Focus Series on Challenges in Granular Matter

Granular (or particulate) matter encompasses a wide range of many-body systems for which the interacting bodies are macroscopic particles: sand, cereals, beans, soil, powders, etc. These grains interact collectively via nonconservative forces such as friction and internal dissipation providing myriad emerging phenomena still poorly understood by physicists, chemists, and engineers. These particles range from soft materials which undergo deformation, to hard particles prone to fracture. This Focus Series brings together a number of contributions that frame some of the most important challenges ahead in the area of granular matter. Each paper focuses on describing an unsolved problem that is important for our community to tackle, and proposes routes to solutions.

The paper from Schroeter et al. surveys a range of techniques, from X-ray to radar to smart tracers, to image granular samples in the inside. Hurley & Zhai take this a step further, and describe techniques and challenges for tracking the interparticle forces in 3D systems, including their temporal evolution. Windows-Yule & Neveu provide an overview and comparison of methodologies developed in the calibration and validation of DEM simulations of powder flows and suggest possible future improvements for them. Casiulis & Martiniani describe how to use the tools of statistical mechanics when you don’t know the probabilities of observing the various valid states, by computing the volume of high-dimensional basins of attraction. Several papers address particles which are far from idealized spheres. Barés et al. summarize recent results in how to model particles that are soft enough that deformation dominates the mechanical response, and challenges in modeling them appropriately. Cantor et al. explore the concept of designing non-convex particles with desired bulk mechanical properties and the challenges related to shape description and measuring their mechanical response. Mollon et al. review the state-of-the-art on modeling hard and soft granular particles in confined shear flows pertaining to tribology and fault mechanics and discuss future challenges on addressing the scaling effect, the fusion of numerical and experimental modeling results, and rigorous computational modeling. And several emphasize the importance of taking granular systems from the ideal to the real, including via the calibration and validation of models. Given the present relevance of sustainable energy, Vidal & Gay survey available techniques, and current challenges, for combining field data, lab experiments and simulations to tackle various problems related to the migration of fluids in subsea basins of saturated sands. Cheng et al. overview the state-of-the-art research on root-zone mechanics and bio-hydrology and suggested an integrated experimental and numerical, bottom-to-top approach for multidisciplinary understanding of soil-water-root interaction.