We are pleased to present this collection dedicated to highlighting the research of our Editorial Board, entitled “Editors’ Showcase 2023: Granular Matter”. This exclusive collection is open to Editorial Board members and selected external researchers. Specialty Chief Editor, Prof. Lou Kondic, leads this initiative which will focus on new insights, novel developments, current challenges, latest discoveries, recent advances, and future perspectives in the field of Granular Matter. The work presented here celebrates the broad diversity of research performed across the section and aims to put a spotlight on all areas of interest to our Editors. This collection aims to further support Frontiers’ strong community by recognizing and promoting the work of highly deserving Editors and external researchers. Additionally, work submitted in this collection will be taken into consideration when nominating the candidates for our Frontiers in Soft Matter 2023 Awards.

New Jersey Institute of Technology

Frontiers in Soft Matter

Granular Matter

Editors’ Showcase 2023: Granular Matter

Granular matter research spans a number of different techniques, measures, and approaches. On the theoretical front, there are (still) many questions regarding fundamental aspects such as packing and structure. Such considerations are commonly based on the discrete particle-scale approach, although significant inroads have also been reached in the development of continuum models. One obvious question is how to reach new insight allowing for bridging the scales from micro (particle) to macro (system) scale. Another set of questions involves various simplifications that are often considered and could be related to particle shape and interactions or the number of physical dimensions, among others. How useful are the results obtained using an idealized system to describe and understand physical experiments carried out in less ideal setups? On the experimental side, one could ask on one side what the best ways to incorporate theoretical findings are, and on the other one, how to upscale from the laboratory experiment scale to the one relevant to industrial-size applications.The papers in this Research Topic touch on the number of questions listed above, illustrating the breadth of the field of granular matter research. They show both the progress made and the potential for further research.The paper by <a href="https://doi.org/10.3389/frsfm.2023.1326756">Santos et al.</a> focuses on one of the significant challenges in granular research: protocol dependence. The authors consider a seemingly very simple system of spherical granular particles exposed to various compression and relaxation protocols, motivated by the goal of understanding how and to which degree the considered protocols influence the results. An even more basic follow-up question involves the formulation of state variables: which and how many variables need to be specified to define precisely a granular system? While the answer to this question may depend on the specific system considered, the authors show that the question (and the answer) are not straightforward: even for the simple system considered, packing fraction and coordination (contact) number, for example, are shown not to be sufficient.The paper by <a href="https://doi.org/10.3389/frsfm.2024.1340744">Carlevaro et al.</a> follows further along the line of asking essential questions that may not be easy to answer. In this contribution, the authors consider one of the most common granular setups, a silo in a gravitational field with an opening at the bottom, and ask the following question: does the discharge rate depend on the size distribution of the particles? They consider mono- and bi-disperse distributions and measure the flow rate through the opening. The finding is that the particle size distribution does play a role, and even further, that the dependence on the parameter describing the deviation from mono-disperse distribution is non-monotonous. Such a finding is not captured by the established Beverloo equation describing the flow and is also at odds with some of the more recent results in the literature. Once again, there is much more to be done!The approach taken in the work by <a href="https://doi.org/10.3389/frsfm.2023.1302367">Arevalo</a> attempts to bridge the gap from theory to applications by considering more involved mechanisms driving granular dynamics. The paper considers a mechanical stirrer that mixes the particles; the applications of interest are the systems where particles are used for temperature equilibration. The question is how to mix the particle efficiently, another question that has been around for a long time and has yet to receive a general answer. It is found in the paper, for example, that faster dynamics of the stirrer lead to faster mixing, which is not surprising. However, such dependence is discovered to be nonlinear, with unclear origins of nonlinearity. In any case, the paper provides precise predictions and will inspire experimental verifications that should explore the generality of the conclusions and explore another essential question related to upscaling.The fourth contribution by <a href="https://doi.org/10.3389/frsfm.2023.1143958">Armanious et al.</a> considers an application of granular matter: a granular-based filter, illustrating the ability of granular particles to serve as an absorbent of, in the considered case, airborne virus such as SARS-COV-2. The main finding is that granular matter (iron particles and protein nanofibrils are considered) provides high filtration efficiency. However, the price to pay is a large pressure drop across the filter and, therefore, a large energy expenditure. One hopes that simulations and theoretical approaches could be useful here to reduce the pressure drop but keep efficiency high. This paper clearly illustrates the significant potential of granular matter for developing useful devices, but also that much more needs to be learned about these systems.The four papers in this Research Topic illustrate the significant progress of granular research and the number of unanswered questions in this exciting field of soft matter physics.LK: Writing&#x2013;original draft, Writing&#x2013;review and editing.The author(s) declare that no financial support was received for the research, authorship, and/or publication of this article.The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Editorial: Editors’ showcase 2023: granular matter

Granular matter takes many paths to pack in natural and industrial processes. The path influences the packing microstructure, particularly for frictional grains. We perform discrete element modeling simulations of different paths to construct packings of frictional spheres. Specifically, we explore four stress-controlled protocols implementing packing expansions and compressions in various combinations thereof. We characterize the eventual packed states through their dependence of the packing fraction and coordination number on packing pressure, identifying non-monotonicities with pressure that correlate with the fraction of frictional contacts. These stress-controlled, bulk-like particle simulations access very low-pressure packings, namely, the marginally stable limit, and demonstrate the strong protocol dependence of frictional granular matter.

Granular matter takes many paths to pack in natural and industrial processes. The path influences the packing microstructure, particularly for frictional grains. We perform discrete element modeling simulations of different paths to construct packings of frictional spheres. Specifically, we explore four stress-controlled protocols implementing packing expansions and compressions in various combinations thereof. We characterize the eventual packed states through their dependence of the packing fraction and coordination number on packing pressure, identifying non-monotonicities with pressure that correlate with the fraction of frictional contacts. These stress-controlled, bulk-like particle simulations access very low-pressure packings, namely, the marginally stable limit, and demonstrate the strong protocol dependence of frictional granular matter.

Protocol-dependent frictional granular jamming simulations: cyclical, compression, and expansion

A long-standing debate regarding the dynamics of silo discharge revolves around the use of mono-dispersed circular or spherical grains in simplified two-dimensional models. It is well-known that granular systems composed of particles of the same size can generate crystal or quasi-crystal domains with specific structural and dynamic behaviors. Can this ordering affect the flow rate to an extent that monosized systems cannot be good models for realistic materials? In this work, we present simulations of the discharge of two-dimensional silos filled with binary mixtures of circular grains that conserve the same mean particle size. We address the question of how ordering affects the mass flow rate, in particular considering the limit of mono-sized systems. We find that the typical hexagonal order observed does not affect the flow rate significantly. However, the flow rate does exhibit a weak, nonmonotonic dependence on packing bidispersity that correlates with changes in the outpouring speed of grains in the vicinity of the orifice.

A long-standing debate regarding the dynamics of silo discharge revolves around the use of mono-dispersed circular or spherical grains in simplified two-dimensional models. It is well-known that granular systems composed of particles of the same size can generate crystal or quasi-crystal domains with specific structural and dynamic behaviors. Can this ordering affect the flow rate to an extent that monosized systems cannot be good models for realistic materials? In this work, we present simulations of the discharge of two-dimensional silos filled with binary mixtures of circular grains that conserve the same mean particle size. We address the question of how ordering affects the mass flow rate, in particular considering the limit of mono-sized systems. We find that the typical hexagonal order observed does not affect the flow rate significantly. However, the flow rate does exhibit a weak, nonmonotonic dependence on packing bidispersity that correlates with changes in the outpouring speed of grains in the vicinity of the orifice.

Flow rate in 2D silo discharge of binary granular mixtures: the role of ordering in monosized systems

Predicting the mixing or segregation of granular of dense materials is difficult due to their athermal nature and lack of general theories. The goal of this research is to improve the design (diameter of the cylinder) and operation (rotational frequency of the screw conveyor) of the stirrer to attain good mixing. We characterize the mixing by some macroscopic measures and relate its behavior with the design parameters. Afterward, we compute some microscopic measurements that characterize the movement of the spheres and relate them with the operating conditions. The quality of mixing is found to depend on the rotation frequency of the screw conveyor and the gap between the latter and the enclosing cylinder. Besides, some relationships between micro- and macroscopic measures are drawn. This approach can provide guidance on the design or operation of devices to handle granular media when their microscopic behavior is previously known.

Predicting the mixing or segregation of granular of dense materials is difficult due to their athermal nature and lack of general theories. The goal of this research is to improve the design (diameter of the cylinder) and operation (rotational frequency of the screw conveyor) of the stirrer to attain good mixing. We characterize the mixing by some macroscopic measures and relate its behavior with the design parameters. Afterward, we compute some microscopic measurements that characterize the movement of the spheres and relate them with the operating conditions. The quality of mixing is found to depend on the rotation frequency of the screw conveyor and the gap between the latter and the enclosing cylinder. Besides, some relationships between micro- and macroscopic measures are drawn. This approach can provide guidance on the design or operation of devices to handle granular media when their microscopic behavior is previously known.

Connecting micro- and macroscopic variables in the design of a granular stirrer using simulations

The ongoing COVID-19 pandemic has revealed that developing effective therapeutics against viruses might be outpaced by emerging variants, waning immunity, vaccine skepticism/hesitancy, lack of resources, and the time needed to develop virus-specific therapeutics, emphasizing the importance of non-pharmaceutical interventions as the first line of defense against virus outbreaks and pandemics. However, fighting the spread of airborne viruses has proven extremely challenging, much more if this needs to be achieved on a global scale and in an environmentally-friendly manner. Here, we introduce an aerosol filter media made of granular material based on whey protein nanofibrils and iron oxyhydroxides nanoparticles. The material is environmentally-friendly, biodegradable, and composed mainly of a dairy industry byproduct. It features filtration efficiencies between 95.91% and 99.99% for both enveloped and non-enveloped viruses, including SARS-CoV-2, the influenza A virus strain H1N1, enterovirus 71, bacteriophage Φ6, and bacteriophage MS2. While the filtration efficiencies were relatively high, they came at the cost of high pressure drop (≈0.03 bar). We believe that the methods and results presented here will contribute to advancing our understanding of granular-based aerosol filters, hopefully helping the design of highly-efficient granular media with low-pressure drops.

The ongoing COVID-19 pandemic has revealed that developing effective therapeutics against viruses might be outpaced by emerging variants, waning immunity, vaccine skepticism/hesitancy, lack of resources, and the time needed to develop virus-specific therapeutics, emphasizing the importance of non-pharmaceutical interventions as the first line of defense against virus outbreaks and pandemics. However, fighting the spread of airborne viruses has proven extremely challenging, much more if this needs to be achieved on a global scale and in an environmentally-friendly manner. Here, we introduce an aerosol filter media made of granular material based on whey protein nanofibrils and iron oxyhydroxides nanoparticles. The material is environmentally-friendly, biodegradable, and composed mainly of a dairy industry byproduct. It features filtration efficiencies between 95.91% and 99.99% for both enveloped and non-enveloped viruses, including SARS-CoV-2, the influenza A virus strain H1N1, enterovirus 71, bacteriophage &#x3a6;6, and bacteriophage MS2. While the filtration efficiencies were relatively high, they came at the cost of high pressure drop (&#x2248;0.03&#xa0;bar). We believe that the methods and results presented here will contribute to advancing our understanding of granular-based aerosol filters, hopefully helping the design of highly-efficient granular media with low-pressure drops.