost. The solver is validated against an open-source DSMC solver, viz., SPARTA, the free molecular theory and experimental data for flow over a sphere and right circular cylinder in the free molecular regime. The flow is simulated over ellipsoidal particles at different aspect ratios, angles of attack and speed ratios to estimate the aerodynamic properties. Regression analysis is performed to give correlations for the variation of aerodynamic properties such as drag, lift and torque coefficients. The correlations are rigorously tested against the DSMC results, obtained from our in-house solver as well as those obtained from SPARTA, over a wide range of parameters.

Dynamic analysis of poured packing process of ellipsoidal particles

Packing properties are determined by the dynamic deposition history and affected by many variables. In this work, DEM is used to study the effects of dropping height, deposition intensity and friction coefficient on the dynamic response and stable-state packing properties of ellipsoidal particles. The results demonstrate that with dropping height increasing, packing density increases but the bed becomes less ordered. Dropping height affects the densification process of ellipsoids more significantly than spheres. At high dropping height, the transition period of the orientation angle in the densification process almost disappears for oblate particles, while it becomes longer for prolate particles. With the increase of deposition intensity, packing density decreases significantly, but orientational order changes slightly. With the increase of sliding friction coefficient, packing density and orientational order decrease dramatically. The difference in packing density is mainly caused by dynamic pouring process rather than the densification process.