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Particle morphology has great influence on mechanical behaviour and hydro/thermal/electrical conductivities of granular materials. Surface reconstruction and mesh generation are critical to consider realistic particle shapes in various computational simulations. This study adopts the combined finite- discrete element method (FDEM) to investigate single particle crushing behaviour. Particle shapes were reconstructed with spherical harmonic (SH) in both spherical and Cartesian coordinate systems. Furthermore, the reconstructed surface mesh qualities in two coordinate systems are investigated and compared. Although the efficiency of the two SH systems in reconstructing star-like shapes is nearly identical, SH in Cartesian coordinate system can reconstruct non-star-like shapes with the help of surface parameterisation. Meanwhile, a higher triangular mesh quality is generated with spherical coordinate. In single particle crushing tests, the low mesh quality produces more fluctuations on load–displacement curves. The particles with more surficial mesh elements tend to have a lower contact stiffness due to more contact stress concentrations induced by complexity of morphology features and more volumetric tetrahedral elements. The fracture patterns are also influenced by mesh quality and density, e.g. a particle with fewer mesh elements has a simpler fragmentation pattern. This study serves as an essential step towards modelling particle breakage using FDEM with surface mesh directly from SH reconstruction.

Discrete Element Method (DEM) simulations are carried out to investigate the packing structure of the vibrated mono-sized and polydisperse particle assemblies in slender prismatic containers. Quantities like void fraction distributions, coordination number distribution, Voronoi packing fraction distributions were analyzed. DEM simulations show excellent agreement with the experimental data. The transient response of the packing structure during the vibration of mono-sized spheres reveals that the crystallization starts at the walls. The packing structure's crystallinity enhances during vibration, increasing the packing density of the assembly. Vertical vibration is found to be more effective compared to lateral vibration. The packing structure is found to be independent of the container's orientation during the filling process. The influence of the ratio of the container dimensions to mean particle diameter (X/dm) and polydispersity (λ) on the packing structure is analyzed. The results showed that the increase of X/dm ratio or λ, increases the random arrangement of particles.

The morphological structure of granular materials can dominate their mechanical, hydraulic, electrical, and thermal properties; thus, the formation of order and disorder arrangement of particles is the key characteristic to describe such micro-structures. During the packing procedure, external energy input and perturbations, such as dynamic cyclic load, and the particle size distribution were mentioned by many previous works to be significant in controlling the structure. This study explores order to disorder transition within 2D binary granular assemblies under vibration. A numerical packing method combining particle size growth and dynamic vibration is implemented and verified against experiments to achieve desired packing structure. The Bond orientation order number and pair distribution function are used as indices for characterising the morphological structure. Our packing simulation results show a combination of appropriate vibration and a proper particle size growth rate can facilitate the formation of granular crystallisation. Additionally, for binary packing, the size ratio, and the number fraction of small particles can play a critical role in determining the morphological transformation.

The macroscopic behaviour of an assembly of polydisperse spherical particles is studied using a numerical model based on discrete element method (DEM), which accounts for the microscopic interactions between individual particles and their damage. DEM models particle–particle interactions enabling to understand the influence of microscopic particle–particle interactions on the macroscopic response. The method is used to stimulate the mechanical response of a polydisperse particle assembly under uniaxial compressive load. The influence of damage rate and the initial packing fraction on the macroscopic stress–strain response is investigated. The analysis shows that the initial nonlinear elastic behaviour is influenced by the initial packing factor, whereas the critical stress is influenced by both initial packing fraction and damage rate. It is also observed that critical stress occurs when the assembly reaches a particular damage state. Furthermore, the failure behaviour of different sized particles within a polydisperse assembly is also investigated. The experimental data from the literature show that the crush strength of the particle of given size is observed to vary over a range. Such variation of crush strengths for a given particle size is also implemented in the present work.

International audience; We experimentally investigated the packing of wet monodisperse spheres with controlled falling height. The packing fraction are found to decrease with smaller grain size and free fall heights. A model describing the effects of interparticle force and falling height on packing fraction is developed by introducing a dimensionless length scale, representing the extent of particle rearrangement towards a denser state due to impacts of falling grains. A universal law is observed for both wet particles where capillary forces dominate and dry powders where van der Waals forces govern the packing behaviour. This study deepens the understanding of packing of cohesive spheres and provide a simple experimental method for generating granular media with tailored packing fraction.

The dynamic contact angle of a gas–liquid–solid system depends on the contact line velocity and ignoring this effect could lead to inaccurate estimations of the capillary pressures in microporous media. While most existing coarse-grained molecular dynamics (CGMD) models use one particle to represent a few molecules, we present a novel CGMD framework to model microscale CO2/water flows in silica with each particle representing hundreds of thousands of molecules. The framework can reproduce the densities and viscosities of water and CO2, water–CO2 interfacial tension, and static contact angle over a wide range of pressures. The validated framework is applied to study the velocity-dependency of contact angle of the microscale CO2–water–silica system. The results indicate that the assumption in the molecular kinetic theory that liquid–solid interaction is similar to the reversible work of adhesion between liquid and solid may not hold for CO2–water–silica systems.

Water management remains one of the major challenges in optimising the performance of PEMFCs, in which liquid accumulation and removal in gas diffusion layers (GDLs) and flow channels should be addressed. Here, effects of GDL surface roughness on the water droplet removal inside a PEMFC flow channel have been investigated using the Volume of Fluid method. Rough surfaces are generated according to realistic GDL properties by incorporating RMS roughness and roughness wavelength as the main characteristic parameters. Droplet dynamics including emergence, growth, detachment, and removal in flow channels with various airflow rates are simulated on rough substrates. The influences of airflow rate on droplet dynamics are also discussed by comparing the detachment time and droplet morphology. The liquid removal efficiency subject to different surface roughness parameters is evaluated by droplet detachment time and elongation, and regimes of detachment modes are identified based on the droplet breakup location and detachment ratio. The results suggest that rough surfaces with higher RMS roughness can facilitate the removal of liquid inside flow channel. Whilst surface roughness wavelength is found less significant to the liquid removal efficiency. The results here provide qualitative assessments on identifying the key surface characteristics controlling droplet motion in PEMFC channels.

In this paper, we investigate the initiation and growth of instability patterns arising from the shock loaded internal surfaces of granular rings confined in a Hele-Shaw cell using both experimental and numerical approaches. A variety of patterns are formed in granular media consisting of grains with varying morphologies. When the particle shape becomes increasingly irregular, and/or the gap in the Hele-Shaw cell becomes narrower, it is increasingly hard for confined particles to fluidize. Consequently the emergent pattern transitions from a smooth circle with trivial undulation which grows in a self-similar manner to an unstable finger-like structure with significant tip-splitting. The distinct growth mode of the well-defined instability pattern is closely associated with its inception phase alongside the transmission of the compaction front. The runaway growth of the incipient perturbations gives rise to the unstable growth of the late-time finger-like instabilities. Conversely the minimal growth of the perturbations in the inception phase guarantees the ensuing self-similar growth of the instability patterns featuring insignificant corrugation. The grain-scale simulations reveal the fundamental role played by the heterogeneous non-linear force network inherent to granular media in the stable-to-unstable transition of the instability pattern. The present work reveals the correlation between the grain-scale physics underpinning the formation of surface instability upon shock loading granular media and the nature of the resulting macro-scale instability patterns. The macroscopic flowability of particles through the confined space is found to be the foremost indicator of the nature of the shock induced granular instability pattern.

Normal contact behaviour between non-adhesive fractal rough particles is studied using a finite element method (FEM). A series of spherical grain surfaces with distinguished roughness features are generated by means of Spherical Harmonics. These surfaces are described by two roughness descriptors, namely, relative roughness (Rr) and fractal dimension (FD). The contact behaviour of rough spheres with a rigid flat surface is simulated using FEM to quantify the influences of surface structure and sphere morphology by focusing on contact stiffness and true contact area. The dependence of normal contact stiffness (k) on applied normal force (F) is found to follow a power law (k = αFβ) over four orders of magnitude, with both α and β being highly correlated with Rr and FD. With increasing load, the power exponent converges to that of Hertzian contact, e.g., 1/3, independent of Rr. Regions of true contact evolve through the formation of new microcontacts and their progressive merging, meanwhile the area distributions of contact island induced by various forces tend to obey similar Weibull distributions due to fractal nature in their surfaces. Contacts with larger values of Rr are found to produce contact contours with higher fractal dimension as calculated by a 2D box-counting method. Our results suggest that the correlation between radial lengths in a quasi-spherical particle should be considered in studying contact behaviour.

In granular media, topological features are known to determine the effective material properties and boundary behavior when interacting with other structural components. X-ray computed tomography results are reported on sphere packing structures in slender prismatic containers (X = 20, Y = Z = 80 mm), filled and vibrated with both monosized spheres (diameter d = 2.4 mm), Exp. (M), and polydisperse spheres (1 mm < d < 1.25 mm), Exp. (P). Packing structures were characterized by void fraction distributions, coordination numbers, contact angle distributions and Voronoi packing fractions. In (M), an almost perfect hexagonal dense packing exists in the total volume, associated with a packing fraction γ t≈0.68. In additional packing experiments, large γ t values were achieved as well. Although the d spread in (P) is relatively small, significantly different results are obtained: γ t≈0.62, regular structures are restricted to narrow wall zones and distributions in the container volume are nonhomogeneous. It is argued that the small degree of ordered structure is a characteristic feature of polydispersity for efficiently vibrated sphere packings.

Magnesium alloys usually lack “operative deformation slip mechanisms” because of their hexagonal close-packed structure. Therefore, the mechanical behavior of magnesium alloys at different temperatures is dictated by other deformation mechanisms such as twinning, detwinning, secondary twinning, or dynamic recrystallization (DRX). Twinning and DRX can affect the development of grain size and orientation distribution, as well as the deformation behavior of magnesium alloys. The current understanding of the mechanisms and mechanics of these different deformation modes and their implementation in crystal plasticity-based modeling are highlighted in this article. Future directions in the development of constitutive models are also discussed.

When particle rings/shells are subjected to divergent explosive loadings, a dual overlapping particle jetting structure emerges during the shock interaction timescale which consists of a large number of minor jets initiated from the external interface at very early instants and a much reduced number of major jets formed from the internal interface at delayed times but overtaking the minor jets in later times. In the present work, the formation of the hierarchical particle jetting pattern is investigated numerically by discrete element method (DEM) coupled with finite element method (FEM), which execute the mechanical calculations of particles and the explosive/detonation gases, respectively. The numerical results find that the external jetting arises from the spallation of an outer layer pulled away by inward propagating rarefaction waves. Meanwhile an inner compact band re-compressed by a secondary shock remains densely packed while expanding outward. The fragmentation of the inner compact particle band, preceding the internal particle jetting, is caused by the profuse spiral shear failures expanding from the inner radius to the outer radius. The resultant jetting structure depends on the shear-band spacing and the grouping of the clockwise and counterclockwise shear bands as well. The former is a function of the bulk characteristics of the inner compact band, especially the resistance to the shear flows. The latter markedly varies with the microstructure of particle packing, especially the structural order. In the highly ordered extreme, the particle ring with global crystalline structure exhibits six groups of shear bands, probably giving rise to around six fragments. By contrast, the grouping of shear bands in the amorphous packing is far from definite, suggesting an increased number of much smaller fragments to be generated. The dual jetting structure would degenerate into a single jetting pattern if the inner compact band manages to entrain all the spall particles before the shear failure occurs. (C) 2019 Elsevier Ltd. All rights reserved.

To model the effective conductivity of the solid and the pore phase of a lithium-ion battery (LIB) we make use of the resistor network method (RN). We recall the scheme on how resistor networks can generally be set up and numerically solved. Furthermore, we explain how this general method can be applied to an assembly of spherical particles where for the individual resistances between touching particles an analytical formula is being used. As a new feature, we use the same scheme to setup resistor networks for the pore phase of an assembly of spherical particles where we propose a simple geometric approach for the calculation of the individual resistances of pore throats. For the validation of this method we created several random particle structures with different size distributions and calculated effective conductivities with both the RN and the finite element method (FEM). On the one hand, the comparison between RN and FEM shows a very good performance of the RN because the mean error lies within 4%. On the other hand, the RN results always lie within the well-known theoretical bounds for the effective conductivity in porous media. As an important aspect, the RN has proven to be highly efficient concerning the computation time and the resource costs.

Multiphase flow through a porous medium involves complex interactions between capillarity, wettability and gravity during drainage process. In contrast to these factors, the effect of pore distribution on liquid retention is less understood. In particular, the quantitative correlation between the fluid displacement and the level of disorder has not yet been established. In this work, we employ direct numerical simulation by solving the Navier–Stokes equations and using volume of fluid method to track the liquid–liquid interface during drainage in disordered porous media. The disorder of pore configuration is characterized by an modified index to capture small microstructural perturbation, which is pivotal for fluid displacement in porous media. Then, we focus on the residual volume and morphological characteristics of saturated zones after drainage and investigate the effect of disorder under different wettability (i.e., the contact angle) and gravity (characterized by a modified Bond number) conditions. Pore-scale simulations reveal that the highly-disordered porous medium is favourable to improve liquid retention and provide various morphologies of entrapped saturated zones. Furthermore, the disorder index has a negative correlation to the characteristic curve index (n) in van Genuchten equation, controlling the shape of the retention characteristic curves. It is expected that the findings will benefit to a broad range of industrial applications involving drainage processes in porous media, e.g., drying, carbon sequestration, and underground water remediation.

Dynamic wetting plays an important role in the physics of multiphase flow, and has a significant influence on many industrial and geotechnical applications. In this work, a modified smoothed particle hydrodynamics (SPH) model is employed to simulate surface tension, contact angle and dynamic wetting effects at meso-scale. The wetting and dewetting phenomena are simulated in a capillary tube, where the liquid particles are raised or withdrawn by a shifting substrate. The SPH model is modified by introducing a newly developed viscous force formulation at the liquid–solid interface to reproduce the rate-dependent behaviour of the moving contact line. Dynamic contact angle simulations with the interfacial viscous force are conducted to verify the effectiveness and accuracy of this new formulation. In addition, the influence of interfacial viscous forces with different magnitude on the contact angle dynamics is examined by empirical power-law correlations; the derived constants suggest that the dynamic contact angle changes monotonically with the interfacial viscous force. The simulation results are consistent with experimental observations and theoretical predictions, implying that the interfacial viscous force can be associated with the slip length of flow and the microscopic surface roughness. This work demonstrates that the modified SPH model can successfully account for the rate-dependent effects of a moving contact line, and can be used for realistic multiphase flow simulations under dynamic conditions.

International audience; We experimentally study the drying of loosely packed wet glass beads at low initial water content. The drying rate is found to decrease at the start, corre- sponding to the decreasing rate period controlled by vapor diffusion, followed by a deviation in drying rate from the diffusion limited evaporation. The prop- agation of drying front associated with a sharp saturation gradient is identified through both image analysis and magnetic resonance imaging technique. The drying-induced collapse of granular medium is observed and quantified. The concentrated collapse at the end of drying suggests the existence of liquid in the form of liquid bridges in the apparent dry region until the end of drying process. Collapse event is found to be local, i.e., a clear boundary can be iden- tified for each collapse event, below which the loosely packed medium remains intact. This indicates the existence of a saturation gradient in the apparent dry region. The drying dynamics and collapse statistics suggest that the observed transition of drying regimes is due to Kelvin effect. This work demonstrates for the first time the drying enhancement phenomenon due to Kelvin effect even for grains with size of hundreds of micrometers, and provides insights on the drying process of partially saturated granular materials, especially near the final period of evaporation.

Hypothesis Control of capillary flow through porous media has broad practical implications. However, achieving accurate and reliable control of such processes by tuning the pore size or by modification of interface wettability remains challenging. Here we propose that the flow of liquid by capillary penetration can be accurately adjusted by tuning the geometry of porous media and develop numerical method to achieve this. Methodologies On the basis of Darcys law, a general framework is proposed to facilitate the control of capillary flow in porous systems by tailoring the geometric shape of porous structures. A numerical simulation approach based on finite element method is also employed to validate the theoretical prediction. Findings A basic capillary component with a tunable velocity gradient is designed according to the proposed framework. By using the basic component, two functional capillary elements, namely, (i) flow amplifier and (ii) flow resistor, are demonstrated. Then, multi functional fluidic devices with controllable capillary flow are realized by integrating the designed capillary elements. All the theoretical designs are validated by numerical simulations. Finally, it is shown that the proposed model can be extended to three dimensional designs of porous media

Understanding the coupled thermo-mechanical behaviour of compacted granular beds can benefit various industrial applications, such as pebble bed design in fusion reactors. In this study, a thermo-mechanical discrete element method based on our previous work is improved and adapted to investigate the cyclic thermo-mechanical performance of gas-filled granular materials composed of elastoplastic grains. An interparticle contact model is developed considering the plastic deformation of grains. Through the simulation on a representative volume element of beryllium pebble beds, we provide grain-scale insight into the evolution of thermal conductivity and stress. The simulation results suggest that the network of thermal contacts is impeded by plastic deformation leading to a significant drop of thermal conductivity during cooling. This effect can be suppressed by increasing the initial packing factor. Not limited to pebble bed design, the conclusion of this work can also pave the way for optimizing powder-based manufacturing and energy storage, where combined thermo-mechanical loading conditions and elastoplastic deformation of individual particles are involved.

International audience; We study how grain shapes impact multiphase flow in porous media in the quasi-static regime using an extended pore-network model. The algorithm allows the explicit determination of different types of pore-scale instabilities and tracks the interface motion during fluid-fluid displacement process. It also includes the volume capacitance model, such that both the evolution of capillary pressure signal and sizes of Haines jumps can be captured. Further, it considers the pinning of menisci at sharp edges of grains, through which the distribution of effective contact angles can be obtained. Simulations are carried out across a wide range of wetting conditions for different particle shapes. Our results show that the effective contact angle distribution during displacement widens as the grain becomes more angular, which consequently modifies the macroscopic fluid invasion morphology. By analyzing various characteristic metrics during displacement, including capillary pressure signal, Haines jump size distribution, and fractal dimension, our results highlight the profound influence of particle shape on the multiphase flow.

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.

International audience; Tribological phenomena are governed by combined effects of material properties, topology and surface-chemistry. We study the interplay of multiscale-surface-structures with molecular-scale interactions towards interpreting static frictional interactions at fractal interfaces. By spline-assisted-discretization we analyse asperity interactions in pairs of contacting fractal surface profiles. For elastically deforming asperities, force analysis reveals greater friction at surfaces exhibiting higher fractality, with increasing molecular-scale friction amplifying this trend. Increasing adhesive strength yields higher overall friction at surfaces of lower fractality owing to greater true-contact-area. In systems where adhesive-type interactions play an important role, such as those where cold-welded junctions form, friction is minimised at an intermediate value of surface profile fractality found here to be in the regime 1.3-1.5. Our results have implications for systems exhibiting evolving surface structures.https://doi.org/10.1016/j.triboint.2015.09.016; Les phénomènes tribologiques sont régis par les effets combinés des propriétés des matériaux, de la topologie et de la chimie des surfaces. Nous étudions l’interaction des structures de surface multi-échelles avec les interactions à l’échelle moléculaire pour interpréter les interactions frictionnelles statiques aux interfaces fractales. Par discrétisation assistée par spline, nous analysons les interactions d’asperité par paires de profils de surface fractale en contact. Pour les aspérités se déformant élastiquement, l'analyse de la force révèle un frottement plus important sur les surfaces présentant une fractalité plus élevée, le frottement croissant à l'échelle moléculaire amplifiant cette tendance. L'augmentation de la force adhésive engendre un frottement global plus élevé sur les surfaces de fractalité inférieure en raison d'une plus grande surface de contact. Dans les systèmes où les interactions de type adhésif jouent un rôle important, tels que ceux où se forment des jonctions soudées à froid, le frottement est minimisé à une valeur intermédiaire de la fractalité du profil de surface qui se situe autour de 1,3–1,5. Les résultats ont des implications pour les systèmes présentant des structures de surface en évolution.https://doi.org/10.1016/j.triboint.2015.09.016

International audience; The present work explores water permeability of uniformly graded irregular grains using 3D printing with controlled shapes and fractal morphological features at low Reynold's number for viscous flow. From large amount of real 3D granular morphological data, a scaling law, in terms of fractal dimension, is found to be followed. With this universal law, sand grains with controlled fractal morphological features are generated using Spherical Harmonics, and then created using 3D printing technique for water permeability tests. A modified Kozeny-Carman equation is proposed through more accurate determination of specific area, as a function of relative roughness and fractal dimension, than approximation using the volume-equivalent sphere. By isolating the contributions from specific area, the shape coefficient is found to be insensitive to particle morphology. Through benchmarking the model prediction against experiments from both this work and past literature, we demonstrate the validity and wide applicability of the modified Kozeny-Carman equation.

International audience; Drying-induced cracking is widely encountered in nature and is of fundamental interest in industrial applications. During desiccation, the evolution of water content is nonlinear. Considering the inhomogeneous procedure of desiccation, it is worth considering whether water content will affect the crack pattern formation. To address this concern, in this paper, we report an experimental investigation on the effect of water content on the failure mode in drying colloidal films. A distinct failure transition from random cracking to curling is found when the initial water content increases gradually. When the water content is below a critical value for given film thickness, random desiccation cracking driven by shrinkage is observed. Beyond this critical water content, the film curls with the advent of several main cracks. It is also found that the critical water content corresponding to the transition point depends on the film thickness. In order to qualitatively interpret the experimental observation, a theoretical model is established by adopting the fracture mechanics based on the energy method. The model is found to agree well with the experimental results, elucidating the effects of initial water content on the crack patterns and the transition of failure modes.

The effects of wettability on multiphase displacement in porous media have been studied extensively in the past, and the contact angle is identified as an important factor influencing the displacement patterns. At the same time, it has been found that the effective contact angle can vary drastically in a time-dependent manner on rough surfaces due to the Cassie-Wenzel wetting transition. In this study, we develop a theoretical model at the pore scale describing the apparent contact angle on a rough interface as a function of time. The theory is then incorporated into the lattice Boltzmann method for simulation of multiphase displacement in disordered porous media. A dimensionless time ratio, Dy, describing the relative speed of the wetting transition and pore invasion is defined. We show that the displacement patterns can be significantly influenced by Dy, where more trapped defending ganglia are observed at large Dy values, leading to lower displacement efficiency. We investigate the mobilization of trapped ganglia through identifying different mobilization dynamics during displacement, including translation, coalescence, and fragmentation. Agreement is observed between the mobilization statistics and the total pressure gradient across a wide range of Dy values. Understanding the effect of the wetting transition during multiphase displacement in porous media is of importance for applications such as carbon geosequestration and oil recovery, especially for porous media where solid surface roughness cannot be neglected.

Fabric of a granular assembly represents the topology of the contact network. This paper investigates the evolution of contact anisotropy (fabric) and average coordination number for a granular assembly subjected to uniaxial compression through the Discrete Element Method (DEM). A monosize three-dimensional random close-packed granular assembly with periodic boundary conditions under uniaxial compression is considered in this work. The fabric evolution is studied by post-processing the output data of the DEM simulation. The influence of cyclic loading, strain rate, and Young’s modulus on the evolution of contact anisotropy and average coordination number is presented. The Young’s modulus of the particle shows a significant influence on the particle contact creation during compression of the granular assembly with high strain rate. Effect of inertia on the contact anisotropy is observed to be significant during the compression of granular assemblies with different Young’s modulus under high strain rate. The paper concludes with a semi-empirical model to predict the evolution of contact anisotropy as a function of the macroscopic stress state of the assembly during quasi-static uniaxial compaction. The model also introduces two microscopic non-dimensional parameters that are independent of friction between the particles and can be used to relate the macroscopic stresses with the contact anisotropy.

Liquid bridges are commonly encountered in nature and the liquid transfer induced by their rupture are widely used in various industrial applications. In this work, with the focus on the porous tip, we studied the impacts of capillary effects on the liquid transfer induced by the rupture through numerical simulations. To depict the capillary effects of a porous tip, a time scale ratio, R_T, is proposed to compare the competing mechanisms of spontaneous imbibitiona and external drag. In terms of R_T, we then develop a theoretical model for estimating the liquid retention ratio considering the geometry, porosity and wettability of tips. The mecahnism presented in this work provides a possible approach to control the liquid transfer with better accuracy in microfluidics or microfabrications.

This paper presents a new data processing method for calculating more reliable thermophysical properties of liquid using Green-Kubo Method based molecular dynamics (MD) simulation. In this study, MD simulations of water are first performed using three common water models. A new approach for analysing the simulation data is then developed to obtain statistically meaningful thermophysical properties such as thermal conductivity and viscosity. It is demonstrated that for a given desired standard deviation for a specific simulation, a suitable simulation box size can be determined. The proposed approach can assure the repeatability and reliability of the calculated thermophysical properties of liquid.

Uniaxial compression test (UCT) experiments were conducted along with Discrete Element Method (DEM) simulations. The objective was to investigate the effect of the material properties and of the blanket operational parameters (in terms of packing factor, pebble material/size, compressive load and bed height) on the mechanical response of breeder beds subjected to cyclic loading. UCTs were performed with the EU advanced and reference ceramic breeder materials. To investigate the wall effects on the cyclic response of packed beds, a parametric study was performed varying the bed height to pebble size ratio (H/d). To this end monosized commercial zirconia pebbles with different sizes were also used. The numerical experiments were carried out with the KIT-DEM code on pebble assemblies using mixed boundary conditions (periodic and rigid planes). The influence of the bed height, pebble size and pebble material were systematically evaluated to gain an insight about their influence on the macro and micro response of the beds. Thanks to the microscale numerical modelling the macroscale response is presented together with the micro response at the pebble scale. Good agreement was found between experiments and simulations and thus, the KIT-DEM was confirmed to be a reliable predictive tool for the study breeder bed related problems.

As a typical multiphase fluid flow process, drainage in porous media is of fundamental interest in nature and industrial applications. During drainage processes in unsaturated soils and porous media in general, saturated clusters, in which a liquid phase fully occupies the pore space between solid grains, affect the relative permeability and effective stress of the system. In this study, we experimentally studied drainage processes in unsaturated granular media as a model porous system. The distribution of saturated clusters is analysed by an optical imaging method under different drainage conditions, in which pore-scale information from Voronoi and Delaunay tessellation was used to characterise the topology of saturated cluster distributions. By employing statistical analyses, the observed spatial and temporal information of multiphase flow and fluid entrapment in porous media are described. The results indicate that the distributions of both the crystallised cell size and pore size are positively correlated to the spatial and temporal distribution of saturated cluster sizes. The saturated cluster size is found to follow a lognormal distribution, in which the generalised Bond number correlates negatively to the scale parameter and positively to the shape parameter. These findings can be used to connect pore-scale behaviour with overall hydro-mechanical characteristics in partially saturated porous media, using both the degree of saturation and generalised Bond number., Comment: 18 pages, 6 figures

Fruit and nut shells can exhibit high hardness and toughness. In the peninsula of Yucatan, Mexico, the fruit of the Cocoyol palm tree (Acrocomia mexicana) is well known to be very difficult to break. Its hardness has been documented since the 1500 s, and is even mentioned in the popular Maya legend The Dwarf of Uxmal. However, until now, no scientific studies quantifying the mechanical performance of the Cocoyol endocarp has been found in the literature to prove or disprove that this fruit shell is indeed “very hard”. Here we report the mechanical properties, microstructure and hardness of this material. The mechanical measurements showed compressive strength values of up to ~150 and ~250 MPa under quasi-static and high strain rate loading conditions, respectively, and microhardness of up to ~0.36 GPa. Our findings reveal a complex hierarchical structure showing that the Cocoyol shell is a functionally graded material with distinctive layers along the radial directions. These findings demonstrate that structure-property relationships make this material hard and tough. The mechanical results and the microstructure presented herein encourage designing new types of bioinspired superior synthetic materials.

An analytical estimate of the effective thermal conductivity of a pebble bed with mono-sized pebbles in the presence of a stagnant gas is presented. The Discrete Element Method (DEM) is used to simulate granular assemblies under uniaxial compression and Resistor Network (R-N) model is applied for estimating the effective thermal conductivity numerically. In this paper, two heat transfer paths, i.e., conduction through pebble–pebble contact and pebble–gas–pebble interface are considered. The effect of the cutoff range for gap and the strain on the effective conductivity is studied. The validity of the analytical model is verified with the results from the R-N Model for different solid-to-gas thermal conductivity ratios and for different packing fractions. The analytical and numerical models show a good agreement with the experimental results of the effective thermal conductivity for lithium orthosilicate pebble beds in the presence of helium and air at different temperatures.

In this paper, a Discrete Element Method (DEM) for the evaluation of the effective thermal conductivity of pebble beds in fusion blankets is presented. Pebble beds are multiphase materials in which both the solid and the gas phase filling the voids between particles coexist. The effective thermal conductivity of a pebble bed depends on the thermal properties of the two phases as well as on the system properties (e.g. gas pressure, temperature etc.). In particular, the pressure of the system is a key parameter for the heat transfer in a packed granular assembly since the thermal conductivity of a confined gas decreases with decreasing pressure (known as Smoluchowski effect). In this work, the influence of the gas pressure on the effective thermal conductivity in the Knudsen domain was implemented, to our knowledge, for the first time in a DEM code. The heat transfer mechanisms implemented are: when two particles touch each other the conduction through the contact area between them and, in any case, the conduction through the gas phase in the gap between neighbouring solid particles, may they be touching or not. These mechanisms are expected to dominate the heat transfer in a fusion breeder packed bed. Parametric studies were carried out to investigate the influence of the solid and gas materials, temperature, pressure and compression state. Numerical results were compared with existing experimental literature data and recent experiments carried out at Karlsruhe Institute of Technology (KIT).

We study experimentally the formation of a dual hierarchical jetting pattern in dry dense particle media subjected to the radially divergent shock loadings in a radial Hele-Shaw cell. The distinct internal and external jetting patterns were formed on the internal and external surfaces of a ring at different times, respectively. The former features dozens of radially aligned fine filaments. By contrast, the latter consists of a large number of small spikes. Once the internal jets are fully developed, a novel proportionate growth is observed, in which distinguishable structures of the overall pattern all grow at the same rate until the interaction between the internal and external jets becomes significant, leading to the inversion of the internal jetting pattern. Although the external jetting is found to be an instability of a Rayleigh-Taylor (RT) type, the internal jetting exhibits a much delayed onset and slower early-stage growth compared with the RT instability, indicating different underlying physics.

Interfacial transition zones (ITZs) of cementitious concrete are highly heterogeneous, which cause many challenges in accurately obtaining their properties. In this paper, regular aggregates were applied to prepare modelled geopolymer composites, in which ITZs exhibited neat boundaries. Nanoscratch technique with the ability to quickly scan a long distance was adopted to investigate mechanical properties of ITZ and geopolymer paste. To compare the properties of the ITZs and paste, abundant scratch data were analyzed in the form of histograms and Gaussian mixture models. The results showed that the ITZs in geopolymer with silica modulus of 1.5 presented similar properties with the paste, while the ITZs in geopolymer with silica modulus of 1.0 showed significantly higher scratch hardness but lower scratch friction coefficient than paste. Deconvolution analysis revealed that the abnormal hardness and friction coefficient of the paste in geopolymer with silica modulus of 1.0 could be caused by the defects related points. Compared with the traditional scratch scheme, the parallel scratch scheme based on modelled ITZ gave more stable results with a given number of test data, which can provide in-depth information for comparative studies.

The breeder materials proposed for the solid tritium breeding blanket concepts are ceramic lithium-based compounds in the form of pebble beds. Different fabrication processes have been developed to produce pebbles with a high sphericity. However, a small deviation from a perfectly spherical shape exists. In this paper the influence of non-sphericity on the mechanical behaviour of a pebble bed is assessed representing the currently produced pebbles by means of ellipsoidal particles. To this end, the in-house Discrete Element Method code KIT-DEM was further extended. The multi-sphere approach was implemented to generate the ellipsoidal particles while the existing random close packing algorithm was modified to create the assemblies. Uniaxial compression of the assemblies, under periodic boundary conditions, was simulated to investigate the bulk stress-strain behaviour of the bed. Sensitivity studies were carried out with different packing factors of the assembly and several aspect ratios of the particles. In agreement with previous studies carried on assemblies of spherical pebbles, the initial packing factor was found to noticeably affect the mechanical response of the investigated assemblies. Moreover, a remarkable influence of the shape on the mechanical behavior of the simulated assemblies was observed. Therefore it is concluded that for production techniques that result in poor sphericity, DEM simulations with non-spherical particles are necessary to reproduce realistic stress-strain behavior of pebble beds.

In this paper, we study the contact stiffness of a fractal rough surface compressed by a rigid flat plane. A numerical model based on the analysis of flat punch indentation is proposed for simulated hierarchical surfaces, which are generated using statistical and fractal descriptors collected by surface profilometry. The contact stiffness of surfaces under increasing normal load is determined on the basis of the total truncated area at varying heights. The results are compared with experimental data from nanoindentation on four types of treated rough surfaces, showing good agreement with experimental observations below a certain truncation depth. Furthermore, the limits of the model's validity are discussed by focusing on surface geometries and deformation of contacting asperities. With this proposed truncation method, we present a parametric analysis to establish a correlation between contact stiffness and surface roughness descriptors. The contact stiffness shows a unified power-law scaling with respect to the applied load over a wide range for simulated surfaces with distinct sets of roughness descriptors. The exponent of the power-law relationship is found to correlate positively to the fractal dimension while its amplitude is inversely correlated to the surface roughness amplitude. This study provides an easily implemented and computationally efficient method to connect mechanical behaviour with multi-scale surface structure, which can be utilized in design and optimization of engineering applications involving rough contacts.

The structure of mono-sized sphere packings (diameter d) in cylindrical containers (diameter D and height H) both with and without inner cylinders (diameter Di) has been investigated in detail by means of advanced X-ray computed tomography. The geometrical parameters were varied in a wide range; in all experiments 1d vertical vibration was applied. Five experiments were selected with characteristically differing local packing structures. The influence of container geometry, filling and vibration procedures on the formation of regular packings is discussed and a simple correlation is presented to assess whether structured packings occupy a significant fraction of the total packed volume. For a packing with moderate densification, the regular structures are restricted to small wall zones and a random packing exists in the largest part of the packing volume. By selecting appropriate vibration parameters, the zones with regular structures can increase considerably and can persist in the total packed volume. The increasing crystallisation causes an increase of the container packing fraction. For cylinders with H/D ≫ 1 and moderate D/d, regular structures develop preferentially in radial direction from a hexagonal layer at the concave wall. For H/D < 1 and D/d ≫ 1, hexagonal dense structures grow preferentially above the flat bottom plate and can occupy a great portion of the total volume. The role of granular convection on these crystallisation processes has been addressed. Previous statements that the thickness of wall zones is ≈(4–5)d are not generally valid for mono-sized sphere packings; the development of a comprehensive correlation is the task of a future work. Structural details of the packings close to concave, plane and convex walls are analysed via void fraction distributions, sphere centre positions, contact angle distributions, coordination numbers, radial distribution function and Voronoi tessellation. The combination of these methods provides a comprehensive understanding of structural details. Only a few characteristic results are presented; special topics will be the subject of forthcoming publications.

Lithium ceramics have been considered as tritium breeder materials in many proposed designs of fusion breeding blankets. Heat generated in breeder pebble beds due to nuclear breeding reaction must be removed by means of actively cooled plates while generated tritiums is recovered by purge gas slowly flowing through beds. Therefore, the effective thermal conductivity of pebble beds that is one of the governing parameters determining heat transport phenomenon needs to be addressed with respect to mechanical status of beds and purge gas pressure. In this study, a numerical framework combining finite element simulation and a semi-empirical correlation of gas gap conduction is proposed to predict the effective thermal conductivity. The purge gas pressure is found to vary the effective thermal conductivity, in particular with the presence of various sized gaps in pebble beds. Random packing of pebble beds is taken into account by an approximated correlation considering the packing factor and coordination number of pebble beds. The model prediction is compared with experimental observation from different sources showing a quantitative agreement with the measurement.

An analytical model including the Smoluchowski effect to estimate the effective thermal conductivity of a monosized granular assembly with interstitial stagnant gas from the microstructural parameters of the assembly, as well as the bulk properties of the granules and the gas is proposed in this paper. The discrete element method (DEM) is used for the generation and compaction of the granular assemblies. In the granular systems with interstitial gas, heat transfer occurs through the conduction between particles by solid overlap contacts and through the surrounding stagnant gas. Thermal radiation is included in the analytical model to study the influence of radiation on the thermal behavior of the assembly. The effective thermal conductivity is strongly influenced by the pressure of the gas as the conductivity of the gas decreases with decreasing pressure when confined in small gaps. This influence of the gas pressure is implemented in the analytical model. The effect of cyclic loading on the microstructural parameters is investigated through DEM simulations. Parametric correlations are developed to predict the values of microstructural parameters from experimentally measurable parameters so as to bypass any simulations for the estimation of effective thermal conductivity. A good agreement is observed between the analytical model and the experimental results for different assembly parameters and materials. The proposed analytical model can be used to get a first hand estimate of the effective thermal conductivity of a granular assembly to aid in the design process of such systems.

Porous media with hierarchical structures are commonly encountered in both natural and synthetic materials, e.g., fractured rock formations, porous electrodes and fibrous materials, which generally consist of two or more distinguishable levels of pore structure with different characteristic lengths. The multiphase flow behaviours in hierarchical porous media have remained elusive. In this study, we investigate the influences of hierarchical structures in porous media on the dynamics of immiscible fingering during fluid-fluid displacement. By conducting a series of numerical simulations, we found that the immiscible fingering can be suppressed due to the existence of secondary porous structures. To characterise the fingering dynamics in hierarchical porous media, a phase diagram is constructed by introducing a scaling parameter, i.e., the ratio of time scales considering the combined effect of characteristic pore sizes and wettability. The findings present in this work provide a basis for further research on the application of hierarchical porous media for controlling immiscible fingerings., 21 pages, 7 figures

International audience; We investigate the effects of topological disorder and wettability on fluid displacement in porous media. A modified disorder index Iv is proposed to characterize the disorder of porous media. By changing Iv, different displacement patterns (stable displacement and fingering) under the same flow condition and fluid property are obtained. We analytically demonstrate how increase in disorder promotes fingering due to uneven distribution of local capillary pressure. It is shown that the displacement efficiency for different wettability conditions and disorder well correlates with the distribution of local capillary pressure. A power law relation between fluid-fluid interfacial length and saturation of invading fluid is proposed by taking geometry into account, where the parameters in power law relation can be predicted by the capillary index, Ic, unifying the effects of topological disorder and wettability.

International audience; Fluid-filled porous materials are widely encountered in natural and artificial systems. A comprehensive understanding of the elastic behavior of such materials and its dependence on fluid diffusion is therefore of fundamental importance. In this work, a multiscale framework is developed to model the overall elastic response of fluid-filled porous materials. By utilizing a two-dimensional micromechanical model with porosity at two scales, the effects of fluid diffusion and the geometric arrangement of pores on the evolution of effective properties in fluid-filled porous materials are investigated. Initially, for a single-porosity model the effective elastic properties of the dry and fluid-filled porous materials with ordered pores are obtained theoretically by considering a geometrical factor, which is related to the distribution of pores in the matrix. Model predictions are validated by finite element simulations. By employing a double-porosity model, fluid diffusion from macro- to micro-scale pores driven by a pressure gradient is investigated, and the resulting time-dependent effective elastic properties are obtained for both constant pressure and constant injection rate conditions. It is found that the presence and diffusion of pressurized pore fluid significantly affect the elastic response of porous materials, and this must be considered when modeling such materials. It is expected that the proposed theoretical model will advance the understanding of the fluid-governed elastic response of porous materials with implications towards the analysis of geophysical, biological and artificial fluid-filled porous systems.https://doi.org/10.1016/j.ijsolstr.2018.11.028; Les matériaux poreux remplis de fluide sont largement rencontrés dans les systèmes naturels et artificiels. Une compréhension globale du comportement élastique de tels matériaux et de sa dépendance à la diffusion de fluide revêt donc une importance fondamentale. Dans ce travail, un cadre multi-échelles est développé pour modéliser la réponse élastique globale des matériaux poreux remplis de fluide. En utilisant un modèle micromécanique bidimensionnel avec une porosité à deux échelles, on étudie les effets de la diffusion de fluide et la disposition géométrique des pores sur l'évolution des propriétés effectives dans les matériaux poreux remplis de fluide. Initialement, pour un modèle à une seule porosité, les propriétés élastiques effectives des matériaux poreux secs et remplis de fluide à pores ordonnés sont obtenues théoriquement en considérant un facteur géométrique, qui est lié à la distribution des pores dans la matrice. Les prévisions du modèle sont validées par des simulations par éléments finis. En utilisant un modèle à double porosité, on étudie la diffusion de fluide à partir de pores de macro à micro échelle, entraînés par un gradient de pression, et on obtient les propriétés élastiques effectives dépendantes du temps qui en résultent, à la fois pour des conditions de pression constante et de vitesse d'injection constante. On constate que la présence et la diffusion de fluide interstitiel sous pression affectent de manière significative la réponse élastique des matériaux poreux, ce qui doit être pris en compte lors de la modélisation de tels matériaux. Le modèle théorique proposé devrait permettre de mieux comprendre la réponse élastique des matériaux poreux régie par les fluides, avec des implications pour l'analyse de systèmes poreux remplis de fluides géophysiques, biologiques et artificiels.

This work investigates interfacial electro-mechanical properties, including electrical contact resistance, interfacial capacitance and characteristic frequency of contacts formed with various surface structures. Fractal rough surfaces were generated and characterised by fractal dimension and root-mean-square (RMS) roughness. The rough surface with a thin oxide layer was compressed by the rigid flat to form a capacitor. Electrical impedances of this contact capacitor were simulated using the finite element method across a wide range of frequencies. A power-law relationship was found between the electrical contact resistance and applied compression load. An analytical model is proposed to capture the interfacial capacitance behaviour with increasing contact loads, revealing a transition of predominated modes for the capacitance. Higher fractal dimension yields smaller overall capacitance in the gap dominant and transition zones. The dependence of the characteristic frequency on compression was found to follow a power-law function at the low load range. It is found that the exponent and magnitude of obtained power-law relations show strong correlations to the fractal dimension and RMS roughness, respectively. Results of this work provide insights into developing a potential impedance measurement protocol to determine the thickness of the oxide layer on conductive fractal rough surfaces.

In this study, we investigate the impact of topological disorder on liquid distribution and transport phenomena in three-dimensional unsaturated granular media using the Shan-Chen lattice Boltzmann method. Distinct samples of disordered media are generated, characterized by the disorder index I v . Under different I v , varied liquid cluster distributions are demonstrated in a gravity-driven vapour-liquid system. Gradually increasing the initial liquid phase in the simulation domain allows the full range of saturation. The focuses are placed on the liquid cluster statistics from the connectivity, total cluster number, largest cluster and mean cluster volume at an increasing saturation. Meanwhile, the interfacial area, liquid retention curves and relative permeability-saturation curves are produced at diverse I v and wettability. It is found that the slopes of retention curves are well correlated with the proposed capillary index I c that unifies both disorder and wettability. The proposed generalized correlation between capillary index and slope index is useful in terms of determining the capillary pressure-saturation curve and relative permeability-saturation curve for a given granular system at varied contact angles. Additionally, the cohesive strength-saturation curves are also obtained with the aid of the interfacial area and negative capillary pressure, which elucidates that a packing with a higher I v experiences a relatively larger cohesive strength. These results enhance the understanding of disorder effect and will be beneficial for the exploration of many retention curves-related phenomena such as liquid transfer and stress-strain relation for wet granular media.

Fly ash-based geopolymers incorporated with 2% nano-SiO2 (NS)/nano-TiO2 (NT) particles were subjected to microstructural and statistical nanoindentation analysis. With the addition of both types of nanoparticles, the compressive strength of geopolymer and the micromechanical properties of N-A-S-H gel were increased. NS exhibited higher reinforcement effect than NT on macro-strength. However, NT more significantly enhanced gel micromechanical properties. NT and especially the NS were found to have a positive effect on the early reaction rate of geopolymer. After 28 days, the gel proportion obtained by Backscattered electron (BSE) images analysis was close values of 49.16%, 55.69% and 54.02% for reference sample and NS, NT reinforced geopolymer, which were more than two times of that from the statistical nanoindentation. The effects of NS and NT on microstructure, gel proportion and gel micromechanical properties were discussed to reveal the macro-strength reinforcement mechanism. The results obtained from different techniques were also compared and discussed.

The size effect of a quasi-brittle fracture is associated with the size of fracture process zone relative to the structural characteristic length. In numerical simulations using damage models, the nonlocal enhancement is commonly adopted to regularize the softening response. However, the conventional nonlocal enhancement, both integral and gradient approaches, induces a spurious spreading of damage zone. Since the evolution of fracture process zone cannot be captured well, the conventional nonlocal enhancement cannot predict the size effect phenomenon accurately. In this paper, the localizing gradient enhancement is adopted to avoid the spurious spreading of damage. Considering the three-point bend test of concrete beams, it is demonstrated that the dissipation profiles obtained with the localizing gradient enhancement compare well with those of reference meso-scale lattice models. With the correct damage evolution process, the localizing gradient enhancement is shown to capture the size effect phenomenon accurately for a series of geometrically similar concrete beams, using only a single set of material parameters.

The mechanical response of a granular system is not only influenced by the bulk material properties but also on various factors due to it’s discrete nature. The factors like topology, packing fraction, friction between particles, particle size distribution etc. influence the behavior of granular systems. For a reliable design of such systems like fusion breeder units comprising of pebble beds, it is essential to understand the various factors influencing the response of the system. Mechanical response of a binary assembly consisting of crushable spherical pebbles is studied using Discrete Element Method (DEM) which is based on particle–particle interactions. The influence of above mentioned factors on the macroscopic stress–strain response is investigated using an in-house DEM code. Furthermore, the effect of these factors on the damage in the assembly is investigated. This present investigation helps in understanding the macroscopic response and damage in terms of microscopic factors paving way to develop a unified prediction tool for a binary crushable granular assembly.

All solid breeder concepts considered to be tested in ITER (International Thermonuclear Experimental Reactor), make use of lithium-based ceramics in the form of pebble-packed beds as tritium breeder. A thorough understanding of the thermal and mechanical properties of the ceramic pebble beds under fusion relevant conditions is essential for the design of the breeder blanket modules of future fusion reactors. In this study, the effect of cyclic loading on the mechanical behaviour of pebble bed assemblies was investigated using a Discrete Element Method (DEM) code. The numerical simulations were compared with the experimental outcomes. The results of numerical simulations show that the pebble size distribution affects noticeably the stress-strain behaviour of the assemblies. A good qualitative agreement between experimental and simulation results was found in terms of difference between residual strains of consecutive cycles. An increase of the oedometric modulus with the compressive load was observed for all investigated compositions in both experimental and DEM simulations. The numerical results show an increase of the oedometric modulus (E) with progressive compaction of the assemblies due to the cycling loading, while no significant influence of the pebbles size distribution was observed.