Particle Aspect Ratio Research Articles

Wave propagation in granular material: What is the role of particle shape?

Wave propagation in granular materials is one of the most fundamental problems in mechanics and physics, with both scientific fascination and practical importance. Whether elastic waves in granular media are affected by particle morphology and, if yes, how they are affected remain open questions. Here we present a novel grain-scale model to address these fundamental questions. Efficient techniques are incorporated in the model to cope with a huge number of non-spherical particles which are randomly packed to propagate elastic waves. The variability of particle shape is mathematically described using the superquadric function and a family of geometric shapes is produced so that a systematic investigation of the effect of particle shape becomes viable. The difficulty with the detection of particle contacts that are represented by nonlinear Hertzian contact law is tackled using an efficient algorithm. A marked finding from the multiple series of simulations using this new model is that: an increase in the aspect ratio of particles (i.e. particles changing from spherical to ellipsoidal) leads to a notable rise in the elastic wave velocity, for both compression and shear waves, whereas for non-spherical particles with a given aspect ratio, an increased particle blockiness causes a moderate reduction in the wave velocity. Moreover, it is found that an assembly of particles with higher aspect ratio is associated with a broader range of transmitted frequencies while an assembly of particles with magnified blockiness holds back the conduction of higher frequencies. Based on statistical analyses, we further show that the transition from spherical to non-spherical particles is associated with a broader range of void ratios and increased coordination numbers whereas inflated blockiness brings an opposite impact, and these changes are linked with the observed effect of particle shape on the characteristics of elastic waves, in both time and frequency domains.

Drag on a spheroidal particle at clean and surfactant-laden interfaces: effects of particle aspect ratio, contact angle and surface viscosities

Abstract

Sand-sand and sand-steel interface grain-scale behavior under shearing

The behavior of a sand-steel interface can be different than the shearing behavior of a sand. This study focuses on the effect of shear loading on a sand-to-sand and a sand-steel interface as well as on particle breakage. A series of ring shear and simple shear tests were conducted to measure the effects of particle shape and surface roughness characteristics before and after shearing at the shear zones within a sand-to-sand and at a sand-steel plate interface. It was found that sand density had a large effect on the interface shearing behavior. While interface sliding began at a shear displacement of 0.9 mm (corresponding to 22% of the total shear displacement) in dense specimens, this occurred at a much higher displacement of 5.6 mm (62% of total displacement) in loose sand at rough steel interface. Sliding displacement prevailed at the sand-steel plate interface in simple shear tests as the sand sample became denser. Critical states were observed in the sand-to-sand ring shear tests after shear displacements of 65–79 mm and 87–98 mm in medium-dense and loose samples, respectively. Comparing sand-to-sand and sand-steel interface results indicated that sand-steel interface behaves less dilative with the critical state occurring at much smaller shear displacements of about 12–14 mm at the interface compared to that occurring within the sand at 65–79 mm. Both loose and dense sands reached a residual strength of 98 kPa at a shear displacement of 7 mm in simple shear tests. The inter-particle shear resistance and particle crushing, combined with particle rearrangement/rotation were the mechanisms which developed shear resistance. An interface friction angle of 27° was subsequently mobilized at the critical states of the simple shear tests, while particle crushing and abrasion resulted in a slightly elevated (by 2°) interface friction angle in the ring shear tests. The difference between sand-steel interface and sand-to-sand interaction mechanism highly influenced particle damage induced by particle fracturing and abrasion. As a result of particle crushing, roughness and aspect ratio of sand particles sheared at the interface with a steel plate reduced by respectively 39.7% and 12–14% sheared at normal stresses of 50 to 200 kPa. In particular, particles' circularity increased from 0.685 to 0.817, indicating symmetrically rounded particles. The maximum size of particles in all tests reduced to about 16.9–18.1% of the original maximum particle size (Dmax = 0.85 mm). While the skewness of particles sheared within the sand increased from −0.231 to 0.656 (indicating increased surface roughness), those of particles collected from the sand-steel interface decreased to −1.31 reflecting smoother particle surfaces.

Effects of volume fraction and particle shape on the rheological properties of oblate spheroid suspensions

Coupled lattice Boltzmann and discrete element methods were employed to investigate the rheological properties of oblate spheroid suspensions in a Newtonian fluid. The volume fraction of the particles is varied along with the particle aspect ratio. As the particle shape is varied from sphere to oblate, we observe an increase in the relative viscosity as well as an increase in the particle contacts and the contact distance. The more oblate particles in denser suspensions are observed to reorient systematically subject to the shear flow. We recast the viscosity data using the Krieger–Dougherty formula and report the modified Einstein coefficients.

Combined FIB/SEM tomography and TEM analysis to characterize high aspect ratio Mg-silicate particles inside silica-based optical fibres

The evolution of particles, during the fabrication step of particle-containing glass fibres, was investigated. The aim of this study was to characterize quantitatively the composition, size and shape distribution of Mg-silicate particles within the initial silica-based preform and the final fibre, these information being crucial to better understand the mechanisms involved during the drawing step. A combined experimental approach using Scanning electron microscopy, Transmission electron microscopy and FIB/SEM tomography was implemented. The particle composition was found to stay unchanged during the drawing step. On the other hand, strong modifications of their shape within the fibre is observed and discussed. • The microstructure of high aspect ratio particles within optical fibres is analysed by electron microscopy techniques • The measured composition of particles is the one of the enstatic phase • 3D imaging of the particles by FIB/SEM tomography reveals their morphology changes induced by the drawing step

Viscoelastic behavior of suspensions of reduced graphene oxide nanoparticles in epoxy

The viscoelastic behavior of dilute suspensions of reduced graphene oxide (RGO) nanoparticles in a liquid SU8 epoxy dissolved in gamma butyrolactone was investigated at RGO concentrations in the range from 0.16 to 1.07 vol. %. Our results revealed that the addition of only 1 vol. % of RGO can increase the complex viscosity of the composites by up to 3 orders of magnitude, which is crucial for processing purposes. A marked shear thinning behavior with a yield stress was, moreover, found to emerge at low RGO concentrations for sonicated homogenized suspensions. This behavior was attributed to the partial polymerization of the epoxy and bonding between the epoxy rings and the residual functional groups on the RGO induced by the energy intensive ultrasound process that is key to achieve a good dispersion of the nanoparticles, in combination with the formation of a gel-like, percolated nanocomposite network. The percolation threshold and the aspect ratio of the RGO particles were derived from the analysis of the elastic shear modulus of the suspensions and found to be equal to 9 × 10−4 and 500, respectively. We attribute this extremely with low percolation threshold to the restriction of the polymer chain mobility within the well dispersed, high aspect ratio RGO network.

The role of intermetallic particles on mode I crack propagation mechanisms in metal plates

In metal plates, the crack propagation mechanism sets the amount of the plastic deformation before failure: a slanted or a cup–cone crack typically yields limited plate thinning within the fracture process zone, while large deformation precedes cup–cup crack propagation. The present work investigates the effect of intermetallic particles on the propagation mechanisms and the associated fracture surface morphologies when tearing Al 1050 plates under far-field mode I loading. Both single edge notched and double edge notched tension specimens, with thicknesses ranging from 0.5 to 5 mm, were tested. The chemical compositions of intermetallic particles were determined by performing energy dispersive X-ray measurements, and their morphological features were characterized by Scanning Electron Microscopy (SEM). Likewise, SEM images were taken to display the fracture surfaces, and the details of the surface morphology were visualized in three dimensions by using X-ray Tomography scanning. The experimental results indicate that an increase in the volume fraction, size, and aspect ratio of the intermetallic particles all promote slanted/cup–cone cracks, while a low amount of small, circular particles leads to cup–cup cracks. Furthermore, two-dimensional finite element simulations for mode I crack propagation support the experimental findings.

Numerical Modeling and Parametric Study of the Melting Behavior of Ice Crystal Particles

Ice crystal icing has been identified as a risk to flight safety, due to its ability to reduce engine performance and its potential to cause engine damage and flameout. A critical factor in the sticking efficiency of ice crystals is the melting behavior of ice crystals. This paper presents an ice melting model integrated with surface blowing and porosity. A parametric study is performed to understand the effects of flow conditions (total pressure, humidity, total temperature, Mach number, and slip velocity) and particle properties (particle size, aspect ratio, and porosity factor) on ice melting behavior. The model is compared against the melting time of single ice particles in an acoustic levitator. The results show that employing sphericity and porosity could improve prediction performance. Surface blowing from evaporation increases melting time, and its effect is larger with a higher gas temperature. Ice melting time increases with pressure at high humidity, whereas an opposite trend is observed at low humidity. Increasing gas temperature, humidity, slip velocity, aspect ratio, and particle temperature can decrease ice melting time, whereas increasing Mach number, particle size, and porosity factor can increase ice melting time. Although wet bulb temperature can be used to evaluate ice melting potential when only one flow parameter is varied, it appears insufficient alone under scenarios where multiple flow parameters are changing simultaneously.

Bioinspired particle engineering for non-invasive inhaled drug delivery to the lungs

Pulmonary drug delivery is governed by several biophysical parameters of delivery carriers, such as particle size, shape, density, charge, and surface modifications. Although much attention has been given to other parameters, particle shape effects have rarely been explored. In this work, we assess the influence of particle shape of inhaled delivery carriers on their aerodynamic properties and macrophage uptake by using polymeric microparticles of different geometries ranging in various sizes. Doxorubicin was conjugated to the polymer particles and the bioconjugates were characterized. Interestingly, the results of in-vitro lung deposition, performed using a next generation impactor, demonstrated a significant improvement in the aerodynamic properties of the rod-shaped particles with a high aspect ratio as compared to spherical particles with the same equivalent volume. The results of a macrophage uptake experiment demonstrate that the high aspect ratio particles were phagocytosed less than spherical particles. Furthermore, the cytotoxicity of these doxorubicin-conjugated particles was determined against murine macrophages, resulting in reduced toxicity when treated with high aspect ratio particles as compared to spherical particles. This project provides valuable insights into the influence of particle shape on aerodynamic properties and primary defense mechanisms in the peripheral lungs, while using polymeric microparticles of various sizes and geometries. Further systematic development can help translate these findings to preclinical and clinical studies for designing efficient inhalable delivery carriers.

Impacts of particle shape on sedimentation of particles

The effects of particle shape on settling behavior of sand particles under the influence of gravity in an initially quiescent water environment have been studied by a two-dimensional high-resolution numerical experiment. The numerical method is based on an immersed boundary method. The accuracy of the code is well demonstrated against experimental data of a fluttering rectangular plate. The particle is either elliptical or rectangular of size about 1 mm, and the particle aspect ratio is varied without changing the volume. The particle-to-fluid density ratio is chosen to be 2.65, and the resulting Galileo number is about 127. It is found that the terminal settling velocity tends to decrease with an increase in the aspect ratio and that the settling velocity of a rectangle is always lower than that of the corresponding ellipse in both the isolated and multiple particle systems. While the presence of surrounding particles makes the influence of the particle shape less clear, the circular particle still presents significant differences in the terminal settling velocity compared with the other particles. It, therefore, calls for caution in the spherical assumption of particle shape and encourages a deeper understanding of the particle shape effects.

Experimental production of charcoal morphologies to discriminate fuel source and fire type: an example from Siberian taiga

Abstract. The analysis of charcoal fragments in peat and lake sediments is the most widely used approach to reconstruct past biomass burning. With a few exceptions, this method typically relies on the quantification of the total charcoal content of the sediment. To enhance charcoal analyses for the reconstruction of past fire regimes and make the method more relevant to studies of both plant evolution and fire management, the extraction of more information from charcoal particles is critical. Here, I used a muffle oven to burn seven fuel types comprising 17 species from boreal Siberia (near Teguldet village), which are also commonly found in the Northern Hemisphere, and built on published schemes to develop morphometric and finer diagnostic classifications of the experimentally charred particles. I then combined these results with those from fossil charcoal from a peat core taken from the same location (Ulukh-Chayakh mire) in order to demonstrate the relevance of these experiments to the fossil charcoal records. Results show that graminoids, Sphagnum, and wood (trunk) lose the most mass at low burn temperatures (<300 ∘C), whereas heathland shrub leaves, brown moss, and ferns lose the most mass at high burn temperatures. This suggests that species with low mass retention in high-temperature fires are likely to be under-represented in the fossil charcoal record. The charcoal particle aspect ratio appeared to be the strongest indicator of the fuel type burnt. Graminoid charcoal particles are the most elongate (6.7–11.5), with a threshold above 6 that may be indicative of wetland graminoids; leaves are the shortest and bulkiest (2.1–3.5); and twigs and wood are intermediate (2.0–5.2). Further, the use of fine diagnostic features was more successful in separating wood, graminoids, and leaves, but it was difficult to further differentiate these fuel types due to overlapping features. High-aspect-ratio particles, dominated by graminoid and Sphagnum morphologies, may be robust indicators of low-temperature surface fires, whereas abundant wood and leaf morphologies as well as low-aspect-ratio particles are indicative of higher-temperature fires. However, the overlapping morphologies of leaves and wood from trees and shrubs make it hard to distinguish between high-intensity surface fires, combusting living shrubs and dead wood and leaves, and high-intensity crown fires that have burnt living trees. Distinct particle shape may also influence charcoal transportation, with elongated particles (graminoids) potentially having a more heterogeneous distribution and being deposited farther away from the origin of fire than the rounder, polygonal leaf particles. Despite these limitations, the combined use of charred-particle aspect ratios and fuel morphotypes can aid in the more robust interpretation of fuel source and fire-type changes. Lastly, I highlight the further investigations needed to refine the histories of past wildfires.

Dispersion of Natural Airborne TiO2 Fibres in Excavation Activity as a Potential Environmental and Human Health Risk.

Titanium is the ninth most abundant element, approximately 0.7% of the Earth crust. It is used worldwide in large quantities for various applications. The IARC includes TiO2 in Group 2B as possibly carcinogenic to humans suggesting that pathological effects correlate to particle size and shape. This study case quantifies the release of natural TiO2 particles during mining activity, involving meta-basalt and shale lithologies in the Ligurian Alps, during excavation of the Terzo Valico as part of the Trans-European Transport Network. Type, width, length, aspect ratio, and concentration of TiO2 particles in needle habit were determined. The different samplings have reported that airborne concentrations in meta-basalt were 4.21 ff/L and 23.94 ff/L in shale. In both cases, the concentration never exceeds the limits established by various organizations for workers health protection. Nevertheless, TiO2 elongated particles, recognized as rutile, showed the dimensional characteristic of fibres, as reported by WHO. These fibres deserve particular attention because they can reach the alveolar space and trigger inflammation and chronic diseases. The results indicate that monitoring the TiO2 in both working environments and Ti-rich geological formations, associated with epidemiological studies, may represent a useful tool to determine the exposure risk of workers and the general population.

Application of the Folgar–Tucker model to predict the orientation of particles of different aspect ratios in polymer suspensions

Abstract The increasing requirements on plastic parts demand a rising use of combined functional and reinforcing materials. Therefore, often reinforcing particles with different aspect ratios are added to the plastic as additive mixtures. However, the engineering design process of reinforced parts requires an early knowledge of the expected orientation of the reinforcing particles. Numerous models try to predict the orientation of particles in polymer suspensions. However, the interaction coefficient strongly depends on the aspect ratio of the particles and a prediction of the orientation behavior of additive mixtures with differently shaped particles has not been validated using conventional methods. In this work, the orientation of differently shaped particle mixtures in polymer suspensions is investigated for different fluid channel geometries. Finally, the Folgar–Tucker model is applied to filler mixtures and implemented into OpenFOAM®, which enables the comparison of filler orientation in different fluid channel geometries. Regarding the experiments a characteristic increase of the interaction coefficient was observed at a filling level of 5%. Furthermore, it was shown that a balanced mixing ratio yields higher interaction coefficients. With regard to the performed simulations, it was possible to show qualitatively how a considered interaction between fibers and platelets affects the orientations.

Understanding the Mechanical Reinforcement of Metal-Organic Framework-Polymer Composites: The Effect of Aspect Ratio.

The aspect ratio (AR) of filler particles is one of the most critical determinants for the mechanical properties of particle-reinforced polymer composites. However, it has been challenging to solely study the effect of particle AR due to the difficulties of controlling AR without altering the physical and chemical properties of the particle. Herein, we synthesized PCN-222, a zirconium-based porphyrinic metal-organic framework (MOF) with preferential longitudinal growth as a series of particles with ARs increasing from 3.4 to 54. The synthetic MOF conditions allowed for the chemical properties of the particles to remain constant over the series. The particles were employed as reinforcers for poly(methyl methacrylate) (PMMA). MOF-polymer composite films were fabricated using doctor-blading techniques, which facilitated particle dispersion and alignment in the PMMA matrix, as revealed by optical microscopy and wide-angle X-ray diffraction. Mechanical measurements showed that both elastic and dynamic moduli increased with particle AR and particle concentrations but started to decrease as particle loading increased beyond 0.5 wt % (1.12 vol %). The data obtained at low particle loadings were fitted well with the Halpin-Tsai model. In contrast, the percolation model and the Cox model were unable to adequately fit the data, indicating the mechanical reinforcement in our system mainly originated from efficient load transfer between particles and the matrix in the particle orienting direction. Finally, we showed that the thermal stability of composite films increased with the addition of MOF particles because of the high thermal degradation temperature and restricted polymer chain mobility.

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.

Wear Assessment of Tibial Inserts Made of Highly Cross-Linked Polyethylene Supplemented with Dodecyl Gallate in the Total Knee Arthroplasty

Background: the wear of tibial insert is still one of primary factors leading to failure of total knee arthroplasty (TKA). Dodecyl gallate (DG) has shown improvements in the oxidation stability of highly cross-linked polyethylene (HXLPE). This study aimed to assess the application of HXLPE supplemented with DG (HXLPE-DG) on the tibial insert in TKA concerning the wear resistance and the potential impact on implant fixation; Methods: tibial inserts made of HXLPE-DG were subjected to a 3 million loading-cycle wear test following ISO 14243-1:2009. The loss of mass and wear rate of the tibial inserts were calculated. The quantity, size,- and shape of wear particles were recorded; Results: the test specimens lost an average mass of 16.00 mg ± 0.94 mg, and were on an average wear rate of 3.92 mg/million cycles ± 0.19 mg/million cycles. The content of wear particles in the calf serum medium was 3.94 × 108 particles/mL ± 3.93 × 107 particles/mL, 96.66% ± 0.77% of the particles had an equivalent circular diameter less than 0.5 μm. The aspect ratio of wear particles was 1.40 (min: 1.01; max: 6.42). Conclusions: HXLPE-DG displayed advantages over the commonly used materials for tibial inserts and presented the potential of application in TKA.

Drag and heat transfer coefficients for axisymmetric nonspherical particles: A LBM study

In this work, the thermal lattice Boltzmann method with immersed moving boundary conditions has been employed to calculate the drag coefficients and Nusselt numbers for isolated axisymmetric nonspherical particles in uniform flow for a wide range of Reynolds numbers (Re) and aspect ratios (Ar). The simulation results demonstrate that the drag coefficient for ellipsoids and a spherocylinder evolves as sin2θ, viz. θ being the incident angle of the particle. However, the Nusselt number for prolate ellipsoids evolves as sinmθ, viz. m being a function of particle aspect ratio. A correlation for m has been developed for 1<Ar≤4 based on the simulation data. The Nusselt number for a spherocylinder of aspect ratio 4 evolves as sin1.278θ. Moreover, we observe that the drag coefficients and Nusselt numbers for a prolate ellipsoid of aspect ratio 4 and a spherocylinder of aspect ratio 4 are comparable with a maximal relative deviation 5%. Based on the simulation data and the data from literature, a drag correlation for oblate (0.2≤Ar<1 and 1≤Re≤100) and prolate (1≤Ar≤5 and 1≤Re≤2000) ellipsoids has been developed to broaden the range of applicability of existing drag correlations. The average relative deviation between the drag correlation and the fitting data is 9.4%. The Nusselt number correlation for prolate ellipsoids is valid for 1≤Ar≤4 and Re≤500. On the other hand, the Nusselt number correlation proposed for a spherocylinder is valid for Ar=4 and Re≤500. The new drag and Nusselt number correlations are expected to improve the accuracy of Euler-Lagrangian simulations of non-isothermal particulate flows comprising ellipsoids or spherocylinders.

A new microstructure‐based multiaxial fatigue life prediction model for A319 casting alloys

AbstractThis paper describes a microstructure‐based multiaxial nonproportional fatigue life prediction model applied to A319 alloy. The materials made with different casting cooling rates and Sr modification are characterized and quantified in terms of secondary dendrite arm spacing, size, and aspect ratio of eutectic Si particles. Multiaxial nonproportional fatigue tests have been performed on six groups of A319 alloys to systematically analyze the effect of microstructure and loading path on the fatigue properties of Al–Si cast alloy. The first part of the paper is focused on microstructure quantitative characterization to determine the influence of different casting conditions, followed by stress response behavior and fatigue fracture analysis. Finally, a new multiaxial fatigue life prediction model for Al–Si alloy is proposed, for which 56% of the data points fall in the bound lines of factor of 2 and 83% of the data points fall in the bound lines of factor of 3.

An In Situ and Real-Time Plasmonic Approach of Seed/Adhesion Layers: Chromium Buffer Effect at the Zinc/Alumina Interface

The effect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated films by a combination of surface science techniques, among which a very flexible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al2O3(0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn films during their growth are analyzed from sub-monolayer to multilayer thicknesses. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. With regard to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the sub-monolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coefficient from 0 to nearly 1 due to a diffusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr film percolates, it forms an adhesion layer on which Zn grows in a nearly two-dimensional mode. In all cases, Cr films are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces, which, in line with atomistic simulations, is assigned to the formation of more favorable Cr–Al2O3 and Cr–Zn than Zn–Al2O3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of films is of great practical interest. It shows that noncontinuous and partially oxidized films of additives, closer to realistic cases of application, can strongly enhance the sticking of films. Also, anchoring a functional film by discrete predeposited seeds can keep its properties intact.

Yield stress and wall slip of kaolinite networks

In this work, the nonlinear rheological properties of kaolinite suspensions as a model fluid were studied. Three kaolinites with different average particle, aspect ratio and asperity were used to gain a deeper understanding of the effects of particle's geometry on yielding, rheology, and apparent slip velocity over a wide range of concentrations in the semi-concentrated to concentrated regime. To reveal the impact of geometry on kaolinite suspension's rheology, atomic force microscopy was employed, thus revealing unique information about the particle's structural differences. The apparent slip velocity was studied through the application of the Mooney technique using experimental data from parallel-plate geometry at various gap spacings. Sandpaper of varying grit was used to eliminate slip and estimate the true rheological parameters including the yield stress of these materials. Kaolinite suspensions were found to follow a Herschel–Bulkley model with yield stress that increases with concentration irrespective of the type of kaolinite particles. The aspect ratio and asperity of particles were found to alter their slip behavior by enabling the kaolinite network to have a tunable yield stress. It was also determined that the existence of resilient larger clusters at higher shear rates often induce higher slip effects.