C. Lee, E. Bililign, E. Azéma, K. Daniels, Loading-dependent microscale measures control bulk properties in granular material: An experimental test of the stress-force-fabric relation, Phys. Rev. E, 112, 035401(2025).

The bulk behavior of granular materials is tied to its mesoscale and particle-scale features: Strength properties arise from the buildup of various anisotropic structures at the particle-scale induced by grain connectivity, force transmission, and frictional mobilization. More fundamentally, these anisotropic structures work collectively to define features like the bulk friction coefficient and the stress tensor at the macroscale and can be explained by the stress-force-fabric (SFF) relationship stemming from the microscale arrangement of the forces and fabric. Although the SFF relation has been extensively verified by discrete numerical simulations, a laboratory real- ization has remained elusive due to the challenge of measuring both normal and frictional contact forces. In this study, we analyze experiments performed on a photoelastic granular system under four different loading conditions: uniaxial compression, isotropic compression, pure shear, and annular shear. During these experi- ments, we record particle locations, contacts between particles, and normal and frictional forces to measure the particle-scale response to progressing strain. We experimentally assess the SFF relation across multiple loading conditions in a two-dimensional photoelastic granular system. We track microscale measures like the packing fraction, average coordination number, and average normal force, along with angular distributions of the interparticle contacts and the interparticle forces. We then track the anisotropy in the angular distributions of contacts and forces and connect these particle-scale anisotropies to bulk behavior using the SFF relation. The SFF relation provides compact expressions for both the stress tensor and the bulk friction coefficient in terms of fabric and force anisotropies. Our results demonstrate that these expressions accurately capture the bulk stress and friction across different loading histories, validating the predictive power of the SFF framework. Additionally, we test the assumption that contact and force anisotropies contribute equally to load transmission in our granular packings and show that this assumption is sufficient at large strain values and can be applied to areas like rock mechanics, soft colloids, or cellular tissue where force information is inaccessible.

W. Wang, J. Bares, M.Renouf, E. Azéma, Flow regimes and repose angle in a rotating drum filled with highly concave particles, Phys. Rev. E, 112, 015417(2025).

We present a series of experiments investigating the flow regimes and repose angles of highly concave particle packings in a rotating drum. By varying grain geometry from spherical to highly nonconvex shapes, adjusting frictional properties and the particle number of branches, we examine how these parameters and the drum speed influence the flow behavior. Our study identifies two distinct flow regimes: the rolling regime, where granular matter exhibits solid-like behavior near the walls and flows like a liquid near the free surface, and the slumping regime, characterized by cyclic avalanches and solid body rotations. Using quantitative criteria such as the repose angle difference and the area ratio of particle packings, we construct phase diagrams delineating the crossover between these regimes. Our findings highlight the significant effects of particle concavity, friction, and rotation speed on the flow dynamics of granular materials, providing new insights into the mechanical behaviors of metagranular matter.

O. Polania, M.Renouf, M. Cabrera, N. Estrada, E. Azéma, Monodisperse behavior of polydisperse flows, Phys. Rev. E, 111, L043401(2025).

Granular flows can occur under low inertia conditions, called the quasi-static regime, and extend to highly inertial systems, called the inertial regime. In the latter, granular flows, particularly those having a variety of grain sizes—property known as polydispersity—have not been extensively studied. Existing rheological laws for monodisperse flows effectively capture volume and friction variations across inertial ranges, assuming the grains diameter as the flow characteristic length. For polydisperse materials, this assumption is less intuitive, and rheological laws cannot be extended straightforwardly. In this work, we employed the Discrete Element Method to study granular flows across varying inertial levels, aiming to identify a physically based length scale that represents the grain scale for polydisperse flows. We show that the average branch length (i.e., distance between the centers of contacting grains) is a representative value of the material’s grain size distribution, remaining nearly constant across the explored range of inertia. Moreover, we show that monodisperse and polydisperse flows follow common inertial volume and friction laws when the average branch length is considered as the characteristic length. The findings of this work propose a new perspective for understanding the characteristic length of granular flows, providing a comprehensive interpretation based on the grains contacts. They also permit to extend rheological laws, initially proposed for monodisperse flows, to polydisperse flows by considering the characteristic length scale as the average branch length. Finally, Our results are useful for choosing the characteristic length that controls large-scale flows where polydispersity plays an important role.

T. Binaree, P. Jitsangiam, M.Renouf, E. Azéma, Packing of cohesive angular particles: Cohesive strength, structure, and effects of angularity, Phys. Rev. E, 111, 15407 (2025)

Employing extensive 2D contact dynamics simulations, we analyze the effects of grain shape angularity on the quasistatic shear strength properties of cohesive granular packings. We consider sticky irregular polygons ranging from disklike shapes to triangle. We find that the Mohr-Coulomb cohesion (i.e., the cohesive strength) is an increasing function of grain angularity. Meanwhile, the macroscopic friction angle increases with angularity and saturates for the most angular shapes, similar to dry cases. Using an effective stresslike approach, we show that the Mohr-Coulomb cohesion emerges from the cohesive force network for all shapes. From this, a micromechanical model reminiscent of the so-called “Rumpf” formula, is derived and reveals the increasing competition between the macroscopic friction, the anisotropy of the cohesive contacts network, and the grain shape parameter (i.e., the number of sides of the polygons) in the variation of Mohr-Coulomb cohesion with increasing angularity.

W. Wang, J. Barés, M.Renouf, E. Azéma, Steady granular flow in a rotating drum: A theoretical nonlocal model for characterizing stress, velocity, and packing fraction profiles, encompassing grain shape effects from convex to highly concave, Phys. Rev. Research, 7, 043310 (2024). https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.6.043310

In this work, we investigate the effect of particle shape, ranging from spherical to highly concave, on steady flows in a rotating drum, a system that facilitates a continuous transition from a jamming state at greater depths to an unjamming state at shallower regions. We develop an analytical model to elucidate granular behavior within the rotating drum: (i) First, by decomposing the shear stress, we reconcile the discrepancy between simulation data and theoretical predictions, establishing a relationship with the angle of repose. (ii) Second, we extend the generalized Bagnold scaling, coupled with a nonlocal fluidity relation based on packing fraction, providing a framework for a correlation between shear stress, shear rate, and packing fraction. Additionally, we introduce a characteristic length to quantify the influence of particle shape and drum speed. This analytical model offers explicit functional forms for physical quantity profiles, which are validated experimentally in a thin rotating drum and numerically in a two-dimensional rotating drum. Our results demonstrate that this model accurately describes the change in velocity from the bottom of the drum to the free surface. Moreover, for different shapes of particle and drum speeds, the characteristic length captures the interplay between shear stress, shear rate, and the variation of packing fraction.

 P. Quiroz-Rojo, D. Cantor, M.Renouf, C. Ovalle, E. Azéma, Rev assessment of granular materials with varied grading based on macro- and micro-mechanical statistical data, Acta Geotechnica (2024)

To assess the mechanical behavior of granular materials in triaxial tests, a mandatory condition is to guarantee a repre- sentative elemental volume (REV) sample. This is achieved by limiting the minimum sample size and the coarsest particle in the sample (dmax). The common geotechnical practice is based on the sample scales H/D and a 1⁄4 D=dmax, where D is the sample diameter and H is its height. While, it is widely accepted that H/D should be between 2 and 2.5, international standards do not agree on the minimum a, and the recommended values vary widely between 5 and 20. Moreover, the impact of particle size distribution on REV is not well understood and is consequently overlooked by most standards. In this paper, we present a study of the effects of a and grading on the critical shear strength of granular materials. We conducted DEM simulations of triaxial tests on samples with values of a ranging from 5 to 20 and grading that varied from mono-size particle assemblies to samples, where the ratio between the coarsest and finest particle was dmax=dmin 1⁄4 4. The results show that the minimum a required to obtain an REV depends on grading. While, for mono-size particle assemblies REV conditions are obtained for a 12:5, better graded samples behave as REV once a  8. A detailed analysis of macro and microscopic parameters reveals that a is not necessarily the most suitable parameter to assess REV scales. We discover that, in our samples, a unique relationship between critical shear strength and the number of grains carrying interparticle forces (Np) exists independently of grading. In effect, REV can be systematically defined as long as Np  3000. The physical source of this observation is linked to the evolution of contact arrangement and force transmission mechanisms, which evolve according to the number of particles engaged in load bearing.

D. Aponte, N. Estrada, J. Barés, M. Renouf, E. Azéma, Geometric cohesion in two-dimensional systems composed of star-shaped particles, Phys. Rev. E, 1009, 044908 (2024)

Using a discrete element method, we investigate the phenomenon of geometric cohesion in granular systems composed of star-shaped particles with 3 to 13 arms. This was done by analyzing the stability of columns built with these particles and by studying the microstructure of these columns in terms of density and connectivity. We find that systems composed of star-shaped particles can exhibit geometric cohesion (i.e., a solidlike behavior, in the absence of adhesive forces between the grains), depending on the shape of the particles and the friction between them. This phenomenon is observed up to a given critical size of the system, from which a transition to a metastable behavior takes place. We also have evidence that geometric cohesion is closely linked to the systems’ connectivity and especially to the capability of forming interlocked interactions (i.e., multicontact interactions that hinder the relative rotation of the grains). Our results contribute to the understanding of the interesting and potentially useful phenomenon of geometric cohesion. In addition, our work supplements an important set of experimental observations and sheds light on the complex behavior of real, three-dimensional, granular systems.

D. Cantor, E. Azéma, C. Ovalle, Failure of an effective stress approach in polydisperse wet granular materials, Phys. Rev. Research, 6, L022008 (2024)

In this Letter, the effective stress principle (ESP)—originally developed for granular materials saturated with water and recently extended to unsaturated ones via simulations—is shown to fail once the material presents wide grain size distributions. We demonstrate that the current ESP approaches cannot capture the Mohr-Coulomb strength parameters as soon as the grain size span exceeds dmax/dmin∼4. This failure is attributed to significant differences in the fabric generated by solid interactions in the wet material, which are supposedly capable of matching the characteristics of the dry material. We show that a generalization of the ESP requires not only macroscopic considerations but also direct attention to the nature of contact and force networks.

O. Polania, N. Estrada, E. Azéma, M. Renouf, M. Cabrera Polydispersity effect on dry and immersed granular collapses: an experimental study, Journal of Fluid Mechanics, 983 A40, (2024)

The column collapse experiment is a simplified version of natural and industrial granular flows. In this set-up, a column built with grains collapses and spreads over a horizontal plane. Granular flows are often studied with a monodisperse distribution; however, this is not the case in natural granular flows where a variety of grain sizes, known as polydispersity, is a common feature. In this work, we study the effect of polydispersity, and of the inherent changes that polydispersity causes in the initial packing fraction, in dry and immersed columns. We show that dry columns are not significantly affected by polydispersity, reaching similar distances at similar times. In contrast, immersed columns are strongly affected by the polydispersity and packing fraction, and the collapse sequence is linked to changes of the basal pore fluid pressure P. At the collapse initiation, negative changes of P beneath the column produce a temporary increase of the column strength. The negative change of P lasts longer in polydisperse columns than in monodisperse columns, delaying the collapse sequence. Conversely, during the column spreading, positive changes of P lead to a decrease of the shear strength. For polydisperse collapses, the excess of P lasts longer, allowing the material to reach farther distances, compared with the collapses of monodisperse materials. Finally, we show that a mobility model that scales the final runout with the collapse kinetic energy remains true for different polydispersity levels in a three-dimensional configuration, capturing the scaling between the micro to macro controlling features.

T. Binaree, S. Kwunjai, P. Jitsangiam, E. Azéma and G. Jing Assessment of macro and micro mechanical properties of fresh and deteriorated ballast combining laboratory tests and 2D-discrete element methods, Construction and Building Materials, 420, 135525 (2024)
https://www.sciencedirect.com/science/article/pii/S0950061824006664

This research initially investigates the mechanical behavior of fresh and used ballast grain assemblies through a combination of laboratory experiments and discrete element modeling (DEM). The extended Los Angeles abrasion (LAA) test generated the used ballast grains defined at a fouling index (FI) of 40%, while fresh ballast grains serve as the reference material. Morphological analysis of angularity properties of fresh and used ballast grains was employed in producing 2D polygonal shapes implemented in a discrete element code. Numerical simulations using the contact dynamics method examined the bi-axial compression of the 2D-equivalent fresh and used ballast grain assemblies under varying confining pressures. Results show that both fresh and used ballast grain packings exhibit similar stress-strain curves, with fresh ballast packing displaying slightly higher shear strength (5%) at the residual state. Fresh ballast packing is denser than the used one at the peak shear state, while residual states show nearly identical volume fractions. Microscale analysis of contact and force networks reveals that the used ballast assembly is more anisotropic in contact at peak stress. At the same time, both packings exhibit similar contact anisotropy at the residual state. Fresh ballast packing exhibits higher force anisotropy, particularly in frictional forces, explaining the excess shear stress observed at the residual state. These findings suggest that used ballast grains mobilize local friction less efficiently than fresh ballast grains, potentially impacting the lateral strength properties of ballasted tracks. This study provides valuable insights into the behavior of used and fresh ballast grains, serving as a foundation for further investigations.