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.

A. Bignon, M. Renouf, R. Sicard and E. Azéma Nonlinear effect of grain elongation on the flow rate in silo discharge, Phys. Rev. E, 108, 054901 (2023)

By means of two-dimensional numerical simulations based on contact dynamics, we present a systematic analysis of the joint effects of grain shape (i.e., grain elongation) and system size on silo discharge for increasing orifice sizes D. Grains are rounded-cap rectangles whose aspect ratio are varied from 1 (disks) to 7. In order to clearly isolate the effect of grain shape, the mass of the grains is keeping constant as well as the condition of the discharge by reintroducing the exiting grains at the top of the silo. In order to quantify the possible size effects, the thickness W of the silos is varied from 7 to 70 grains diameter, while keeping the silos aspect ratio always equal to 2. We find that, as long as size effects are negligible, the flow rate Q increases as a Beverloo-like function with D, also for the most elongated grains. In contrast, the effects of grain elongation on the flow rate depend on orifice size. For small normalized orifice sizes, the flow rate is nearly independent with grain elongation. For intermediate normalized orifice sizes the flow rate first increases with grain elongation up to a maximum value that depends on the normalized size of the orifice and saturates as the grains become more elongated. For larger normalized orifice size, the flow rate is an increasing function of grains’ aspect ratio. Velocity profiles and packing fraction profiles close to the orifice turn out to be self-similar for all grain shapes and for the whole range of orifice and system sizes studied. Following the methodology introduced by Janda et al. [Phys. Rev. Lett. 108, 248001 (2012)], we explain the nonlinear variation of Q with grain elongation, and for all orifice sizes, from compensation mechanisms between the velocity and packing fraction measured at the center of the orifice. Finally, an equation to predict the evolution of Q as a function of the aspect ratio of the grains is deduced.

J. Barés, M. Cárdenas-Barrantes, G. Pinzón, E. Andò, M. Renouf, G. Viggiani, and E. Azéma Compacting an assembly of soft balls far beyond the jammed state: Insights from three-dimensional imaging, Phys. Rev. E, 108, 044901 (2023) https://journals.aps.org/pre/abstract/10.1103/PhysRevE.108.044901

Very soft grain assemblies have unique shape-changing capabilities that allow them to be compressed far beyond the rigid jammed state by filling void spaces more effectively. However, accurately following the formation of these systems by monitoring the creation of new contacts, monitoring the changes in grain shape, and measuring grain-scale stresses is challenging. We developed an experimental method that overcomes these challenges and connects their microscale behavior to their macroscopic response. By tracking the local strain energy during compression, we reveal a transition from granular-like to continuous-like material. Mean contact geometry is shown to vary linearly with the packing fraction, which is supported by a mean field approximation. We also validate a theoretical framework which describes the compaction from a local view. Our experimental framework provides insights into the granular micromechanisms and opens perspectives for rheological analysis of highly deformable grain assemblies in various fields ranging from biology to engineering.

N. Coppin, M. Henry, M. Cabrera, E. Azéma, F. Dubois, V. Legat, J. Lambrechts, Collapse dynamics of two-dimensional dry and immersed granular columns of elongated grains, Phys. Rev. Fluids, 8,094303 (2023)

The collapse dynamics and runout of columns of elongated grains in two dimensions are numerically investigated in dry and immersed conditions, by means of an unresolved finite elements/discrete elements model. The elongated grains are modeled as rigid aggregates of disks. The column aspect ratio is systematically varied from 0.125 to 16 in order to span short and tall columns. To analyze the effect of the initial grain orientation, columns with an initial grain orientation that is either random or aligned with a given direction are both considered. Collapse dynamics, both in dry and immersed cases, are found analogous to that previously observed for circular grain columns, particularly with respect to the power law dependency for the runout as a function of the column aspect ratio. The effect of the fluid mainly results in a decrease of the runout distance. Inter- estingly, the collapse dynamics and runout are not significantly affected by the initial orientation of the grains, except maybe in the extreme case where the grains are all horizontally oriented, which geometrically prevents the collapse. Finally, a scaling based on the front propagation energy is proposed allowing one to unify the runout of short to tall and dry to immersed columns in a single description, regardless of the initial grain orientation.

O. Polania, M. Cabrera, M. Renouf, E. Azéma and Nicolas Estrada, Grain size distribution does not affect the residual shear strength of granular materials: An experimental proof, Phys. Rev. E-Letter, 107, L052901 (2023)

Granular materials are used in several fields and in a wide variety of processes. An important feature of these materials is the diversity of grain sizes, commonly referred to as polydispersity. When granular materials are sheared, they exhibit a predominant small elastic range. Then, the material yields, with or without a peak shear strength depending on the initial density. Finally, the material reaches a stationary state, in which it deforms at a constant shear stress, which can be linked to the residual friction angle φr. However, the role of polydispersity on the shear strength of granular materials is still a matter of debate. In particular, a series of investigations have proved, using numerical simulations, that φr is independent of polydispersity. This counterintuitive observation remains elusive to experimentalists, and especially for some technical communities that use φr as a design parameter (e.g., the soil mechanics community). In this Letter, we studied experimentally the effects of polydispersity on φr . In order to do so, we built samples of ceramic beads and then sheared these samples in a triaxial apparatus. We varied polydispersity, building monodisperse, bidisperse, and polydisperse granular samples; this allowed us to study the effects of grain size, size span, and grain size distribution on φr. We find that φr is indeed independent of polydispersity, confirming the previous findings achieved through numerical simulations. Our work fairly closes the gap of knowledge between experiments and simulations.

O. Polania, M. Cabrera, M. Renouf, and E. Azéma,Collapse of dry and immersed polydisperse granular columns: A unified runout description, Phys. Rev. Fluids, 7, 084304 (2022).
https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.7.084304

The granular column collapse is a simplified version of granular flows such as landslides, avalanches, and other industrial processes mobilized in air or within a fluid. In this configu- ration, the particles collapse in an accelerating phase, reaching a state of constant spreading velocity until they decelerate and stop. Granular flows commonly involve particles of different sizes, a property termed polydispersity. Understanding the role of polydispersity remains a challenging task that is often analyzed with nearly monodisperse systems and demanding a series of simplifications when coupled with a fluid in a numerical model. Here, we study the effect of particle-size polydispersity in dry and immersed granular columns, using a finite element method-discrete element method model for fluid-particle interactions. We show that the velocity of the column collapse and runout distance decrease with an increase in the level of polydispersity in immersed conditions, and remain nearly independent of the level of polydispersity in dry conditions. Moreover, we find that the runout scales with the spreading front kinetic energy, weighted by the ratio between the particles’ density and the density difference between particles and fluid. This scaling helps in identifying the governing processes in polydisperse granular columns, unifying the runout description of both dry and immersed collapses, and indicating that the column initial packing fraction is the governing parameter.

M. Cárdenas-Barrantes, J. Barés, M. Renouf, and E. Azéma,Experimental validation of a micromechanically based compaction law for mixtures of soft and hard grains, Phys. Rev. E, 106, L022901 (2022)

In this Letter, we report on an experimental study which analyzes the compressive behavior of two-dimensional bidisperse granular assemblies made of soft (hyperelastic) and hard grains in varying proportions (κ is the portion of soft grains). By means of a recently developed uniaxial compression setup [Vu and Barés, Phys. Rev. E 100, 042907 (2019)] and using an advanced digital image correlation method, we follow, beyond the jamming point, the evolution of the main mechanical observables, from the global scale down to the strain field inside each deformable grain. First, we validate experimentally and extend to the uniaxial case a recently proposed micromechanical compaction model linking the evolution of the applied pressure P to the packing fraction ϕ [Cantor  et al., Phys. Rev. Lett. 124, 208003 (2020)]. Second, we reveal two different linear regimes depending on whether the system is above or below a crossover strain unraveling a transition from a discrete to a continuous-like system. Third, the evolution of these linear laws is found to vary linearly with κ. These results provide a comprehensive experimental and theoretical framework that can now be extended to a more general class of polydisperse soft granular systems.