Vanessa Magnanimo

Effect of cohesion on shear banding in quasistatic granular materials.

Cohesive powders have widely different bulk behavior due to their peculiar interactions. We use discrete element simulations to investigate the effect of contact cohesion on the steady state flow of dense powders in a slowly sheared split-bottom Couette cell, which imposes a wide stable shear band. The intensity of cohesive forces can be quantified by the granular Bond number (Bo), namely the ratio between maximum attractive force and average force due to external compression. We find that the shear banding phenomenon is almost independent of cohesion for Bond numbers Bo<1, however for Bo≥1 cohesive forces start to play an important role, as both width and center position of the band increase. Inside the shear band, the mean normal contact force is independent of cohesion and depends only on the confining stress. In contrast, when the behavior is analyzed focusing on the eigendirections of the local strain rate tensor, a dependence on cohesion shows up. Forces carried by contacts along the compressive and tensile directions are symmetric about the mean force (larger and smaller respectively), while the force along the third, neutral direction follows the mean force. This anisotropy of the force network increases with cohesion, just like the heterogeneity in all (compressive, tensile and neutral) directions.

Role of gravity or confining pressure and contact stiffness in granular rheology

The steady-state shear rheology of granular materials is investigated in slow quasistatic and inertial flows. The effect of gravity (thus the local pressure) and the often-neglected contact stiffness are the focus of this study. A series of particle simulations are performed on a weakly frictional granular assembly in a split-bottom geometry considering various magnitudes of gravity and contact stiffnesses. While traditionally the inertial number, i.e., the ratio of stress to strain-rate time scales, is used to describe the flow rheology, we report that a second dimensionless number, the ratio of softness and stress time scales, must also be included to characterize the bulk flow behavior. For slow, quasistatic flows, the density increases while the effective (macroscopic) friction decreases with increase in either particle softness or local pressure. This trend is added to the

Experiments and discrete element simulation of the dosing of cohesive powders in a simplified geometry

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Tuning the bulk properties of bidisperse granular mixtures by small amount of fines

rheology and can be traced back to the anisotropy in the contact network, displaying a linear correlation between the effective friction coefficient and deviatoric fabric in the steady state. When the external rotation rate is increased towards the inertial regime, for a given gravity field and contact stiffness, the effective friction increases faster than linearly with the deviatoric fabric.

Deformation Modes for Assemblies of Frictionless Polydisperse Spheres

We perform experiments and discrete element simulations on the dosing of cohesive granular materials in a simplified geometry. The setup is a simplified canister box where the powder is dosed out of the box through the action of a constant-pitch screw feeder connected to a motor. A dose consists of a rotation step followed by a period of rest before the next dosage. From the experiments, we report on the operational performance of the dosing process through a variation of dosage time, coil pitch and initial powder mass. We find that the dosed mass shows an increasing linear dependence on the dosage time and rotation speed. In contrast, the mass output from the canister is not directly proportional to an increase/decrease in the number coils. By calibrating the interparticle friction and cohesion, we show that DEM simulation can quantitatively reproduce the experimental findings for smaller masses but also overestimate arching and blockage. With appropriate homogenization tools, further insights into microstructure and macroscopic fields can be obtained. This work shows that particle scaling and the adaptation of particle properties is a viable approach to overcome the untreatable number of particles inherent in experiments with fine, cohesive powders and opens the gateway to simulating their flow in more complex geometries.

Effect of Friction and Cohesion on Anisotropy in Quasi-static Granular Materials under Shear

We study the bulk properties of isotropic bidisperse granular mixtures using discrete element simulations. The focus is on the influence of the size (radius) ratio of the two constituents and volume fraction on the mixture properties. We show that the effective bulk modulus of a dense granular (base) assembly can be enhanced by up to 20% by substituting as little as 5% of its volume with smaller sized particles. Particles of similar sizes barely affect the macroscopic properties of the mixture. On the other extreme, when a huge number of fine particles are included, most of them lie in the voids of the base material, acting as rattlers, leading to an overall weakening effect. In between the limits, an optimum size ratio that maximizes the bulk modulus of the mixture is found. For loose systems, the bulk modulus decreases monotonically with addition of fines regardless of the size ratio. Finally, we relate the mixture properties to the ‘typical’ pore size in a disordered structure as induced by the combined effect of operating volume fraction (consolidation) and size ratio.

From particles in steady state shear bands via micro-macro to macroscopic rheology laws

The challenge of dealing with cohesive powders during storage, handling and transport are widely known in the process and pharmaceutical industries. Simulations with the discrete element method (DEM) provide further insight into the local microstructure of bulk materials. In this work, the DEM approach is presented to investigate the flow behavior of granular systems subjected to different modes of deformations. When uniaxial compression is applied of frictionless, polydisperse spheres above jamming (transition from fluid-like state to solid-like state), the evolution of coordination number (average number of contacts per particle) and pressure as functions of the volume fraction are, astonishingly, identical to results obtained for purely isotropic compression. Analytical predictions for the evolution of pressure and coordination number under isotropic strain can thus be separated from different deformation modes, as applied in this study. After two different modes of volume-conserving deviatoric shear, the results still compare quite well with results for purely isotropic compression. The difference between the two deviatoric modes and uniaxial deformation is examined with respect to the anisotropic stress response as a function of deviatoric strain.

Force correlations, anisotropy, and friction mobilization in granular assemblies under uniaxial deformation

We study the effect of particle friction and cohesion on the steady-state shear stress and the contact anisotropy of a granular assembly sheared in a split-bottom ring shear cell. For non-cohesive frictional materials, the critical state shear stress first increases and then saturates with friction. The contact number density is found to decrease monotonically, while the anisotropy of the contact network saturates after an initial increase. For cohesive powders, the relation between shear stress and confining pressure becomes non-linear. Interestingly the contact number density stays almost unaffected, while the structural anisotropy decreases with increasing cohesion, hinting at a redistribution of the network with almost constant contact number density.

Understanding the effects of inter-particle contact friction on the elastic moduli of granular materials

Particulate systems and granular matter are discrete systems made of many particles; they display interesting dynamic or static, fluid- or solid-like states, respectively, or both together. The challenge of bridging the gap between the particulate, microscopic picture towards their continuum description (via the so-called micro-macro transition) is one of today’s challenges of modern research. This short paper gives a brief overview of recent progress and some new insights about local granular flow rules for soft particles.

Slow dynamics near jamming

We study dense, frictional, polydisperse 3D granular assemblies under uniaxial deformation with Discrete Element Method (DEM) simulations. The overall goal – beyond the scope of the present study – is to link microscopic parameters and observations with the macroscopic behavior, for different elementary deformation modes. At present, we focus on the behavior of the force/contact network during uniaxial deformation, for different coefficients of friction. We discuss the stress and structural anisotropy and the relationship between force intensity weighted by contact state (sticking or sliding, at the Coulomb limit) or force strength. Furthermore, we study the orientational distribution of contacts and forces and the contribution of friction to structural anisotropy. We find that initial isotropic states are irrecoverable, since the structural anisotropy is independent of the deviatoric stress behavior both with and without friction. Contacts display an interesting anisotropy of order four in the presence of friction.