Simulation of the effect of bond strength on the breakage pattern of agglomerates by Distinct Element Method

Chemical, pharmaceutical and food industries amongst many others use agglomerates either as intermediate or manufactured products. The mechanical strength of agglomerates under impact or shear deformation during handling and processing is of great interest to these industries for optimising product specification and functionality. The effect of surface energy on agglomerate behaviour under impact has been investigated using Distinct Element Method (DEM). Four different agglomerates were formed and impacted against a target along the direction of gravity for three different values of the surface energy (0.35, 3.5 and 35.0 J/m²). The agglomerate breakage pattern was influenced by the surface energy and a transition in the mode of failure of agglomerates was observed when the surface energy was varied. Based on the previous work, the surface energy is expressed in terms of Weber Number, We=(V-V0²)ρD/γ. Agglomerates showed extensive deformation under impact at the lowest value of surface energy (0.35 J/m²) and no evidence of fragmentation was found for any value of impact velocity. In this case the agglomerates behaved macroscopically in a ductile mode. At values of surface energy larger than 3.5 J/m² the agglomerates fragmented at the same time as local damage around the impact site occurred. This type of behaviour is typical of semi-brittle material failure. Therefore, the breakage pattern of agglomerates is influenced by the surface energy

Analysis of the flowability of cohesive powders using Distinct Element Method

Computer simulations using Distinct Element Method (DEM) have been carried out to investigate the effect of cohesion on the flowability of polydisperse particulate systems. For this purpose, two assemblies with different values of surface energy and made of 3000 spheres with the mechanical properties of glass beads were considered. The analysis of the flowability of the powders is presented in terms of the unconfined yield stress as a function of strain rate for different pre-consolidation loads. For values of the surface energy of 1.0 J/m2 and strain rates lower than 6 s− 1, the unconfined yield stress does not change significantly indicating a quasi-static behaviour of the particulate assemblies during the compression process. For larger strain rates, the unconfined yield stress varies with the power index of 1.2 of the strain rate. The influence of the pre-consolidating stress on the powder behaviour has also been investigated and a flow factor was obtained from the linear relationship between the unconfined yield stress and pre-consolidation stress. The computer simulations show qualitatively a good agreement with the experimental trends on highly cohesive powder flow behaviour

Analysis of the effect of cohesion and gravity on the bulk behaviour of powders using Discrete Element Method

Computer simulations using Distinct Element Method have been carried out to analyse the bulk behaviour of a polydisperse assembly of glass beads. For this purpose an assembly made of 3000 spheres were generated to which the mechanical properties of glass beads were assigned. The system was initially compressed isotropically at a strain rate of 1 s-1 in the absence of gravity and surface energy. Once the assembly reached a packing fraction of about 0.62, the effects of cohesion and gravity on the bulk behaviour were analysed for two different cases. In the first case only gravity was applied, whilst in the second case both gravity and surface energy were acting on the particles. The evolution of the components of the stress tensor for the case in which only gravity was applied indicated that the gravity did not appreciably affect the isotropy of the system. In contrast, the system in which surface energy was introduced became anisotropic. The concept of unconfined yield stress of bulk cohesive powders was used to analyse the effect of surface energy and strain rate. For values of surface energy of 1.0 J/m2 and of strain rate lower than 1 s-1 the unconfined yield strength did not change significantly indicating a quasi-static behaviour for the compression process. However, for values of strain rates larger than 1 s-1 the unconfined yield strength increased with the strain rate, following a power law trend with an index of 1.7

Mechanistic analysis and computer simulation of impact breakage of agglomerates: Effect of surface energy

Agglomerates are ubiquitous as intermediate or manufactured products in chemical, pharmaceutical and food industries. During handling and processing they may suffer breakage if they are weak. On the other hand, if they are too strong, their dispersion and disintegration could be difficult. The control of their mechanical strength is therefore highly desirable. However, the analysis of agglomerate strength is complex due to the large number of parameters that influence agglomerate behaviour, such as the primary particle size, density and elastic modulus, and the interparticle bond strength. A simple mechanistic model is presented here which relates the number of broken contacts in agglomerate due to impact velocity, interparticle adhesion energy and the particle properties of the particles forming the agglomerate. The model is based on the hypothesis that the energy used to break contacts during impact is proportional to the incident kinetic energy of the agglomerate. The damage ratio defined as the ratio of broken contacts to the initial number of bonds is shown to depend on the dimensionless group, Δ, in the form (ρV2D5/3E2/3)/ Γ5/3, where V is the impact velocity, E the elastic modulus, D the particle diameter, ρ the particle density and Γ the interface energy. This dimensionless group, Δ, incorporates the Weber number, (ρDV2/Γ), which was previously shown to be influential in agglomerate breakage, and may be presented in the form, Δ=WeIe2/3 , where Ie = ED/ Γ. The predicted dependency of the damage ratio on the surface energy has been tested using Distinct Element Method (DEM). Four different agglomerates have been formed and impacted against a target for three different values of the surface energy of the primary particles. The simulation results show that the effect of surface energy is better described by the above mechanistic model than by the Weber number alone, as previously used to characterise the impact strength of agglomerates

Impact Fracture of Composite and Homogeneous Nanoagglomerates

It is not yet clear on whether the fracture characteristics of structured composite capsules and homogeneous nanoagglomerates differ significantly under impact loading conditions. Experimental measurement of impact fracture properties of such small agglomerates is difficult, due to the length and time scales associated with this problem. Using computer simulations, here we show that nanoagglomerates are subjected to normal impact loading fracture within a few nanoseconds in a brittle manner. The restitution coefficient of the nanoagglomerates varies nonlinearly with initial kinetic energy. The fracture of nanoagglomerates does not always happen at the moment when they experience the maximum wall force, but occurs after a time lag of a few nanoseconds as characterised by impact survival time (IST) and IST index. IST is dependant on the initial kinetic energy, mechanical and geometric properties of the nanoagglomerates. For identical geometries of the capsules, IST index is higher for capsules with a soft shell than for these with a hard shell, an indication of the enhanced ability of the soft nanocapsules to dissipate impact energy. The DEM simulations reported here based on theories of contact mechanics provide fundamental insights on the fracture behaviour of agglomerates—at nanoscale, the structure of the agglomerates significantly influences their breakage behaviour

COVID-19 outbreaks in a transmission control scenario: challenges posed by social and leisure activities, and for workers in vulnerable conditions, Spain, early summer 2020

Severe acute respiratory syndrome coronavirus 2 community-wide transmission declined in Spain by early May 2020, being replaced by outbreaks and sporadic cases. From mid-June to 2 August, excluding single household outbreaks, 673 outbreaks were notified nationally, 551 active (>6,200 cases) at the time. More than half of these outbreaks and cases coincided with: (i) social (family/friends’ gatherings or leisure venues) and (ii) occupational (mainly involving workers in vulnerable conditions) settings. Control measures were accordingly applied

Computer simulations of particle-bubble interactions using discrete element method

Discrete Element Method computer simulations have been carried out to analyse the kinetics of collision of multiple particles against a central bubble whilst immersed in water. Two hundred particles, with diameters ranging between 24 and 66 mum, were randomly positioned within a maximum distance from the surface of a bubble of 2 mm in diameter. Initial particle velocities were random in direction and value and they followed Gaussian distributions with standard deviations of 1.0, 0.1 and 0.0. Two possible cases corresponding to relationships of the hydrophobic force with the distance between the particles and bubble have been analysed. In the first case the relationship had the form 1/d, whilst in the second case a relationship in the form 1/d2 was considered. The differences in the capture efficiency of the particles predicted by the two models were drastic. All particles were captured by the bubble in the first case for any distance smaller than 1 mm. However, only 60 % of the particles were captured in the second case even for distances smaller than 0.1 mm. This work provides simulation results that will aid in the future determination of a general hydrophobic force model, an approach of great importance to improve mineral separation using froth flotation

Sinking in quicksand: an applied approach to the Archimedes principle

The objective of this paper is to present a laboratory experiment that explains the phenomenon of sinking in quicksand simulated as a fluidized bed. The paper demonstrates experimentally and theoretically that the proportion of a body that sinks in quicksand depends on the volume fraction of solids and the density of the body relative to the density of the quicksand (fluidized bed)

Influence of magnetic and hydrodynamic forces on chain-aggregation and motion of magnetisable particles and composites

The motion of ferromagnetic single particles and linear aggregates of spheres subjected to a gradient magnetic field in the presence of a non-magnetic fluid was investigated through the experimental validation of a new theoretical model. The theoretical model combines the correction of the mutual magnetisation between particles as well as the interaction with the external magnetic gradient and the hydrodynamic drag corrections for linear aggregates for the first time. The drag for linear aggregates of spheres was corrected using a semiempirical equation which was developed from published work on the drag correction for linear clusters of particles. The experimental results show that the ratio of the velocity of aggregates to the velocity of a single particle with the same diameter can be described by the aspect ratio of the aggregate. The solution for the motion equation obtained by numerical analysis showed an excellent agreement with the experimental data for linear aggregates of spheres, while very similar results were obtained for non-spherical particles. This study also examined the influence of the thickness of a non-magnetic coating around the ferromagnetic particles. Core–shell composites were created by dip-coating ferromagnetic particles in a non-magnetic material. When a pair of the core–shell composite particles (doublet) was formed, the velocity of the doublet was found to be lower than for the aggregates of the non-coated particles. This effect was examined as a function of the film thickness. When the thickness of the shell was approximately 50% of the particle radius, the velocity of the doublet was similar to that of a single uncoated particle. This work establishes an understanding of the fundamental effects of thin coatings on magnetic aggregation. Particle coatings provide a novel and simple means for controlling the separation between the magnetic cores and in turn the kinetics of the aggregation and subsequent magnetisation of the final aggregates