Assembly Of Spherical Particles Research Articles

ZSM-5 monolithic microreactors with hierarchical porosity: Maximizing selective production of para-xylene

ZSM-5 monoliths with Si/Al ∼40 were achieved by steam assisted conversion of micro-/macroporous silica-alumina monoliths synthesized by an optimized combination of spinodal decomposition and sol-gel process. ZSM-5 monoliths exhibit a homogeneous and interconnected network of macropores of ∼30 μm diameter, with a macropore volume of ∼1 mL g−1 and a micropore volume of 0.16 mL g−1, as waited for pure ZSM-5. The skeleton of ZSM-5 monoliths featured a 3D aggregation of spherical particles of 6 μm in diameter. The latter were composed by a stacking of thin plate-like shaped crystallites. The catalytic properties of the ZSM-5 monoliths were assessed through their use as catalytic microreactor in the continuous flow alkylation of toluene with methanol. An outstanding for para-xylene selectivity of 81 % was achieved possibly ascribable due to the maximization of pore openings of sinusoidal channels as a result of the plate-like morphology of the ZSM-5 crystallites.

Connectivity in binary mixtures of spherical particles

Mono- and poly-disperse assemblies of spherical particles are investigated in terms of their average and partial coordination numbers by means of X-ray microtomography using a novel morphology-based image processing method to statistically distinguish true contacting particles from very close ones having apparent contacts arising from image artefacts. This technique is shown to reduce overestimations given by the laborious liquid-bridge method while corroborating theoretical predictions such as Z¯≤6 for random beds of mono-sized spheres and trends of the partial coordination numbers in binary mixtures of spheres. This method also provides a detailed and unbiased visualization of the long-range connectivity between similar particles which suggests that for partial coordination numbers Z¯ii>3, chains of particular contact-category are formed throughout the assembly.

Effects of Dy on structural, morphology, optical, and resistive switching properties of HfO2 thin films

This paper studies the effect of Dy on the structure, morphology, and optical properties of HfO2 thin films deposited on fused silica substrates and the optical transparency and resistive switching (RS) behavior of the thin films as an active insulator layer in Ag/active insulator layer/ITO/glass memories. Thin films were deposited by spin-coating using a precursor solution obtained by the Pechini-type sol-gel method. The structural analysis revealed that the insertion of Dy ions into the HfO2 lattice caused the phase transformation from the monoclinic to the cubic phase. Morphological analysis revealed that the thin films had thicknesses between 98 and 101 nm, and their surfaces were composed of 3D islands formed by aggregates of spherical particles ranging from 8 to 5 nm. The active insulator layers had thicknesses ranging from 235 to 218 nm, with no cavities or cracks between them. The optical transparency of the active insulator layer/ITO/glass memory structures was between 70 and 87 %. Optical transmittance, optical bandgap, and optical constants were modified incorporating Dy ions into HfO2 thin films compared to the undoped HfO2 thin films. Incorporating Dy ions into HfO2 increased oxygen vacancies in the active insulator layers. The memory with 10 w% of Dy had the highest resistance off/on ratio of 9.04 compared with undoped HfO2 memory. Also, the conduction mechanism in the high resistive state (HRS) of the memory with 10 w% of Dy was related to ohmic conduction and space charge limited conduction (SCLC).

Single-particle fabric tensors for assemblies of spherical particles

A novel method to calculate single-particle fabric tensors and related geometric characteristics (contact, free and particle area, particle volume and contact centroid) of assemblies of spherical particles has been developed. A standard contact detection stage is used to identify overlapping particles. The shape of each contact region is obtained as the intersection of a circle, obtained in the absence of any contact impingement, and a convex polygon, determined by half-plane intersection based on directed lines obtained from the intersection of contact planes. An underlying power diagram is used to subdivide volume and area between particles, but calculations are performed particle-wise, resulting in an efficient algorithm with a structure ideally suited for parallelisation. Numerical tests performed on loose and dense particle assemblies and comparison (of single-particle free area and particle volume) with a reference method (POWERSASA) indicate that the method is robust and highly accurate. The method will be useful not only for geometric analysis but also as a basis for subsequent developments of strain measures and computational procedures for highly deformed granular materials. Moreover, it constitutes a viable alternative for calculation of solvent accessible surface areas, considering its robustness, efficiency and ease of parallelisation.

Kinetic model describing self-limiting CO2 diffusion in supported amine adsorbents

A reaction–diffusion shrinking core model describing the decay in diffusivity of supported amine sorbents upon CO2 sorption under both simulated direct air capture and point source capture conditions is described. The decay in CO2 diffusivity is associated with crosslinking in the aminopolymer samples and general pore blockage in the amino-silane derived samples, which occurs as CO2 is adsorbed. The model is used to extract four kinetic parameters that govern the CO2 uptake kinetics and working capacity: an apparent reaction rate constant, an initial effective diffusivity, and two dimensionless decay parameters. Ideally, an initially reaction limited system would allow for direct determination of the intrinsic reaction rate constant; however, sorption experiments suggest mass transfer resistances related to gas mixing, external boundary layers and intraparticle diffusion are present. Reaction rate constants are determined and agree well with theoretical values predicted with the Eyring equation parameterized using density functional theory energies from literature sources. The kinetic performance is expressed as the average effective diffusivity as a function of average conversion, which can be correlated to the dispersion of sorption sites on the support and the morphology of the active sorbent phase. Four supports are impregnated or grafted with amines, SBA-15, single-walled zeolite nanotubes (ZNT), Syloid SiO2, and γ-Al2O3. Due to its pore structure, γ-Al2O3 supported amines can be modeled at the μm scale or at the nm scale, where the shell balance is on the μm-sized macroporous particle aggregate or on the nm-sized amine film on the surface of the Al2O3 nanoparticles, which comprise the spherical particle aggregates. Faster diffusion rates are maintained under 400 ppm rather than 10% CO2 due to a slower reaction rate giving a slower decay in diffusivity. This work provides a first principles kinetic analysis of CO2 sorption where previous models are semi-empirical and use arbitrary kinetic parameters.

Improving the prediction accuracy of small-strain shear modulus of granular soils through PSD: An investigation enabled by DEM and machine learning technique

This study presents a numerical investigation into the effect of particle size distribution (PSD) on the small-strain shear modulus (G0) of granular soils using the discrete element method (DEM). Drained triaxial tests were conducted on a set of spherical particle assemblies covering a wide variety of PSDs. The tests demonstrated that the G0 decreases with increasing the coefficient of uniformity (Cu), assuming the confining pressure (p’) and the void ratio (e) remain constant. A corrected void ratio (e4), which regards mechanically unstable particles with less than four contacts as void, was developed. Analyses performed afterward revealed that e4 is more strongly correlated with G0 than e. Consequently, a Hardin-based G0 model parameterized by e4 and p’ was established and it is more accurate than two commonly used models, which are parameterized by e, p’ and Cu. Furthermore, micromechanical and microstructural features were investigated to acquire a better understanding of why models based on e and Cu yield less accurate results than models based on e4. Finally, a hybrid model combining the high explanatory power of statistical models with the high predictive power of machine learning models were proposed. The hybrid model which only uses measurable macroscopic parameters performs on par with the microstructural model and is more accurate than conventional empirical models. The proposed model offers a simple and streamlined approach to predicting G0 with satisfactory accuracy, which is a feasible strategy for maximizing the utilization of copious experimental data.

Can Differently Stabilized Silver Nanoparticles Modify Calcium Phosphate Precipitation?

Calcium phosphates (CaPs) composites with silver nanoparticles (AgNPs) attract attention as a possible alternative to conventional approaches to combating orthopedic implant-associated infections. Although precipitation of calcium phosphates at room temperatures was pointed out as an advantageous method for the preparation of various CaP-based biomaterials, to the best of our knowledge, no such study exists for the preparation of CaPs/AgNP composites. Motivated by this lack of data in this study we investigated the influence of AgNPs stabilized with citrate (cit-AgNPs), poly(vinylpyrrolidone) (PVP-AgNPs), and sodium bis(2-ethylhexyl) sulfosuccinate (AOT-AgNPs) in the concentration range 5-25 mg dm-3 on the precipitation of CaPs. The first solid phase to precipitate in the investigated precipitation system was amorphous calcium phosphate (ACP). The effect of AgNPs on ACP stability was significant only in the presence of the highest concentration of AOT-AgNPs. However, in all precipitation systems containing AgNPs, the morphology of ACP was affected, as gel-like precipitates formed in addition to the typical chain-like aggregates of spherical particles. The exact effect depended on the type of AgNPs. After 60 min of reaction time, a mixture of calcium-deficient hydroxyapatite (CaDHA) and a smaller amount of octacalcium phosphate (OCP) formed. PXRD and EPR data point out that the amount of formed OCP decreases with increasing AgNPs concentration. The obtained results showed that AgNPs can modify the precipitation of CaPs and that CaPs properties can be fine-tuned by the choice of stabilizing agent. Furthermore, it was shown that precipitation can be used as a simple and fast method for CaP/AgNPs composites preparation which is of special interest for biomaterials preparation.

Liquefaction characteristics in triaxial tests under various gravity environments – DEM analyses

Liquefaction of sands occurs when the effective stress becomes zero during undrained loading, and the state of liquefaction is often considered that individual particles do not transmit contact forces. In shaking table model tests, seepage flows towards the surface occur during liquefaction; this potentially maintains individual particles floated as the gravitational forces can be cancelled out by the upward seepage forces. In laboratory element tests, however, undrained conditions induce no seepage flows during liquefaction. This raises a key question of how the individual particles behave during liquefaction in element tests, and how the macroscopic mechanical response is affected due to gravity. To answer these questions, this contribution conducts a series of triaxial loading simulations using the discrete element method (DEM) considering the effect of gravity on the mechanical response of spherical particle assemblies. The results reveal that the presence of gravity affects both packing properties and liquefaction characteristics. Inter-particle contacts are maintained during liquefaction under non-zero gravity conditions. Although the liquefaction resistance is merely affected due to gravity, the stress–strain relation and stress path are measurably altered. This contribution highlights the importance of considering the potential effect of gravity when laboratory or DEM results on liquefaction phenomena are interpreted.

Deciphering the role of water and a zinc-doping process in a polyol-based approach for obtaining Zn/Co/Al-based spinels: toward "green" mesoporous inorganic pigments.

Two new families of zinc/cobalt/aluminum-based pigments, with a unique composition, were obtained through the polyol method. The hydrolysis process of a mixture of Co(CH3COO)2, Zn(acac)2 and Al(acac)3 (acac- = acetylacetonate ion) in 1,4-butanediol afforded dark blue gels (wPZnxCo1-xAl), in the presence of a supplementary amount of water, and light green powders (PZnxCo1-xAl), respectively, for the water-free procedure (x = 0, 0.2, 0.4). The calcination of the precursors yielded dark green (wZnxCo1-xAl) and blue (ZnxCo1-xAl) products. XRD measurements and Rietveld refinement indicate the co-existence of three spinel phases, in different proportions: ZnxCo1-xAl2O4, Co3O4 and the defect spinel, γ-Al2.67O4. The Raman scattering and XPS spectra are in agreement with the compositions of the samples. The morphology of wZnxCo1-xAl consists of large and irregular spherical particle aggregates (ca. 5-100 mm). Smaller agglomerates (ca. 1-5 mm) with a unique silkworm cocoon-like hierarchical morphology composed of cobalt aluminate cores covered with flake-like alumina shells are formed for ZnxCo1-xAl. TEM and HR-TEM analyses revealed the formation of crystalline, polyhedral particles of 7-43 nm sizes for wZnxCo1-xAl, while for ZnxCo1-xAl, a duplex-type morphology, with small (7-13 nm) and larger (30-40 nm) particles, was found. BET assessment showed that both series of oxides are mesoporous materials, with different pore structures, with the water-free samples exhibiting the largest surface areas due, most likely, to the high percent of aluminum oxide. A chemical mechanism is proposed to highlight the role of the water amount and the nature of the starting compounds in the hydrolysis reaction products and, further, in the morpho-structural features and composition of the resulting spinel oxides. The CIE L*a*b* and C* colorimetric parameters indicate that the pigments are bright, with a moderate degree of luminosity, presenting an outstanding high blueness.

Effect of coefficient of uniformity on cyclic liquefaction resistance of granular materials

Using three-dimensional discrete element method, we analyze the particle size distribution (PSD) effect on the cyclic liquefaction resistance of spherical particle assemblies. For the same mean particle size and log-linear type PSD, the coefficient of uniformity (Cu) is chosen as a descriptor of the PSD. Samples with five levels of Cu are isotropically compressed to the same pressure and two relative densities (Dr) informed by the maximum and minimum achieved void ratios determined for each Cu. The ten samples are subjected to constant volume cyclic simple shearing at different cyclic stress ratios until reaching initial liquefaction, in 56 simulations. The simulations suggest that at each Dr the evolution pattern of excess pore pressure ratio against the number of loading cycles normalized by the number of cycles to liquefaction is minimally affected by the Cu. For the samples with lower Dr, increasing the Cu in the range 1–3 first increases and then decreases the liquefaction resistance; this trend reverses at the higher Dr. Two critical state parameters based on the void ratio and the coordination number at the pre-shearing state of the samples correlate well with the cyclic liquefaction resistance for the ranges of Cu and Dr considered in this study.

An advanced pore-scale model for simulating water retention characteristics in granular soils

Accurately estimating the soil water characteristics curve (SWCC) has become indispensable in successfully implementing unsaturated soil mechanics into engineering practice. The objective of this paper is to present a pore-scale model for the prediction of the hysteretic SWCC of granular soils using basic properties like grain size distribution (GSD) and porosity. The Discrete Element Method (DEM) is used to model the soil structure by generating a stable assembly of spherical particles to attain a predefined grain size distribution and porosity. A network of pores and throats is extracted from this assembly using a medial axis-based network extraction technique, and adequate pore-scale mechanisms are availed to simulate two-phase flow along drying and wetting paths. The current work is further expanded to model SWCC under different void ratios. In addition, SWCCs exhibit stress-dependent behavior, and a numerical framework for modeling SWCCs under various stress conditions is proposed. The effect of particle shapes used for generating DEM samples on SWCC is also investigated. The modeled SWCC compares well with measured curves obtained for a range of granular soils from the literature. The model effectively estimates the hysteretic drying-wetting response and can capture the variation in SWCC with change in net stress and initial void ratio.

Classification of Aggregates Using Multispectral Two-Dimensional Angular Light Scattering Simulations.

Airborne particulate matter plays an important role in climate change and health impacts, and is generally irregularly shaped and/or forms agglomerates. These particles may be characterized through their light scattering signals. Two-dimensional angular scattering from such particles produce a speckle pattern that is influenced by their morphology (shape and material composition). In what follows, we revisit morphological descriptors obtained from computationally generated light scattering patterns from aggregates of spherical particles. These descriptors are used as inputs to a multivariate statistical algorithm and then classified via supervised machine learning algorithms. The classification results show improved accuracy over previous efforts and demonstrate the utility of the proposed morphological descriptors.

Hydroxyapatite from Natural Sources for Medical Applications.

The aim of this work is to study the physical-chemical, mechanical, and biocompatible properties of hydroxyapatite obtained by hydrothermal synthesis, at relatively low temperatures and high pressures, starting from natural sources (Rapana whelk shells), knowing that these properties influence the behavior of nanostructured materials in cells or tissues. Thus, hydroxyapatite nanopowders were characterized by chemical analysis, Fourier-transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). In vitro studies on osteoblast cell lines (cytotoxicity and cell proliferation), as well as preliminary mechanical tests, have been performed. The results showed that the obtained powders have a crystallite size below 50 nm and particle size less than 100 nm, demonstrating that hydrothermal synthesis led to hydroxyapatite nanocrystalline powders, with a Ca:P ratio close to the stoichiometric ratio and a controlled morphology (spherical particle aggregates). The tensile strength of HAp samples sintered at 1100 °C/90 min varies between 37.6–39.1 N/mm2. HAp samples sintered at 1300 °C/120 min provide better results for the investigated mechanical properties. The coefficient of friction has an appropriate value for biomechanical applications. The results of cell viability showed that the cytotoxic effect is low for all tested samples. Better cell proliferation is observed for osteoblasts grown on square samples.

Critical appraisal of pore network models to simulate fluid flow through assemblies of spherical particles

Coupled numerical models considering fluid flow and particle movement enable fundamental analyses of a variety of phenomena in geomechanics including seepage-induced instabilities. Amongst the various CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) coupled frameworks which have been proposed, Pore Network Models (PNMs) have the potential to simulate fluid flow in granular materials accurately with a low computational cost to enable simulations on Representative Volume Elements (RVEs). However, the current models of the local conductance between the connected pores are very simple, limiting the accuracy of PNMs. This study develops novel local conductance models by detailed analysis of existing analytical studies of fluid flow through different 3D lattice packings of uniform spheres. The performance of these new models relative to existing, simpler models is demonstrated using CFD simulations in which the flow in the pore space of random assemblies of polydisperse spheres is accurately resolved. The analyses show that the new models proposed here can more accurately predict the local and global permeabilities of specimens with a wide range of void ratios and polydispersities. These models do not require any optimisation via merging pores so that they can efficiently simulate systems with an evolving pore space topology.

DEM modelling of the influence of initial stress state on the discharge rate of spherical particles from a model silo

Formulation of a proper physical law describing the dynamics of discharge of granular solids has been undertaken by researchers for decades. A frequently expressed opinion is the existence of the free-fall arch region and independence of the discharge rate on pressure. The objective of this study was to explore the impact of dynamic particle–particle contact interactions in the vicinity of the orifice on the discharge flow rate from a model silo by means of DEM simulations with special attention paid to the impact of the bedding height and bottom pressure. The DEM simulations were performed with an assembly of spherical particles poured on a flat bottom of the cylindrical model silo with a height-to-diameter ratio of 1.14. The bottom pressure was changed by changing the height of the bedding deposit and depositing an additional layer of very dense particles on the top of the bedding of primary particles. The results indicate that the discharge rate was affected by both the orifice diameter and mean bottom pressure during storage. The correction coefficient derived from the integral of the dimensionless acceleration of particles along the dimensionless movement path allowed obtaining very good approximation of the discharge rate using the well known Beverloo equation in the entire range of change of the orifice size and bottom pressure.

Digital Assembly of Spherical Viscoelastic Bio‐Ink Particles

Abstract3D bioprinting additively assembles bio‐inks to manufacture tissue‐mimicking biological constructs, but with the typical building blocks limited to 1D filaments. Here, it is developed a voxelated bioprinting technique for the digital assembly of spherical particles (DASP), which are effectively 0D voxels—the basic unit of 3D structures. It is shown that DASP enables on‐demand generation, deposition, and assembly of viscoelastic bio‐ink droplets. A two‐parameter diagram is developed to outline the viscoelasticity of bio‐inks required for printing spherical particles of good fidelity. Moreover, a strategy is developed for engineering bio‐inks with independently controllable viscoelasticity and mesh size, two of the most important yet intrinsically coupled physical properties of biomaterials. Using DASP, mechanically robust, multiscale porous scaffolds composed of interconnected yet distinguishable hydrogel particles are created. Finally, it is demonstrated the application of the scaffolds in encapsulating human pancreatic islets for sustained responsive insulin release. Together with the knowledge of bio‐ink design, DASP might be used to engineer highly heterogeneous, yet tightly organized tissue constructs for therapeutic applications.

Deformability of polydisperse granular materials: the effect of size span and shape of grain size distribution

This paper investigates the effect of the grain size distribution (GSD) on deformability of spherical particle assemblies using the distinct element method. The GSD is modeled by a normalized beta function, which allows variation of the size span (nearly monodisperse to highly polydisperse) and the shape (approximately linear to strongly curved) of GSD. A series of triaxial compression tests is simulated on a set of assemblies with varying GSDs under different packing fractions and confining pressures. The compressibility under isotropic loading and deformation modulus during shearing are examined for various GSDs. The results show that the size span and shape of GSDs affect the packing conditions of the assemblies but have negligible influence on the compressibility and deformation modulus of the assemblies under the same packing conditions. A unique correlation is found between the deformation modulus and the mechanical coordination number, which can be estimated from the effective void ratio.

A Discrete Element Framework for the modelling of rock-filled gabions

Gabions are steel mesh cages filled with stones. Recently, they have had a great diffusion for numerous applications such as erosion control and soil retention along cuts and natural slopes. The study of their mechanical behaviour under different loading conditions is a fundamental prerequisite for an improved understanding of their overall performance and in a design perspective. In this respect, the Discrete Element Method (DEM) appears a robust and effective technique. The DEM is particularly suited for modelling granular materials, and recently it has efficiently been applied for modelling deformable structures such as welded meshes. This work presents a discrete framework for the modelling of rock-filled gabions. The gabion module is modelled as an assembly of deformable cylindrical elements, while the filling material is represented using rigid aggregates of spherical particles. The tensile behaviour of the gabion elements is set in order to fit the mechanical response of the steel bars adopted in practice. The model is applied to analyse a uniaxial compression test on a single gabion.

Yttria-Stabilized Zirconia Assisted Green Electrochemical Preparation of Silicon from Solid Silica in Calcium Chloride Melt

A novel integrated cell with O2-|YSZ|Pt|O2(air) reference and counter electrodes was constructed using a short yttria-stabilized zirconia solid electrolyte (YSZ) tube. Combining with cyclic voltammetry and potentiostatic electrolysis methods, green electrochemical preparation of Si from solid SiO2 in CaCl2 melt at 1173 K was studied via an experimental apparatus containing the novel integrated cell under completely carbon-free conditions; the effect of electrolysis time on the morphology of the Si product was also investigated by scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS). The results show that the morphology of the product obtained from potentiostatic electrolysis at a low overpotential (− 1.6 V) undergoes an evolution from SiO2 raw powder with different sizes to aggregates of spherical particles and small particles with partial reduction, and then to Si nuclei, and finally to Si wires or flakes. The morphology of electrolytic products has little relation with that of SiO2 raw powder and may be controlled by applying different potentials. Furthermore, the longer the electrolysis time, the more the Si wires grow, and the higher the Si purity overall. It is feasible that the experimental apparatus without the sealed stainless steel reactor and any carbonaceous materials can be used to prepare Si from solid SiO2 in CaCl2 melt and release O2 gas at the same time.

DEM analysis on the stress wave response of spherical particle assemblies under triaxial compression

It is challenging to understand the stress wave response of cohesionless particle assemblies including compression and shear wave velocities (Vp and Vs) due to complicated particle-scale interactions particularly during a loading process. This contribution adopts the discrete element method to simulate triaxial compression experiments using spherical particles. Four samples are isotropically confined at various initial packing densities and then sheared monotonically up to the critical state. Small-amplitude wave propagation simulation is performed during shearing along the axis of loading. The results reveal that Vp is affected by the major principal stress and the coordination number contributing to the vertical direction rather than horizontal direction, while Vs is more influenced by the geometric mean stress and the mean coordination number. The wave velocity ratio (Vp/Vs) is well correlated with the fabric anisotropy, having a linear relationship. This enables prediction of microscopic change in fabric from macroscopic wave velocity values. Besides, a material-specific relationship between stress levels and wave velocities is found at the critical state with a unique fabric anisotropy, independent of initial packing. Vs in the isotropic state is found to be always larger that at the critical state under an equivalent stress level.