Influence of cohesive forces on the macroscopic properties of granular assemblies (original) (raw)

Effect of cohesion on local compaction and granulation of sheared soft granular materials

EPJ Web of Conferences

This paper results from an ongoing investigation of the effect of cohesion on the compaction of sheared soft wet granular materials. We compare dry non-cohesive and wet moderately-to-strongly cohesive soft almost frictionless granular materials and report the effect of cohesion between the grains on the local volume fraction. We study this in a three dimensional, unconfined, slowly sheared split-bottom ring shear cell, where materials while sheared are subject to compression under the confining weight of the material above. Our results show that inter-particle cohesion has a considerable impact on the compaction of soft materials. Cohesion causes additional stresses, due to capillary forces between particles, leading to an increase in volume fraction due to higher compaction. This effect is not visible in a system of infinitely stiff particles. In addition, acting oppositely, we observe a general decrease in volume fraction due to increased cohesion for frictional particle, which we attribute to the role of contact friction that enhances dilation.

Compaction Dynamics of Wet Granular Assemblies

Physical Review Letters, 2010

The extremely slow compaction dynamics of wet granular assemblies is studied experimentally. The cohesion, due to capillary bridges between neighboring grains, is tuned using different liquids having specific surface tension values. The compaction dynamics of a cohesive packing obeys an inverse logarithmic law, like most dry random packings. However, the characteristic relaxation time grows strongly with cohesion. A model, based on free volume kinetic equations and the presence of a capillary energy barrier, is able to reproduce quantitatively the experimental curves.

From liquid to solid bonding in cohesive granular media

Mechanics of Materials, 2011

We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: 1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; 2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; 3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the consolidation of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.

Force transmission in cohesive granular media

2010

We use numerical simulations to investigate force and stress transmission in cohesive granular media covering a wide class of materials encountered in nature and industrial processing. The cohesion results either from capillary bridges between particles or from the presence of a solid binding matrix filling fully or partially the interstitial space. The liquid bonding is treated by implementing a capillary force law within a debonding distance between particles and simulated by the discrete element method. The solid binding matrix is treated by means of the Lattice Element Method (LEM) based on a lattice-type discretization of the particles and matrix. Our data indicate that the exponential fall-off of strong compressive forces is a generic feature of both cohesive and noncohesive granular media both for liquid and solid bonding. The tensile forces exhibit a similar decreasing exponential distribution, suggesting that this form basically reflects granular disorder. This is consistent with the finding that not only the contact forces but also the stress components in the bulk of the particles and matrix, accessible from LEM simulations in the case of solid bonding, show an exponential fall-off. We also find that the distribution of weak compressive forces is sensitive to packing anisotropy, particle shape and particle size distribution. In the case of wet packings, we analyze the self-equilibrated forces induced by liquid bonds and show that the positive and negative particle pressures form a bi-percolating structure.

The influence of grain shape, friction and cohesion on granular compaction dynamics

The European Physical Journal E, 2007

This article is a review of our recent and new experimental works on granular compaction. The effects of various microscopic parameters on the compaction dynamics are addressed, in particular the influence of the grain shape, the friction and the cohesion between the grains. Two dimensionnal and three dimensionnal systems are analysed. And the role of dimensionality will be emphasized. Theoretical and numerical investigations provide additional informations about that phenomenon. Indeed numerical models permit us to study the influence of some parameters not easily accessible experimentally. Our results show that the above mentioned parameters have a deep impact on the compaction dynamics. Anisotropic grains lead to two different compaction regimes separated by a "burst" of the packing fraction. Friction is observed to modify how the grains are arranged in the pile. This is confirmed by numerical simulations. Cohesive forces between particles inhibit compaction and lead to extremely low values of the packing fraction.

Predicting granule behaviour through micro-mechanistic investigations

International Journal of Mineral Processing, 2003

A novel micro-manipulator device has been developed to observe and measure, directly, the behaviour of binder liquid bridges between pairs of solid particles. The objective has been to develop the fundamental understanding of the role of the liquid and solid properties in the growth and consolidation of granules, from the initial contact between the liquid and particles to the resultant multi-particle bodies. On the particle level, it is the liquid bridges that are responsible for the strength of ''wet'' agglomerates, since they hold the particles together. In this paper, results of experiments will be reported that identify the role of liquid surface tension, bridge Laplace pressure, liquid viscosity and, hence, wetting behaviour, in the axial strength of the bridges. In particular, the differences in bridge shape when particles of different surface properties come together (i.e. in mineral mixtures) provides a crucial insight into whether granules will grow successfully or not. A parabolic approach to describing the shapes adopted by the liquid bridges, from which parameters such as resistance to deformation can be calculated, will be shown. From the theoretical behaviour of individual bridges, determined through the direct experimental observations, a simple model has been constructed, which relates granule porosity, liquid content and the physicochemical properties of the materials to the agglomerate hardness. Some experimental measurements using spherical particles and powders commonly granulated in the pharmaceutical industry, such as lactose, will be compared to the model predictions and the role of interparticle friction and liquid surface tension and viscosity will be shown quantitatively.

Macroscopic bulk cohesion and torque for wet granular materials

2015

Wet granular materials in steady-state in a quasi-static flow have been studied with discrete particle simulations. The total torque is an experimentally accessible macroscopic quantity that can be used to investigate the shear strength, bulk cohesion and other properties of the materials. We report in this paper how the macroscopic bulk cohesion and torque required to rotate the system change with the liquid content. Consequently, micro-macro correlations are obtained for the macro properties as a function of the microscopic liquid bridge volume which is one factor dominating the contact force.

Structure of Sheared Cohesive Granular Bulk

Particles in Contact

The particle-particle interactions on micro scale determine the macroscopic flow behaviour of bulk solids as in shear testers and in industrial facilities. However, although the flow behaviour can be measured on macro scale and bulk solid facilities as silos can be designed based on reliable engineering knowledge, the microscopic physics causing the wide fluctuation in flow properties of the different bulk solids is still not deeply understood. Therefore, the motion of individual particles in shear testers was determined experimentally as well as by discrete element method (DEM) simulations. The experimental detection of the particle motion was achieved by an own-built micro torsional shear tester which can be placed into a X-ray tomography device (µCT) and a customized statistical analysis method to extract the individual trajectories of almost all particles even at large angle increments of up to 5° between the single tomographic measurements. The two bulk solids, borosilicate glass beads and potassium chloride, with particle sizes in the range of 10 to 100 µm show very different contact behaviour, on one side viscoelastic with constant adhesion force and on the other side elastoplastic with time dependent adhesion. By a careful calibration of the DEM contact model parameters using among others shear and nanoindentation tests the microscopic behaviour of the two different model materials could be simulated successfully to predict the shear bands and to determine the macroscopic flow properties. Moreover, a theory for the rate dependent rheology of granular materials showing time consolidation has been developed.

Steady state rheology of homogeneous and inhomogeneous cohesive granular materials

Granular Matter

This paper aims to understand the effect of different particle/contact properties like friction, softness and cohesion on the compression/dilation of sheared granular materials. We focus on the local volume fraction in steady state of various non-cohesive, dry cohesive and moderate to strong wet cohesive, frictionless-to-frictional soft granular materials. The results from (1) an inhomogeneous, slowly sheared split-bottom ring shear cell and (2) a homogeneous, stress-controlled simple shear box with periodic boundaries are compared. The steady state volume fractions agree between the two geometries for a wide range of particle properties. While increasing inter-particle friction systematically leads to decreasing volume fractions, the inter-particle cohesion causes two opposing effects. With increasing strength of cohesion, we report an enhancement of the effect of contact friction i.e. even smaller volume fraction. However, for soft granular materials, strong cohesion causes an inc...

Microscopic and macroscopic compaction of cohesive powders

A novel method to investigate the compaction behaviour of cohesive powders is presented. As a sample, a highly porous agglomerate formed by random ballistic deposition (RBD) of micron sized spherical particles is used. A nanomanipulator deforms this small structure under scanning electron microscope observation, allowing for the tracking of individual particle motion. Defined forces are applied and the resulting deformations are measured. The hereby obtained results are compared to results from threedimensional discrete element simulations as well as macroscopic compaction experiments. Relevant simulation parameters are determined by colloidal probe measurements.