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The paper describes a micromechanical approach that explores the anisotropic nature of the capillary stress tensor and its evolution in pendular granular materials via Discrete Element Modeling (DEM) simulations. Dimensionless parameters are used to address the conditions under which the contribution of capillarity (or cohesive interparticle forces) to the stress transmission within a Representative Elementary Volume (REV) is expected to be considerable. From a series of suction-controlled conventional triaxial tests, numerical results show that the significance of the capillary stress and the relative magnitude of its mean to deviatoric components is directly connected to the characteristic particle size and applied stress. In addition, it is shown that the anisotropic character of the capillary stress tensor intensifies with increasing suction. Furthermore, a simple shear test is conducted at constant mean stress to reveal the development of deviatoric capillary stresses in the absence of any change in mean stress, which cannot be captured by the commonly used Bishop’s stress expression.

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Anisotropic nature of the capillary stress tensor

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[EPJ Web of Conferences, Volume 249 (2021)](https://www.epj-conferences.org/articles/epjconf/abs/2021/03/contents/contents.html)

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Powders & Grains 2021

Powders & Grains 2021 will be held in Buenos Aires (Argentina, VIRTUAL) July 5, 13, 21, 29 and Aug 6, 2021. The aim of the conference is to give an up-to-date picture of the broad research field of granular media, powders and grains. Contributions from experts around the world will focus on research topics that cover a wide range of open problems in the field.

Characterization of soil cohesion is crucial for the understanding of geomechanical phenomena such as erosion, excavation, or sediment transport. Moreover, cohesive granular flows occur in many industrial processes ranging from agriculture to pharmaceutical. In the present study, we experimentally investigated cohesion of artificially bonded granular materials made of spherical glass beads cemented by solid paraffin bridges. By means of laboratory tests designed and carried out for investigation at different scales, we measured the tensile yield strength for solid bonds both at the inter-particle micro-scale and at the meso-scale of a cemented sample.

Multi-scale cohesion force measurements for cemented granular materials

Controlling the powder flow by the surface properties of the particles as well as understanding their flowability under different processing dynamics is amongst the major challenges of the powder industry. Indeed, handling large quantities needs powders with good flowability, adequate compressibility, and few electrostatic charges. We have performed a chemical treatment in order to obtain hydrophobic glass bead and its bulk behavior has been compared with raw glass bead. We characterized flow properties under different processing conditions. Both powders presented similar flow behavior in the free fall measurement, however,
hydrophobic glass bead showed lower flow behavior under flow with a rotating drum. Also, hydrophobic glass bead presented negative and raw glass bead almost zero electrostatic charges.

Hydrophobic and raw glass beads flow behavior under different processing conditions

We perform numerical simulations of a two-dimensional frictional granular system under oscillatory shear confined by constant pressure. We found that the system undergoes dilatancy as the strain increases. We confirmed that compaction also takes place at an intermediate strain amplitude for a small mutual friction coefficient between particles. We also found that compaction depends on the confinement pressure while dilatancy little depends on the pressure.

Dilatancy of frictional granular materials under oscillatory shear with constant pressure

Granular materials whether dry or immersed in fluid show dilation or compaction depending upon
the initial conditions, solid fraction and normal stress. Here we probe the transient response of a dense granular
suspension subjected to change of applied normal stress under simple shear. In this aim, normal-stress-imposed
discrete element particle simulations are developed considering the contributions arising from the drag induced
on the particles by fluid phase. These pressure-imposed simulations show transient behaviors of dense granular
suspensions such as dilation or compaction before reaching a steady state following the μ(J) rheology. Less
expectedly, the transient behavior, in particular the height of the system as a function of applied strain, can also
be described by assuming that the system follows the steady μ(J) rheology at all times.

Transients in pressure-imposed shearing of dense granular suspensions

. Magnetic beads attract each other forming rather stable chains. We consider such chains formed by
magnetic beads and push them into a Hele-Shaw cell either from the boundary or from the center. When such
a chain is pushed into a cavity, it bends and folds spontaneously forming interesting unreported patterns. These
patterns are self-similar and an effective fractal dimension can be defined. As found experimentally and with
numerical simulations, the numbers of beads, loops and contacts follow power laws as a function of packing
fraction and, depending on the injection procedure, even energetically less favorable triangular configurations
can be stabilized

Patterns formed by chains of magnetic beads

Particle segregation in geophysical and industrial granular flows is typically driven by gravity and shear. While gravity-induced segregation is relatively well understood, shear-induced segregation is not. In par- ticular, what controls segregation in the absence of gravity and the interplay between shear- and gravity-driven segregation remain unclear. Here, we explore the shear-induced segregation force on an intruder particle in controlled-velocity granular flows where the shear profile is systematically varied. The shear-induced segrega- tion force is found to be proportional to the shear rate gradient, which effectively pushes the large intruder from lower to higher shear rate regions. A scaling law is developed for the segregation force that is accurate over a wide range of overburden pressures and shear rates, and hence inertial numbers.

Exploring shear-induced segregation in controlled-velocity granular flows

Swelling of grains due to water absorption is ubiquitous in many natural materials and industrial products. Hence, a thorough understanding of grain swelling is of great scientific importance. An experimental investigation can only provide limited information, whereas great insight could be gained from numerical modelling, rigorous numerical models for describing particle swelling are essential. Thus, the objective of this study is to develop and validate a discrete element method (DEM) model for swelling of grains. A first order kinetic model was introduced to describe the swelling of a single grain, and subsequently implemented into the DEM code LIGGGHTS. Model validation was performed by comparing the time evolution of the expansion of a packed bed made of super absorbent polymer (SAP) particles obtained numerically and experimentally. It was demonstrated that the developed model can accurately predict the bed expansion. The validated model was then used to investigate the effect of material properties on the swelling behaviour using rice and SAP as the model materials. It is shown that the swelling depends significantly on material properties, as expected; the expansion of the powder bed made of rice is much lower than that of SAP. The developed model could be further advanced to study consequences of swelling phenomena in granular materials, such as segregation and heat generation.

DEM modelling of swelling of grains

Stress propagation in granular impacts is a key problem in physics and many other related disciplines, particularly in civil engineering. However, this topic has been poorly addressed in the literature so far, motivating the present study where the pressure distribution was measured at the bottom of a sandy bed, impacted by a rigid sphere. Exploring different bed thicknesses and intruder’s dropping height, we have found that the structure of this distribution is very similar to the Boussinesq model, initially proposed for a static point-force acting over an isotropic-elastic medium. This surprising result opens up many challenging questions that could help validate or refute this model in other scenarios.

Granular impacts: how far the Boussinesq model can go?

It is well recognised that particle breakage in granular materials is affected by stress level, stress path, initial density, and particle size distribution (PSD), amongst others. Furthermore, it has been shown that breakage has a significant influence on the stress-strain behaviour of soils. This paper compares a commonly used breakage parameter with grading entropy coordinates. Such coordinates enable for the representation of any PSD as a single point in a Cartesian coordinate plane. Hence, the evolution of PSD changes may be easily tracked. This paper aims to demonstrate that grading entropy coordinates are as (or more) effective than other breakage parameters, whilst providing additional insight. On the basis of limited data it is shown that grading entropy coordinates are able to capture the dependence of breakage on stress level, stress path and initial PSD.

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