Granular Matter Webinar Series

The Editors of Granular Matter are proud to present a series of webinars organized by the journal.

Stay tuned for the next webinar announcement!

If you have any questions, contact Jack Manzi at


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

Watch it here.

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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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

Watch it Here.

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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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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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

Originally Presented on: 5 January 2021

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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

Watch it here.

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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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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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

Watch it here.

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)