Runyu Yang

Rolling friction in the dynamic simulation of sandpile formation

The contact between spheres results in a rolling resistance due to elastic hysteresis losses or viscous dissipation. This resistance is shown to be important in the three-dimensional dynamic simulation of the formation of a heap of spheres. The implementation of a rolling friction model can avoid arbitrary treatments or unnecessary assumptions, and its validity is confirmed by the good agreement between the simulated and experimental results under comparable conditions. Numerical results suggest that the angle of repose increases significantly with the rolling friction coefficient and decreases with particle size.

Microdynamic analysis of particle flow in a horizontal rotating drum

The flow of particles in a horizontal rotating drum is studied based on the results generated by Distinct Element Method (DEM). The simulation conditions are comparable to those measured by means of Positron Emission Particle Tracking (PEPT), with a drum being 100 mm in diameter, 35% filled by spheres of 3 mm diameter, and rotating at a speed from 10 to 65 rpm. The simulation method is validated from its good agreement with the PEPT measurement in terms of the dynamic angle of repose and spatial velocity fields. The dependence of flow behaviour on rotation speed is then analysed based on the DEM results, aiming to establish the spatial and statistical distributions of microdynamic variables related to flow structure such as porosity and coordination number, and force structure such as particle interaction forces, relative collision velocity and collision frequency. An attempt has also been made to explain the effect of rotation speed on agglomeration based on the present findings.

On the relationship between porosity and interparticle forces

This paper presents an attempt to quantify the relationship between porosity and interparticle forces for mono-sized spheres. Two systems are considered: the packing of wet coarse spheres where the dominant interparticle force is the capillary force, and the packing of dry fine spheres where the dominant force is the van der Waals force. The interrelationships between porosity, capillary force and liquid content are first discussed based on the well-established theories and experimental observations. The resultant relationship between porosity and capillary force is then applied to the packing of fine particles to quantify the van der Waals force in a packing. A generalised relationship between porosity and interparticle forces results as an extension of this analysis. The usefulness of this relationship is finally demonstrated in depicting the fundamentals governing the relationship between porosity and particle size.

Dynamics of wet particles in rotating drums: Effect of liquid surface tension

A numerical model based on the discrete element method was developed to simulate the wet particle flow in a rotating drum. The model explicitly considered the capillary force between particles and liquid distribution within the packed bed. Physical experiments under similar conditions were carried out to validate the model, showing that the simulation and experiment results were quite comparable in terms of the flow patterns, maximum flow repose angle, and the frequency of avalanching. Flow properties in two different states were investigated with the focus on the effect of liquid surface tension. In the quasistatic state with the drum rotating at very low speeds, discrete avalanches were observed after the flow reached the maximum repose angle. However, flow properties had changed well before avalanches occurred. The microscopic analysis indicated that the strength caused by the capillary force reached a minimal when avalanches started. The maximum repose angle increased with increasing capillary force a...

Effect of material properties on the packing of fine particles

The packing of fine particles differs from that of coarse particles because of the strong cohesive interparticle forces. Consequently, material properties of particles have a strong effect on the packing structure of fine particles. This article presents a study of this effect by means of discrete element method. Variables considered include sliding and rolling friction coefficients related to the surface forces, and Hamaker constant and particle density related to the body forces acting on a particle. The results are analyzed in terms of porosity, mean coordination number and radial distribution function. It is shown that porosity increases with sliding and rolling friction coefficients as well as the Hamaker constant, but decreases with particle density, and coordination number and radial distribution function vary with these variables in line with porosity. The results have also been used to link the macrostructural property such as porosity to microstructural properties such as coordination number and radial distribution function. It is demonstrated that porosity can be described as a function of the force ratio between the van der Waals force and gravity force on a particle but the relationship varies with the sliding and rolling friction coefficients.

Critical states and phase diagram in the packing of uniform spheres

This paper presents a study of the structural transition of the packing of uniform spheres with a wide range of packing fractions. Several critical states are identified including the onset of local jamming, onset of global jamming and maximally random jammed (MRJ) states. With the increase of packing fraction ρ, the packing structure transforms from one-dimensional chains to two-dimensional triangles and finally three-dimensional tetrahedra, correspondingly undergoing phase changes from non-jamming to local jamming, global jamming, MRJ and finally crystal structure. There is a competition between FCC (face-centred cubic) and HCP (hexagonal closed packed) in the transition from disordered to ordered structure with the increase of ρ. HCP can transform to FCC after reaching its maximum at ρ=0.7.

Simulation of the packing of cohesive particles

Abstract The packing behaviour of cohesive particles differs from that of coarse, cohesionless particles. To understand the underlying fundamentals, we have conducted a series of studies by means of discrete particle simulation. The forces involved vary with packing conditions. The packing structures are characterised in terms of porosity, coordination number and radial distribution function, and the particle–particle or pore–pore connectivity is analysed with the Voronoi or Delaunay tessellation. The force structures and their link with jamming states are also discussed. Structural properties are shown to be correlated with porosity which links directly to the cohesive force acting on a particle.

DEM study of crystallization of monosized spheres under mechanical vibrations

Abstract The crystallization (disorder-order transition) of monosized spheres under three-dimensional (3D) mechanical vibrations is studied using discrete element method (DEM). The crystallization dynamics and final structure are analyzed for two selected conditions: i.e. the packing of rough spheres (glass beads) with interval vibration and batch-wise feeding (Case I) and the packing of smooth spheres with continuous vibration and total feeding (Case II). The final packing densities are 0.728 and 0.712 for Cases I and II, respectively, higher than that of random close packings. Partial crystallization characterized by the {111}-oriented face centered cubic (FCC) structure can be observed in both packings, which is further confirmed from the analyses of coordination number, radial and angular distribution functions, and Q 6 bond order. Through the tracing of the particles (e.g. the evolutions of velocity and force fields), two crystallization mechanisms are identified: engulfed growth of two adjacent small crystals and epitaxial growth from existing ordered structures (nuclei).

Scanning White-Light Interferometry as a Novel Technique to Quantify the Surface Roughness of Micron-Sized Particles for Inhalation

A novel approach of measuring the surface roughness of spherical and flat micron-sized drug particles using scanning white-light interferometry was applied to investigate the surface morphology of micron-sized active pharmaceutical ingredients (APIs) and excipient particles used for inhalation aerosols. Bovine serum albumin (BSA) and alpha-lactose monohydrate particles were chosen as model API and excipient particles, respectively. Both BSA and lactose particles were prepared with different degrees of surface corrugation using either controlled spray drying (four samples of BSA) or decantation (two samples of lactose). Particle size distributions were characterized by laser diffraction, and particles were imaged by scanning electron microscopy (SEM). Surface roughness of the BSA and lactose particles was quantified by white-light optical profilometry using vertical scanning interferometry (VSI) at full resolution using a 50x objective lens with 2.0x and 0.5x fields of view for BSA and lactose, respectively. Data were analyzed using Vision software (version 32, WYKO), and surface roughness values are expressed as root-mean-square roughness ( Rrms). Furthermore, data were compared to topographical measurements made using conventional atomic force microscopy. Analysis of the optical profilometry data showed significant variation in BSA roughness ranging from 18.58 +/- 3.80 nm to 110.90 +/- 13.16 nm for the smoothest and roughest BSA particles, respectively, and from 81.20 +/- 15.90 nm to 229.20 +/- 68.20 nm for decanted and normal lactose, respectively. The Rrms values were in good agreement with the AFM-derived values. The particle morphology was similar to SEM and AFM images. In conclusion, scanning white-light interferometry provides a useful complementary tool for rapid evaluation of surface morphology and roughness in particles used for dry powder inhalation formulation.

Self-diffusion of wet particles in rotating drums

Axial mixing of wet particles in rotating drums was investigated by the discrete element method with the capillary force explicitly considered. Different flow regimes were observed by varying the surface tension of liquid and keeping other conditions unchanged. The analysis of the concentration and mean square displacement of particles indicated that the axial motion of wet particles was a diffusive process characterised by Ficks law. Particle diffusivity decreased with increasing inter-particle cohesion and drum filling level but increased with increasing drum rotation speed. Two competing mechanisms were proposed to explain these effects. A theoretical model based on the relation between local diffusivity and shear rate was developed to predict particle diffusivity as a function of drum operation conditions. It was also observed that despite the high inhomogeneity of particle flow in rotating drums, the mean diffusivity of flow exhibited a strong correlation with granular temperature, defined as the mean square fluctuating velocity of particles.