United States

The effect of liquid viscosity on sliding friction coefficient of wet granular materials

The mechanical properties of wet granular materials are different to those of dry granular matter. If the wet granular medium is not completely saturated, the capillary bridges form and the surface tension changes the elastic properties of the medium. We studied the sliding friction of a sledge over wet granular media and found that surface tension and viscosity have a crucial effect on the dynamic friction coefficient of the wet granular media. The higher viscosity of the interstitial liquid results in a higher dynamic friction coefficient. Furthermore, viscous interstitial liquid shows time dependency behaviour. Viscosity makes the friction coefficient to increase since causes more energy loss.

Powders &amp; Grains 2021

[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.

technical paper

We use bi-dimensional non-smooth contact dynamics simulations to analyze the isotropic compaction of mixtures composed of rigid and deformable incompressible particles. Deformable particles are modeled using the finite-element method and following a hyper-elastic neo-Hookean constitutive law. The evolution of the packing fraction, bulk modulus, and particle connectivity, beyond the jamming point, are characterized as a function of the applied stresses for a different proportion of rigid/soft particles and two values of friction coefficient. Based on the granular stress tensor, a micro-mechanical expression for the evolution of the packing fraction and the bulk modulus are proposed. This expression is based on the evolution of the particle connectivity together with the bulk behavior of a single representative deformable particle. A constitutive compaction equation is then introduced, set by well-defined physical quantities, given a direct prediction of the maximum packing fraction $\phi_{max}$ as a function of the proportion of rigid/soft particles.

A micro-mechanical compaction model for granular mix of soft and rigid particles

Granular dilatancy has been previously characterised through a simple linear relationship between the packing fraction and dimensionless shear rate. However, this relationship was developed for granular flows in a simple shear cell geometry. Here we examine volume changes in a shear cell with gravity and a vertical chute. In so doing, we show that the packing fraction displays both a local and non-local response, analogous to what is typically observed for the stress ratio μ. We then extend this analysis, looking at the direct connection between the packing fraction and the stress ratio, as well as how these variables behave in a more complex system, specifically a pseudo 2D hopper.

Evidence of a non-local φ(I) response

Particle size segregation is ubiquitous in granular systems with differently sized constituents but is found to diminish in the presence of viscous ambient fluids. We study this inhibiting effect through coupled fluid-particle numerical 
simulations. It is found that size segregation is indeed slower in the presence 
of fluid and this effect becomes more significant as fluid viscosity is increased. Direct calculation of segregation forcing terms reveal that the ambient fluids affect segregation in two major ways: buoyant forces reduce contact pressures, while viscous dissipation diminish particle-fluctuation driven kinetic pressures, 
both of which are necessary in driving large particles up. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not directly affect segregation.

Mechanisms of size segregation in granular flows with different ambient fluids

A linear stability analysis of the hydrodynamic equations of a model for confined quasi-two-dimensional granular gases is carried out. The stability analysis is performed around the homogeneous steady state (HSS) reached eventually by the system after a transient regime. In contrast to previous studies (which considered dilute or quasielastic systems), our analysis is based on the results obtained from the inelastic Enskog kinetic equation, which takes into account the (nonlinear) dependence of the transport coefficients and the cooling rate on dissipation and applies to moderate densities. As in earlier studies, the analysis shows that the HSS is linearly stable with respect to long enough wavelength excitations.

Stability of the homogeneous steady state for a model of a confined quasi-two-dimensional granular fluid

The Mpemba effect is a phenomenon in which an initially hotter sample cools sooner. In this paper, we show the emergence of a non-monotonic Mpemba-like effect in a molecular binary mixture immersed in a viscous gas. Namely, a crossover in the temperature evolution when at least one of the samples presents non-monotonic relaxation. The influence of the bath on the dynamics of the particles is modeled via a viscous drag force plus a stochastic Langevin-like term. Each component of the mixture interchanges energy with the bath depending on the mechanical properties of its particles. This discrimination causes the coupling between the time evolution of temperature with that of the partial temperatures of each component. The non-monotonic Mpemba effect—and its inverse and mixed counterparts—stems from this coupling. In order to obtain analytical results, the velocity distribution functions of each component are approximated by considering multitemperature Maxwellian distributions. The theoretical results derived from the Enskog kinetic theory show an excellent agreement with direct simulation Monte Carlo (DMSC) data.

Non-monotonic Mpemba effect in binary molecular suspensions

The critical state of a granular material made of rounded tetrahedral particles is studied through DEM simulations of triaxial compressions. A minimum number of 7,500 particles is first obtained as the rep- resentative volume element (RVE) for the present triaxial simulations. Then, the macroscopic critical state line (CSL) is shown to be increasing in the (stress, density) space for low confining pressures. Such an unexpected behaviour is explained by the existence of a significant proportion of rattlers. Considering rattlers as voids indeed reinstates a classically decreasing CSL.

Rattlers’ involvement for possibly looser critical states under higher mean stress

We study the influence of particle shape on the evolution of particle breakage process taking place inside rotating cylinders. Extensive particle dynamics simulations taking into account the dynamics of the granular flow, particle breakage, and polygonal particle shapes were carried out. We find that the rate of particle breakage is faster in samples composed of initially rounder particles. The analysis of the active flowing layer thickness suggests that for samples composed of rounder particles a relatively lower dilatancy and higher connectivity lead to a less curved free surface profile. As a result, rounder particles rolling down the free surface have a higher mobility and thus higher velocities. In consequence, the faster breakage observed for rounder initial particles is due to the larger particles kinetic energy at the toe of the flow.

Effect of particle shape on particle breakage inside rotating cylinders

Size limitations of geotechnical testing equipment often require that samples of coarse granular materials have to be scaled in order to be tested in the laboratory. Scaling implies a convenient modification of the particle size distribution (PSD) to reduce particle sizes. However, it is well known that particle size and shape may be correlated in nature, due to geological factors (as an example). By means of two-dimensional contact dynamics simulations, we analyzed the e↵ect of altering the size span on the shear strength of granular materials when particle size and shape are correlated. Two di↵erent systems were considered: one made of only circular particles, and the second made of size-shape correlated particles. By varying systematically the size span we observed that the resulting alteration of material strength is not due to the change in particle sizes. It results instead from the variation of the particle shapes induced by the modification of the PSD, when particle size and particle shape are correlated. This finding suggests that particle shape distribution is a higher order factor than PSD for the shear strength of granular materials. It also highlights the importance of particle shape quantification in soil classification and the case for its consideration in activities such as sampling, sub- sampling, and scaling of coarse materials for geotechnical testing

Impact of sample scaling on shear strength: coupled effects of grains size and shape

The granular column collapse is a benchmark configuration for the study of granular flows in dry, saturated, and submerged conditions. The collapse sequence and resultant mobility is acknowledged to be controlled by the column aspect ratio, while grain properties define the relative transition of each stage. Grain shape effects are found to modify the global shear resistance of granular media, with a strong and coupled interaction when interacting with a fluid. In this work, we present the first steps towards the study of grain shape effects in a column collapse when interacting with an ambient fluid. For this purpose, we use a planar configuration and explore the collapse of a column consisting of rod-like grains and study the initial and after collapse grain orientations. On it, the mobilized grains deposit in a preferential horizontal orientation, but further experiments are required to confirm if a nematic configuration can be achieved.

Planar column collapse of elongated grains

To gain an understanding of the factors affecting the interaction of one grain with its environment as it reaches equilibrium, we study a particle bouncing off a flat surface. The bouncing of the particle leads to dissipation that is usually characterized with the coefficient of restitution, defined as the ratio between the velocity component that is normal to the contact surface just before impact (Vn) and the same component, but immediately after the collision (Vn'), i.e. related to a kinetic energy corresponding to motion in the normal direction. We will show how the coeffcient of restitution is affected by energy stored in other degrees of freedom and transferred to kinetic energy that leads to an increase in normal velocity after the impact Vn', and therefore to the coeffcient of restitution is larger than 1. For this purpose, the evolution of potential, translational kinetic energy and rotational kinetic energy is analysed during the whole relaxation process and just before and after each collision for two different types of particle, a disk and a faceted particle.

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Shape or friction? Which of these characteristics drives the shear strength in granular systems?

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