Granular Matter - Granular Matter Webinar Series
The Editors of Granular Matter are proud to present a series of webinars organized by the journal.
See the new Granular Matter Webinar Series Homepage for Updates and Announcements! (this opens in a new tab)
Computational Modeling of Multiphysics Problems in Granular Media
Speaker: Prof. Jidong Zhao, Hong Kong University of Science and Technology
Date: Friday, 26 April
Time:
6:00 AM Colorado, USA
8:00 AM New York, Boston, USA
1:00 PM United Kingdom
2:00 PM Central Europe
5:30 PM Delhi, India
8:00 PM Beijing, Hong Kong
10:00 PM Sydney, Australia
Link: Register for the Webinar Here. (this opens in a new tab)
Registration is free, anyone can register.
Abstract
This talk addresses the emerging challenges of numerical modeling of granular media, specifically those involving multiphysics problems related to contemporary climate change and energy crises. Three examples are used to demonstrate key computational elements in addressing relevant challenges, including the free-thaw of frozen permafrost, the transport, deposition, and mitigation of geophysical flows, and laser powder-bed fusion in additive manufacturing. We emphasize the importance of considering the multiscale nature of these processes, from continuum-scale governing equations to particle and pore scale characteristics, to accurately capture and understand the underlying mechanisms for providing pertinent solutions to engineering problems of varied complexity. We also discuss pending issues related to computational granular mechanics as a whole.
A recording of the webinar will be made available following the presentation. The webinar will include a presentation and a brief Q+A session, and is expected to last approximately 60-90 minutes.
If you have any questions, contact Jack Manzi at jack.manzi@springer.com (this opens in a new tab)
Previous Webinars
Understanding the micro-world of particle-to-particle interactions
Speaker: Prof. Kostas Senetakis, City University of Hong Kong
Originally presented on: Wednesday, 27 March
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Abstract
Contact modeling of particle-to-particle interactions has its own value in the simulation process and the understanding of macroscopic phenomena such as sand dunes, landslides, vibrating granular systems and other problems involving powders and grains. However, the real contact problem of particle interactions, let it be natural crystalline grains or amorphous glass beads, has been highly overlooked by means of experimentation. In this seminar, we will look into this contact problem and specifically on the way particles interact with each other and with their surrounding environment through an experimental approach. We will discuss on the influence of stress history, rate of loading and creep, and predominantly we will see pairs of particles within a general context of Coulomb friction. It will also be attempted to link micro- and macroscopic observations from experimental results on granular systems.
References:
N.S.C. Reddy, H. He, K. Senetakis. DEM analysis of small and small-to-medium strain shear modulus of sands. Computers & Geotechnics 141, 104518 (2022).
J. Ren, S. Li, H. He, K. Senetakis. The tribological behavior of iron tailing sand grain contacts in dry, water and biopolymer immersed states. Granular Matter 23(1), 12 (2021).
C.S. Sandeep K. Senetakis. An experimental investigation of the microslip displacement of geological materials. Computers and Geotechnics, 107, 55-67 (2019).
S.S. Kasyap, K. Senetakis. Experimental investigation of the coupled influence of rate of loading and contact time on the frictional behavior of quartz grain interfaces under varying normal load. International Journal of Geomechanics, 19(10), 04019112 (2019).
A recording of the webinar will be made available following the presentation. The webinar will include a presentation and a brief Q+A session, and is expected to last approximately 60-90 minutes.
Physics and modeling of wind-blown sand dunes
Speaker: Dr. Eric Josef Ribeiro Parteli, University of Duisburg-Essen
Originally Presented On: Thursday, 25 January
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Abstract
Sand dunes occur in desert and coastal areas of our planet thereby interacting with local infrastructure and contributing to desertification. To form, dunes require a supply of granular particles at the surface and a boundary layer of sufficient efficacy to enable transport of these particles by fluid forces. However, different dune shapes occur in nature depending on the wind regimes and the amount of sand available for transport. Numerical simulations of wind-blown sand transport and the concatenated dune migration have been pushing forward our understanding of dune physics, as I will discuss in the talk.
Moreover, dunes have been detected in surprising locations of our solar system, including on Mars and even on Pluto, where the atmosphere is 100,000 times less dense than the Earth’s and the granular material is made of methane ice. Therefore, I will present insights we have gained about the formative processes of dunes on Earth and in extra-terrestrial environments, and discuss open questions and promising applications in the planetary and desertification research.
Key differences in the mechanical response of granular versus continuum solids
Speaker: Prof. Corey O'Hern, Yale University
Originally presented on: Tuesday, 12 December
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Abstract
Granular materials are composed of discrete, macroscopic particles that interact via dissipative, contact interactions. When granular materials are compressed, they undergo a jamming transition from a liquid- to a solid-like state. In this presentation, I will describe the key differences between the mechanical properties of granular solids compared to continuum solids. First, continuum solids (e.g. modeled using Lennard-Jones interactions) can store a significant amount of potential energy during isotropic compression and thus their elastic moduli typically increase with pressure. In contrast, granular solids undergo frequent particle rearrangements during compression, which prevents them from storing significant elastic energy. We find that along elastic segments where granular solids do not undergo particle rearrangements during compression, their shear moduli actually decrease with increasing pressure. Second, granular solids are always anisotropic at jamming onset, even in the large-system limit. In contrast, compressed Lennard-Jones glasses are isotropic in the large-system limit. Finally, we show that jammed solids possess a large number of structural ``defects" that are activated during applied shear and are the source of the collective, non-affine displacement fields in granular solids.
Particle fragmentation in granular materials
Speaker: Dr. Farhang Radjai, University of Montpellier
Originally Presented On: Wednesday, 25 October
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Abstract
Particle fragmentation plays an important role in natural granular flows, soil microstructure evolution, and powder-based industrial processes. The energetic inefficiency of comminution and effects of particle breakage on the strength properties of soils and rockfills are among old issues that have remained poorly understood mainly because of the multiscale nature of the particle breakage process in granular materials. Along with modern experimental tools such as tomography, new insights can however be expected from particle dynamics simulations coupled with methods accounting for crack nucleation and propagation inside the particles. In this seminar, I will use several examples to illustrate the potential of this particle-based approach, allowing me also to discuss several aspects of particle fragmentation in granular materials: 1) Breakage of a single particle and its scaling with impact energy, 2) Evolution of particle size distribution in cascading flows in a rotating drum and its scaling with system parameters, 3) Evolution of particle size distribution in quasi-static flows and its effect on the rheology, 4) Population balance model and cushioning effect, and 5) Self-similar fragment size distributions and particle shapes arising from both single-particle and collective particle breakage.
Granular Matter in Space
Speaker: Prof. Matthias Sperl, University of Cologne and German Aerospace Center DLR
Originally Presented On: 12 June 2023
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Abstract
The investigation of granular matter benefits from experiments in microgravity in several ways and may be categorized into three regimes of increasing density:
(1) Granular gases [1] are dilute systems and subject to sedimentation which can be avoided in microgravity. Their agitation by magnetic excitation allows for the study of cooling, velocity distributions and the onset of particle agglomeration.
(2) Granular fluids [2] are denser than granular gases and are monitored by scattering techniques. On Earth, granular fluids are anisotropic and inhomogeneous. In microgravity, agitated homogeneous granular fluids can be prepared for densities up to the jamming transition.
(3) Granular packings [3] close to the jamming transisionexperience a large pressure gradient on Earth which is absent in microgravity. Hence, sound transmission can be studied at very low confinement. Similarly, microgravityoffers unique opportunities for the rheology on dense granular systems.
[1] Peidong Yu, Matthias Schröter, and Matthias Sperl, Velocity Distribution of a Homogeneously Cooling Granular Gas, Phys. Rev. Lett. 124, 208007(2020).
[2] Philip Born, Johannes Schmitz, and Matthias Sperl, Dense fluidized granular media in microgravity, Nature pj Microgravity 3, 27 (2017).
[3] Karsten Tell, Christoph Dreißigacker, Alberto Chiengue Tchapnda, Peidong Yu, and Matthias Sperl, Acoustic Waves in Granular Packings at Low Confinement Pressure, Rev. Sci. Instr. 91, 033906 (2020).
Impact response of granular materials:
From the origin of the universe to catastrophic asteroid strikes
Speaker: Prof. Xiang Cheng (程翔), Department of Chemical Engineering and Materials Science, University of Minnesota
Originally Presented on: Wednesday, 22 March 2023
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Abstract
Granular materials are large conglomerations of discrete macroscopic particles. Examples include seeds, sand, coals, powder of pharmacy, etc. Though simple, they show unique properties different from other familiar forms of matter. The unusual behaviors of granular materials are clearly illustrated in various impact processes, where the impact-induced fast deformation of granular materials leads to emergent flow patterns revealing distinctive granular physics. Here, we explored the impact response of granular materials in two specific experiments:
First, we investigated impact cratering in granular media induced by the strike of liquid drops—a ubiquitous phenomenon relevant to many important environmental, agricultural and industrial processes. Surprisingly, we found that granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we develop a simple model that quantitatively describes various features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals an interesting analogy between familiar phenomena of raining and catastrophic asteroid strikes.
Second, we performed the granular analog to “water bell” experiments. When a wide jet of granular material impacts on a fixed cylindrical target, it deforms into a sharply-defined sheet or cone with a shape mimicking a liquid of zero surface tension. The jets' particulate nature appears when the number of particles in the beam cross-section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment reveals a universal fluid structure arising from the collision of discrete particles, which has a counterpart in the behavior of quark-gluon plasmas created by colliding heavy ions at the Relativistic Heavy Ion Colliders.
Continuum models for granular statics and flow:
How particle simulations can help in build them and verify the principles that underpin them
Speaker: Prof. Prabhu Nott (IISC)
Originally Presented On: Tuesday, 24 January 2023
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Abstract
We have today the ability to numerically simulate the dynamics of millions of grains, but unless the simulation results are distilled to a continuum theory, we would not have understood the mechanics. In this talk, I will argue that by asking the right questions and using relatively simple analysis, one can use particle simulations to build models and develop an understanding. I will consider two problems, one in granular statics and another in slow flow, and demonstrate how we have used particle simulations to achieve a fundamental understanding of granular mechanics. In granular statics, such an understanding leads to a closure relation for the stress. In slow flow, it provides explanations for the basic principles that underpin continuum models. I will then show experimental verification of the key ideas distilled from the simulations.
Protocols for the Mpemba effect in granular fluids
Speaker: Prof. Andrés Santos, University of Extremadura
Originally Presented on: 20 December 2022
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Abstract
The so-called Mpemba effect (ME) is a counterintuitive memory phenomenon according to which, given two samples of a fluid, the initially hotter one may cool more rapidly than the initially cooler one to a common steady state. Although initially reported in the case of water, its existence for that liquid is still questioned. On the other hand, the ME has been observed to emerge (both theoretically and computationally) in the case of granular fluids, where the role of temperature (T) is played by the mean kinetic energy per particle and the steady state is tuned by a certain set of control parameters. The key point responsible for the ME in granular fluids is the observation that the relaxation of T(t) to its stationary value is governed not only by its initial value T(0) but also by the initial values of one or more additional variables.
In some of the works in the literature on the ME for granular fluids, the state at t=0 was assumed ad hoc, without a description of the prior preparation protocols for t<0. In this talk, I will discuss two classes of granular systems where protocols for the ME can be clearly devised.
In the first class, the system is an inertial suspension made of inelastic and smooth hard spheres under shear [1], the control parameters being the shear rate and the bath temperature. One of the samples (A) is prepared in a quasi-equilibrium unsheared steady state, while the other sample (B) is prepared in a sheared steady state with a bath temperature smaller than that of sample A. Then, at t=0, a common bath temperature (equal to that of sample B) and a common shear rate (smaller than that of sample B) are applied and both systems are allowed to relax to the new steady state.
The second class of systems is defined by a dilute two-dimensional gas of hard and rough hard disks driven by a stochastic force and a stochastic torque, which inject translational and rotational energy, respectively [2]. In this case, the control parameters are the noise temperature associated with the total noise intensity and the fraction of rotational noise. In the preparation protocol for the standard ME, sample A is prepared with a higher noise temperature than sample B, the fraction of rotational noise being 0 and 1 for samples A and B, respectively. At t=0, both samples are subjected to a common noise temperature (smaller than that of sample B) and a common fraction of rotational noise (different from 0 and 1), and their relaxation to the new steady state is analyzed.
- S. Takada, H. Hayakawa, and A. Santos, Phys. Rev. E 103, 032901 (2021).
- A. Megías and A. Santos, Front. Phys. 10, 971671 (2022).
Landscapes of Glass
Speaker: Prof. Douglas Jerolmack, University of Pennsylvania
Originally Presented on: Thursday, 17 November 2022
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Abstract
If cooled sufficiently quickly, the disorder of a liquid can be "quenched" or locked in place; the resulting amorphous solid is glass. Although it appears to be solid on human timescales, glass continues to creep due to thermal vibrations at the molecular scale. Consider now a pile of sand; it too is a disordered system, but the grains are too massive for such thermal effects to be relevant. Yet, soils in nature relentlessly creep, on hillslopes below the angle of repose. The unchallenged dogma is that this creep is driven by churning of soil by (bio)physical disturbances, and diffusion models based on this premise underpin virtually all landscape evolution models (LEMs). River-bed sediments also creep, at flows below the threshold of motion, though this has received far less attention. In this talk I focus on recent work from my group and others that examines the origins of granular creep in hillslope and river systems, and the consequences of these findings for landscape dynamics. Our observations reveal surprises for both geologists and physicists. First, gravity-driven granular creep occurs with minimal disturbance, with rates and styles comparable to field observations. Second, this creep shares deep similarities with the behavior of glass, suggesting that mechanical disturbance in granular systems plays a role akin to thermal fluctuations in molecular systems. Third, fluid-driven creep in bed-load systems has similar behavior to gravity-driven hillslope creep. In both cases this creep acts to "harden" the bed, by compaction and the creation of structures that resist motion. Thus, sediment beds maintain a memory of their history of forcing, that dictates the threshold for landsliding (hillslopes) or entrainment (rivers). I hope to get some ideas from the audience on where to go next!
Discovering gedy's of precursory failure in motion data
Speaker: Prof. Antoinette Tordesillas, University of Melbourne
Originally Presented on: Monday, 20 June 2022
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Abstract
Advancing the basic science of granular failure is challenging, but so is translating these advances from the lab to the field. In Yoda's words, "Difficult to see; always in motion is the future." Could motion be the key? After all, what is measured in the field is motion (or deformation), not stress. Here we share lessons learned from data-driven characterization and modelling of failure across space and time scales from motion data. Attention will be paid to the common intrinsic structure of such data - across different spatial scales - be it formed by individual grain movements in a lab test or by ground motion in the field. We coined this generic dynamics "gedy’s". With the support of partners (e.g., industry and International Consortium on Landslides), we briefly discuss examples of gedy's in some catastrophic landslides and how they can be exploited in applications of AI in practical landslide risk assessment and decision-making.
What is the Normal Compression of Soil?
Speaker: Professor Glenn McDowell, University of Nottingham
Originally presented on: Friday, 27 May 2022
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Abstract
It has long been widely accepted that the normal (plastic) compression of sand and clay are very similar: a linear relationship exists beyond some sort of yield point when voids ratio is plotted against the logarithm of applied stress – at least over one log cycle of stress. However the reason for this similarity in behaviour has not been exposed. We present some early thoughts for sand on the evolution of a fractal distribution of particle sizes as stress increases and particles break. Only recently have we been able to explain the normal compression line more clearly and to propose a new linear relationship in log voids ratio – log stress space. The approach recognises the role of the smallest grains, getting smaller and statistically stronger as a grading with a fractal number of 2.5 emerges. However many questions have remained unanswered: why should the fractal number be 2.5, what are the smallest particles (how small is small?) and what about the rest of critical state soil mechanics (peak strength, dilatancy, state boundary surface and so on)? And the important question as to why clay should behave macroscopically in the same way has remained unanswered. In this talk, using DEM we explain the micro mechanics of normal compression of sand, and propose a space-filling argument for why the fractal number should be 2.5. In addition we explain what we mean by the "smallest" particles (because newly formed dust doesn’t contribute to mechanical behaviour). We then introduce a simple attraction/repulsion model for clay platelets and show that a fractal distribution of macro particles evolves as stress increases in order to fill the void space. We believe this is the first time a unifying picture of soil appears to be emerging.
Discrete complex system modeling: the role of granular mechanics in predicting failure and instability from the grain scale to the structural scale
Speaker: Prof. José Andrade, Caltech
Originally Presented on: Monday, 28 March, 2022
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Abstract
In this talk, we will present collaborative work we have done to model the behavior of discrete complex systems across different scales and applications. We will start with work on grain breakage where discrete models and advanced experiments can shed light into hitherto obscure mechanisms controlling particle failure. we will then present work to understand ant-tunneling and the mechanisms that the natural world seems to utilize to minimize instability. Finally, we will show an example of collaborative work between computational and experimental models to tune particle shape in order to obtain structural behavior that maximizes bending stiffness for applications such as impact protection. A common theme will be the synergistic effect of collaboration, as well as the amplifying effect of experimentation and modeling in complex systems.
Granular flows in reduced gravity environments
Speaker: Prof. Troy Shinbrot, Rutgers University
Originally Presented on: Friday, 14 January 2022
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Abstract
Asteroids, moons, and planetesimals exhibit surprising shapes and patterns that appear to be formed through granular interactions at low gravity. For example, in the absence of wind or other external influences, the small asteroid Itokawa (gravity ~ 10^-5 g) has somehow separated fine sand from large boulders, and the larger asteroid Eros (10^-3 g) has concentrated ponds of apparently pure sand through mechanisms that are not understood. At slightly larger scales, Saturn’s sandy moons Pan and Atlas have developed striking ravioli shapes, while rocky debris on the moon Iapetus (10^-2 g) has formed a 20 km high equatorial ridge. In this talk, we describe both common and unique granular behaviors that play an important role in morphogenesis and that arise predominantly at reduced gravity.
Frustrated Bearings
Speaker: Prof. Hans Herrmann, UFC, Fortaleza, Brazil, Editor-in-Chief of Granular Matter
Originally presented on: Monday, 13 September 2021
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Abstract
A bearing is a system of spheres (or disks) in contact. If in a bearing every loop is even, one can obtain “bearing states”, in which touching spheres roll on each other without slip. We frustrate a system of touching spheres by imposing two different bearing states on opposite sides and search for the configurations of lowest energy dissipation. For Coulomb friction (with random friction coefficients) in two dimensions, a sharp line separates the two bearing states and we prove that this line corresponds to the minimum cut. Astonishingly however, in three dimensions, intermediate bearing domains, that are not synchronized with either side, are energetically more favourable than the minimum-cut surface. This novel state of minimum dissipation is characterized by a spanning network of slip-less contacts that reaches every sphere. Such a situation becomes possible because in three dimensions bearings of loops of size four have four degrees of freedom. By considering spheres of different size, packings with bearing states can even be made space-filling. The construction and mechanical properties of such space-filling bearings will be discussed. Space-filling bearing states can be viewed as a realization of solid turbulence exhibiting Kolmogorov scaling and anomalous heat conduction. Bearings states can be perceived as physical realizations of networks of oscillators with asymmetrically weighted couplings. These networks can exhibit optimal synchronization properties through tuning of the local interaction strength as a function of node degree or the inertia of their constituting rotor disks through a power-law mass-radius relation. As a consequence, one finds that space filling bearings synchronize fastest, when they are hollow.
Granular matter self-organises by entropy-stability competition into non-equilibrium detailed balance states
Speaker: Prof. Raphael Blumenfeld, University of Cambridge, UK
Originally Presented on: 16 June 2021
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Abstract
The large-scale behaviour of granular materials is sensitive to the grain-scale structure. This structure self-organises in a way that depends on the driving dynamic process, often regarded as history-dependence. It is therefore important to find a general underlying principle that applies to a wide range of dynamic processes. In this talk, I present a couple of steps in this direction. I will first describe a recently-developed formalism to model structural organisation of two-dimensional dense granular matter. The formalism makes it possible to predict a number of structural characteristics under any quasi-static process. A particularly surprising discovery is that the steady states of such dynamics satisfy a non-equilibrium detailed balance. I then present evidence that underlying the self-organisation is a general principle of competition between entropy and mechanical stability. It is possible to reduce the effect of mechanical stability, in which case some structural characteristics can be predicted from the maximum entropy principle alone. The theoretical predictions are supported by numerical and experimental observations. These results lend further support to Sir Sam Edwards’s proposal that much of granular science can be modelled by statistical mechanics.
The role of force networks in granular materials
Speaker: Prof. Karen Daniels, NC State University
Originally presented on: 12 May 2021
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Abstract:
Granular materials are inherently heterogeneous, and continuum models of properties such as rigidity and sound speed often fail to quantitatively capture their dynamics. One likely reason for these difficulties is that internal stresses are transmitted via a chain-like network of strong forces, introducing a secondary, meso-scale structure to the system. In my talk, I will describe several experiments on two-dimensional photoelastic granular materials which bridge particle-scale, meso-scale, and continuum-scale approaches. These experiments allow us to both investigate the statistical ensembles from which force networks are drawn, as well as probe their effects on mechanical properties such as sound transmission, rigidity, and rheology.
X-ray tomography study of granular materials
Speaker: Prof. Yujie Wang, Shanghai Jiao Tong University
Originally Presented on: 13 April 2021
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Abstract:
To develop a reasonable constitutive theory for dense granular materials, it is crucial to establish the connections between the microscopic and the macroscopic. This will first require the establishment of a statistical framework for these by nature out-of-equilibrium systems. Equally important is the identification of relevant microscopic processes which can be thermodynamically averaged to yield macroscopic responses. In this webinar, I will first try to test the validity of Edwards ensemble which is the only statistical mechanical framework conjectured to understand granular materials. Currently there exists no experimental validation of Edwards volume ensemble in three dimensions and the role played by friction has not been carefully investigated. We show one of our recent experimental work on this topic using X-ray tomography to investigate tapping-generated 3D granular packings. We validate Edwards volume ensemble as well as establish a granular version of thermodynamic zeroth law. We clarify the influence of friction on granular statistical mechanics as modifying the density of states. Upon the proof of the usefulness of the Edwards volume ensemble, we will then talk about one of its application on dense granular flows. Here we show in order to understand macroscopic response of granular materials upon shear, it is critical to consider structural evolutions on both particle and contact levels, thus differentiate them from ordinary disordered materials where only the particle level structures are important. By using defect concept similar to crystalline materials, we can then account for both volume fraction and shear force change during the whole shear cycle. We find that there exist two microscopic processes which tend to create and annihilate these defective structures and it is reasonable to conjecture that critical state corresponds the state when two processes reach balance.
Polydispersity – implications for granular material behaviour and modelling challenges
Speaker: Prof. Catherine O'Sulivan, Imperial College London
Originally Presented on: 11 March 2021
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Abstract
A lot of studies in granular mechanics have restricted consideration to materials with a relatively narrow range of particle sizes. However engineers have long recognised that the range of sizes in a granular material significantly influences its mechanical behaviour. Quantifying the polydispersity, i.e. determining the particle size distribution, is one of the most basic characterizations we perform on granular materials. However, our understanding of how changes in the particle size distribution influence the mechanical behaviour of granular materials is incomplete. Recently generated DEM data provide a new perspective on how changes in the particle size distribution change the fabric, the distribution of stresses and stress wave propagation in polydisperse granular materials. Gap graded materials (i.e. materials with two distinct size fractions) have attracted a lot of interest in geomechanics over the past decade. Our DEM data indicate that some of the hypotheses that have emerged are not robust; for the concept of a transitional fines content is not supported by our data. Through these studies key challenges associated with using DEM to simulate polydisperse materials have emerged: large numbers of particles must be considered and the accuracy of the coarse grained approach often used in DEM-CFD modelling is compromised.
Heterarchy in Granular Matter
Speaker: Prof. Itai Einav, University of Sydney
Originally Presented on: 11 February 2021
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Abstract
This presentation will deal with `grainsize dynamics' -- the mechanics dealing with the evolution of particle size distributions in space and time, and their governing forces. Typical forces include mixing, segregation and comminution. Considering the particle-scale stochastic processes that govern these dynamics, the scientific aim is to frame equivalent homogenised continua that can handle those processes even when acting simultaneously. A key observation is that mixing and segregation are `open-system' mechanisms that do not lend themselves for hierarchical approach that artificially identifies scales and treat them separately. On the other hand grain crushing may occur even in `closed-system', yet its description requires to preserve the state of nonlocal grainsize fabric. We therefore propose a heterarchical multi-scale approach that does not separate scales, and retain both local and nonlocal grainsize information. Although the talk will focus on particle size, much of the presented philosophy may be adapted for other shape descriptors such as elongation and sphericity.
Collisional Contact Charging in Granular Materials
Speaker: Prof. Heinrich M. Jaeger, The University of Chicago, Editor-in-Chief of Granular Matter
Originally Presented on: 5 January 2021
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Abstract
Collisional contact charging of sub-millimeter particles and the resulting clustering is important in circumstances ranging from the earliest stages of planet formation to aggregation of airborne pollutants to industrial powder processing. Even in systems comprised of grains of identical dielectric material, contact charging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces. Remarkably, rather fundamental aspects of contact charging, such as the type of the charge carriers or the nature of the charge transfer mechanism are still under debate. This webinar focuses on recent work where collision events between individual particles are tracked with high-speed video and the charge on single particles can be extracted. In freely falling granular streams we observe collide-and-capture events between charged particles and particle-by-particle aggregation into clusters. Size-dependent contact charging is found to produce a variety of charge-stabilized “granular molecules”, whose configurations can be modeled by taking many-body dielectric polarization effects into account. I will also introduce a new approach, based on ultrasonic levitation, for studying contact charging where the very same particles can be forced to undergo multiple head-on collisions. This method allows for measurements under a wide range of environmental conditions as well as applying an electric field, and its exquisite sensitivity makes it possible to determine the net charge transferred in a single contact event.
To the Continuum and Beyond!
Speaker: Prof. Ken Kamrin, Massachusetts Institute of Technology
Originally presented on: 24 November 2020
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Abstract
The ability to predict granular flows efficiently has been a major challenge for years. An accurate and robust continuum model would be ideal, as it could lead to fast simulation of industrial and geo-scale problems. However, there are a number of granular flow behaviors that complicate the development of a continuum treatment including coupled history effects, nontrivial phase change, pressure-sensitive yielding, nonlocal effects, and shear banding phenomena. Rather than attempt to combine all these effects together, this talk will begin by identifying a class of problems that tend to be well-predicted using a very simple continuum treatment. These are problems based on intrusion, where the intrusive dynamics of solid objects (e.g. locomotion, impact) is the primary interest. We then discuss two ways to extend this basic continuum framework with nonstandard "add-ons", in order to handle various complications. First, we will discuss the state of affairs in nonlocal modeling approaches, and focus on some new results pertinent to the physics of nonlocality. Secondly, as an alternative to adding more complexity to the continuum model, we will discuss a hybridized DEM/continuum method that allows us to adaptively choose subdomains in a problem to be treated with continuum modeling vs discrete element modeling. This allows us to keep a simple and fast-to-solve continuum model almost everywhere, while providing a more precise DEM treatment in zones that fall outside the scope of the continuum model.
Glass half full: Embracing the unexpected in granular systems
Speaker: Prof. Christine Hrenya, University of Colorado at Boulder
Originally Presented on: 22 October 2020
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Abstract
Granular and multiphase systems containing solid particles display a host of behaviors unlike those of their single-phase counterparts. The unexpected behaviors are often at odds with current hypotheses, which ultimately leads to a greater physical understanding. In this talk, results from our investigations into liquid-coated particles, clustering instabilities, and cohesive-particle flows will be presented. Each topic will be discussed in chronological order, revealing the unexpected results we encountered, the hypotheses we developed to explain said behaviors, and the testing of these hypotheses until a a physical understanding emerged that we were confident in. This presentation echoes considerably the material covered in the 2020 van 't Hoff lecture (September 2020, TU Delft Process Technology Institute), with modifications to target the Granular Matter webinar audience.
How to Convert a Nano-powder into a Nano-crystalline Solid
Speaker: Prof. Dietrich E. Wolf, University of Duisburg-Essen
Originally Presented on: 22 September 2020
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Abstract
Largely unnoticed by theoretical physics, the last 20 years have seen a revolution in nano-particle processing. One example is that powders - although very porous, when freshly produced - can be converted into a dense solid, which still keeps a microstructure at the nanoscale. The processes are new developments related to what traditionally was called "Spark Plasma Sintering", but has nothing to do with sparks and plasmas. Computer simulations predict intermediate steps of these processes and reveal the underlying mechanisms. I will give a review of recent simulations, with a focus on so-called flash-sintering.
Repulsion and rotation: Penetrating granular matter near a wall
Speaker: Ernesto Altshuler, Group of Complex Systems and Statistical Physics, Physics Faculty, University of Havana
Originally Presented on: 28 August 2020
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Abstract
How a solid object penetrates granular matter near boundaries has been rarely studied. In this seminar I will describe detailed experiments showing how a cylindrical object penetrates into a granular bed near a vertical wall. We find two kinds of motion: the intruder separates from the wall as it sinks, and rotates around its symmetry axis. The repulsion is thought to be caused by the asymmetrical loading of force chains, which are stronger between the object and the wall. The rotation is associated to the tangential friction between the grains and the intruder --a fact that has been neglected in previous research. We introduce simple phenomenological models to explain both motions, and DEM simulations to further explore the parameter space. Moreover, we experimentally show the analogy between the penetration of two intruders released side-by-side far from boundaries, and one intruder released near a vertical wall, which suggests the idea that the method of images might be useful in the field of granular matter.
Ref: V. L Díaz-Melián, A. Serrano-Munoz, M. Espinosa, L. Alonso-Llanes, G. Viera-López and E. Altshuler, Phys. Rev. Lett. 125, 078002 (2020)