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 email@example.com
Collisional Contact Charging in Granular Materials
Speaker: Prof. Heinrich M. Jaeger, The University of Chicago
Originally Presented on: 5 January 2021
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
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
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
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
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)