Mechanical characterization of partially crystallized sphere packings

We study grain-scale mechanical and geometrical features of partially crystallized packings of frictional spheres, produced experimentally by a vibrational protocol. By combining x-ray computed tomography, 3D image analysis, and discrete element method simulations, we have access to the 3D structure of internal forces. We investigate how the network of mechanical contacts and intergranular forces change when the packing structure evolves from amorphous to near perfect crystalline arrangements. We compare the behavior of the geometrical neighbors (quasicontracts) of a grain to the evolution of the mechanical contacts. The mechanical coordination number Zm is a key parameter characterizing the crystallization onset. The high fluctuation level of Zm and of the force distribution in highly crystallized packings reveals that a geometrically ordered structure still possesses a highly random mechanical backbone similar to that of amorphous packings

Force-Chain Finder: A software tool for the recursive detection of force-chains in granular materials via minor principal stress

Force transmission in granular media occurs through an inhomogeneous network of inter-particle contacts referred to as force-chains. A thorough understanding of the structure of these chains is indispensable for a better comprehension of the macroscopic signatures they generate. This paper introduces Force-Chain Finder (FCF), an open-source software tool designed for detecting force-chains in granular materials. Leveraging the stress tensor computed for each particle based on its interactions with neighboring particles, the tool effectively identifies the magnitude and direction of the most compressive principal stress. Through a recursive traversal of particles and their neighbours, force-chains are robustly detected based on the alignment of the principal stress directions, which is decided by a parameter α (an angle in radians). The software provides a comprehensive suite of post-processing features, including the exportation of results in different formats, enabling detailed analysis of specific regions and dynamic phenomena. Additionally, the software facilitates the computation of statistical measures pertaining to chain size and population. By streamlining the identification and characterization of force-chains within discrete element method (DEM) simulations, this tool significantly enhances the efficiency and accuracy of force-chain analysis. Thus, the software promotes deeper insights into the behaviour of granular materials by enabling researchers to effortlessly detect and analyse force-chains

Modes de cristallisation granulaire induite par la paroi dans un compactage vibratoire

International audienceGranular crystallisation is an important phenomenon whereby ordered packing structures form in granular matter under vibration. However, compared with the well-developed principles of crystallisation at the atomic scale, crystallisation in granular matter remains relatively poorly understood. To investigate this behaviour further and bridge the fields of granular matter and materials science, we simulated mono-disperse spheres confined in cylindrical containers to study their structural dynamics during vibration. By applying adequate vibration, disorder-to-order transitions were induced. Such transitions were characterised at the particle scale through bond orientation order parameters. As a result, emergent crystallisation was indicated by the enhancement of the local order of individual particles and the number of ordered particles. The observed heterogeneous crystallisation was characterised by the evolution of the spatial distributions via coarse-graining the order index. Crystalline regimes epitaxially grew from templates formed near the container walls during vibration, here termed the wall effect. By varying the geometrical dimensions of cylindrical containers, the obtained crystallised structures were found to differ at the cylindrical wall zone and the planar bottom wall zone. The formed packing structures were quantitatively compared to X-ray tomography results using again these order parameters. The findings here provide a microscopic perspective for developing laws governing structural dynamics in granular matter.https://doi.org/10.1007/s10035-019-0876-8La cristallisation granulaire est un phénomène important dans lequel des structures de garnissage ordonnées se forment dans une matière granulaire sous vibration. Cependant, comparée aux principes bien développés de la cristallisation à l'échelle atomique, la cristallisation dans la matière granulaire reste relativement mal comprise. Pour étudier plus avant ce comportement et relier les domaines de la matière granulaire et de la science des matériaux, nous avons simulé des sphères mono-dispersées confinées dans des conteneurs cylindriques afin d'étudier leur dynamique structurelle lors de la vibration. En appliquant une vibration adéquate, des transitions désordre à ordre ont été induites. Ces transitions ont été caractérisées à l'échelle des particules par le biais de paramètres d'ordre d'orientation de liaison. En conséquence, la cristallisation émergente était indiquée par l'amélioration de l'ordre local des particules individuelles et du nombre de particules ordonnées. La cristallisation hétérogène observée a été caractérisée par l'évolution des distributions spatiales via la granulation grossière de l'indice d'ordre. Les régimes cristallins se développaient par épitaxie à partir de gabarits formés près des parois du conteneur lors de vibrations, appelé ici effet de paroi. En faisant varier les dimensions géométriques des récipients cylindriques, il s'est avéré que les structures cristallisées obtenues différaient au niveau de la zone de paroi cylindrique et de la zone de paroi de fond plane. Les structures de garnissage formées ont été comparées quantitativement aux résultats de tomographie à rayons X en utilisant à nouveau ces paramètres d'ordre. Les résultats présentés ici fournissent une perspective microscopique pour l'élaboration de lois régissant la dynamique structurelle dans la matière granulaire.https://doi.org/10.1007/s10035-019-0876-

Self-assembly of granular particles

Granular particles are ubiquitous in nature and daily life, and have wide applications in various disciplines such as infrastructure engineering, architecture, agriculture, etc. Yet, their fundamentals have not been fully understood by scientists. This is mainly because the structure of granular particles, which determines their properties, is complicated and can experience critical changes from disorder to ordered state. In recent years, understanding the fundamentals of such critical structural transitions of granular materials has become a hot multidisciplinary research topic attracting both scientists and engineers. Generally the transition from disordered to ordered structure can be regarded as a self-assembly process, which happens at different scales. In the nucleation of crystals, atoms or molecules can self-assemble due to thermal energy. For such thermodynamics systems, the theory of self-assembly is well established and is dependent on the Gibbs free energy. However, granular particles are much bigger and can dissipate energy quickly with the collision between particles, so they are normally at athermal or low-thermal states. The granular packings are prone to be disordered in structure, whereas they can also self-assemble with the input of external energy via vibration or shear, which can densify the granular packings and hence improve their properties for different applications. This thesis is devoted to advancing the knowledge of the self-assembly of granular spheres, particularly in better understanding the effects of the energy input and the boundary shape. The thesis has revealed a rich and deep picture for the effect of various factors on the self-assembly of granular particles, including the vibration mode, the container shape, material properties, different wall motions and gravity. The obtained results can improve the current understanding of the structural evolution and phase transition of the granular packings with or without vibration. The findings of this study enhance the knowledge on the self-assembly of granular systems and help take a step forward toward stablishing the mechanism behind the phenomenon. Thorough comprehension of the structure of the granular particles are essential for controlling the behaviour and properties of the granular materials, which can be of paramount importance for both the science and technology and have sensible influence on the mankind’s life

THERMAL CONDUCTION IN GRANULAR MEDIA: FROM INTERFACE, TOPOLOGY TO EFFECTIVE PROPERTY

Granular media are particulate substance featured by their unique discrete structure, which are commonly seen in daily life and extensively used in industry. Differently from those continuum materials whose properties are mostly defined by their chemical formulas and status, granular media further require clarification about the effects of their topology on their properties. Therefore, effective properties are used to emphasise this distinction in measuring and describing granular media. In this study, we focus on the effective thermal conductivity of generalised gas-filled granular media, which is highly related to energy technologies and advanced fabrication processes. With particularly concentration on the topological transitions in vibrated granular media, how the topology influences the effective thermal conductivity is explored. Aiming at revealing the mechanisms governing the heat conduction of granular media, a bottom-up consequence scheme is employed in this study by decomposing the macroscale phenomena into grain-scale interactions. Under such scheme, the objectives of this work are further divided into (1) investigating the heat conduction mechanisms at inter-grain contact interfaces and (2) integrating the thermal contact units based on the topology of granular media. To accomplish the former investigation, the finite element analysis is implemented to model the gas-solid thermal interaction contributed by the Smoluschowski effect that gives rise to coupling dependence of gas pressure and grain size. With a systematic study on the heat conduction of individual units, the later objective is tackled by introducing the grain-scale thermal interaction into discrete element methods. With the combination of these cross-scale studies, a numerical framework is established. Furthermore, the thermal measurement system based on transient plane source techniques is applied to experimentally characterise correlations between the effective thermal conductivity and external mechanical loading. These experimental results as well as available literature data are used to quantitatively verify the proposed numerical method. In order to figure out the topological influence on the effective thermal conductivity, the discrete element method is further employed to examine the mesoscale behaviours of agitated granular media. The grain-scale structural characterisation unravels the topological transitions in vibration. Granular crystallisation, a process prompting the disorder-to-order transition, is identified as the major phenomenon and its boundary dependent mechanisms are iii proposed. Moreover, the topological influence on the effective thermal conductivity can be assessed with respect to the crystallisation, i.e., the degree of structure order, of granular media. With the fundamental research in this thesis on the heat conduction mechanisms and the granular crystallisation, the effective thermal conductivity is studied in a full range of scales from individual grains to bulk media. In summary, we demonstrate and experimentally validate a multiscale framework to solve the thermal problems in granular media that can also be applied to other effective conduction properties

Numerical Simulation and Characterisation of the Packing of Granular Materials

The scientific problems related to granular matter are ubiquitous. It is currently an active area of research for physicists and earth scientists, with a wide range of applications within the industrial community. Simple analogue experiments exhibit behaviour that is neither predicted nor described by any current theory. The work presented here consists of modelling granular media using a two-dimensional combined Finite-Discrete Element Method (FEM-DEM). While computationally expensive, as well as modelling accurately the dynamic interactions between independent and arbitrarily shaped grains, this method allows for a complete description of the stress state within individual grains during their transient motion. After a detailed description of FEM-DEM principles, this computational approach is used to investigate the packing of elliptical particles. The work is aimed at understanding the influence of the particle shape (the ellipse aspect ratio) on the emergent properties of the granular matrix such as the particle coordination number and the packing density. The diff erences in microstructure of the resultant packing are analysed using pair correlation functions, particle orientations and pore size distributions. A comparison between frictional and frictionless systems is carried out. It shows great diff erences not only in the calculated porosity and coordination number, but also in terms of structural arrangement and stress distribution. The results suggest that the particle's shape a ffects the structural order of the particle assemblage, which itself controls the stress distribution between the pseudo-static grains. The study then focuses on describing the stress patterns or \force chains" naturally generated in a frictional system. An algorithm based on the analysis of the contact force network is proposed and applied to various packs in order to identify the force chains. A statistical analysis of the force chains looking at their orientation, length and proportion of the particles that support the loads is then performed. It is observed that force chains propagate less efficiently and more heterogeneously through granular systems made of elliptical particles than through systems of discs and it is proposed that structural diff erences due to the particle shape lead to a signifi cant reduction in the length of the stress path that propagates across connected particles. Finally, the e ffect of compression on the granular packing, the emergent properties and the contact force distribution is examined. Results show that the force network evolves towards a more randomly distributed system (from an exponential to a Gaussian distribution), and it confi rms the observations made from simulations using discs. To conclude, the combined finite-discrete element method applied to the study of granular systems provides an attractive modelling strategy to improve the knowledge of granular matter. This is due to the wide range of static and dynamic problems that can be treated with a rigorous physical basis. The applicability of the method was demonstrated through to a variety of problems that involve di fferent physical processes modelled with the FEM-DEM (internal deformations, fracture, and complex geometry). With the rapid extension of the practical limits of computational models, this work emphasizes the opportunity to move towards a modern generation of computer software to understand the complexity of the phenomena associated with discontinua

Development and calibration of discrete element method inputs to mechanical responses of granular materials

Simulation of soil excavation is difficult. Tools which manipulate soil are difficult to evaluate in a virtual environment prior to prototype or manufacture. Soil behaves as a discontinuous material in normal excavation activities. Therefore, numerical methods which naturally model discontinuous media, such as the Discrete Element Method (DEM), can be used to perform simulations of soil excavation. However, DEM input parameters must be calibrated to accurately model the mechanical behavior of soil. The goal of this research was to develop intelligent methodologies to calibrate DEM input parameters to reproduce the mechanical responses of soil and other granular materials subject to traditional laboratory tests, such as triaxial and direct shear tests. A mechanistic understanding of the interaction between sliding and rolling friction was developed and correlated with the critical state strength of drained granular media. In addition, the fundamental soil mechanics concept of relative density was successfully applied to the DEM calibration methodology to predict peak granular strength and dilatancy. Sensitivity analyses of DEM input parameters were used to enhance the characterization of mechanical behavior of DEM specimens. A calibration algorithm was developed to quickly and mechanistically relate DEM input parameters to laboratory measured mechanical behavior of soils. The algorithm eliminates unnecessary iterations during the DEM parameter calibration by enforcing a sophisticated understanding of the mechanisms of granular shear strength. The outcomes of this research greatly simplify the calibration of DEM parameters of soil for use in industrial excavation problems

Structural and Phase Transition in Wet Granular Media

In granular media, a macro-view deformation reflects the structural change at the particle level. A structural change is widely considered as the phase transition. Due to wide industry applications, such as food storage, mineral transportation, geotechnical engineering, additive manufacturing, and pharmaceutical production, the phase transition of granular media is of great significance. Although researchers have been intensively investigating the phase transition phenomena and the accompanying structure formations in recent decades, challenges still remain in predicting and controlling structures, from ordered structures to random very loose packing, especially under a partially saturated condition where cohesion can be raised. In this thesis, the structure formation process and the granular phase transition with a focus on partially saturated conditions are investigated. Experiments and numerical simulations show how particle size, cohesion and external excitations can be controlled to achieve tailored phase transition and packing structures in a wide packing fraction range from random very loose packing to crystallisation. Furthermore, the combined effects of cohesion, inertia and gravity during the phase transition can be successfully characterised by several bespoke dimensionless numbers. The findings deepen our understanding of phase transition and structural formation mechanisms of granular materials under various conditions. The effective methods to examine and analyse particle movement and interactions provide insights into predicting and controlling phase transitions and structures

Understanding bulk behavior of particulate materials from particle scale simulations

Particulate materials play an increasingly significant role in various industries, such as pharmaceutical manufacturing, food, mining, and civil engineering. The objective of this research is to better understand bulk behaviors of particulate materials from particle scale simulations. Packing properties of assembly of particles are investigated first, focusing on the effects of particle size, surface energy, and aspect ratio on the coordination number, porosity, and packing structures. The simulation results show that particle sizes, surface energy, and aspect ratio all influence the porosity of packing to various degrees. The heterogeneous force networks within particle assembly under external compressive loading are investigated as well. The results show that coarse-coarse contacts dominate the strong network and coarse-fine contacts dominate the total network. Next, DEM models are developed to simulate the particle dynamics inside a conical screen mill (comil) and magnetically assisted impaction mixer (MAIM), both are important particle processing devices. For comil, the mean residence time (MRT), spatial distribution of particles, along with the collision dynamics between particles as well as particle and vessel geometries are examined as a function of the various operating parameters such as impeller speed, screen hole size, open area, and feed rate. The simulation results can help better understand dry coating experimental results using comil. For MAIM system, the magnetic force is incorporated into the contact model, allowing to describe the interactions between magnets. The simulation results reveal the connections between homogeneity of mixture and particle scale variables such as size of magnets and surface energy of non-magnets. In particular, at the fixed mass ratio of magnets to non-magnets and surface energy the smaller magnets lead to better homogeneity of mixing, which is in good agreement with previously published experimental results. Last but not least, numerical simulations, along with theoretical analysis, are performed to investigate the interparticle force of dry coated particles. A model is derived and can be used to predict the probabilities of hose-host (HH), host-guest (HG), and guest-guest (GG) contacts. The results indicate that there are three different regions dominated by HH, HG, and GG contacts, respectively. Moreover, the critical SAC for the transition of HG to GG contacts is lower than previously estimated value. In summary, particle packing, particle dynamics associated with various particle processing devices, and interparticle force of dry coated particles are investigated in this thesis. The results show that particle scale information such as coordination number, collision dynamics, and contact force between particles from simulation results can help better understand bulk properties of assembly of individual particles

DEM-CFD analysis of micromechanics for dry powder inhalers

Dry powder inhalers (DPIs) are widely used for the therapy of respiratory and pulmonary diseases. In this study, a coupled discrete element method and computational fluid dynamics (DEM-CFD) is employed to investigate the micromechanics of carrier-based DPIs. The effects of van der Waals forces and electrostatic forces on the mixing process, and the influences of air flow and particle-wall impact on the dispersion process are examined. For the mixing of carrier and active pharmaceutical ingredient (API) particles in a vibrating container, it is found that vibration conditions affect the mixing performance. While there is an optimal mixing condition to maximise the number of API particles attaching to the carrier (i.e. contact number) for van der Waals cases, the contact number decreases with increasing vibration velocity amplitude and frequency for electrostatic force cases. It is also revealed that van der Waals forces (short range) and electrostatic forces (long range) result in different mixing behaviours. For the air flow induced and impact induced dispersion, it is found that the dispersion performance improves with increasing air velocity, impact velocity and impact angle, and reduces with increasing work of adhesion. The dispersion performance can be approximated using the cumulative Weibull distribution function governed by the ratio of air drag force to adhesive force or the ratio of impact energy to adhesion energy