Discrete particle modeling and micromechanical characterization of bilayer tablet compaction.

A mechanistic particle scale model is proposed for bilayer tablet compaction. Making bilayer tablets involves the application of first layer compaction pressure on the first layer powder and a second layer compaction pressure on entire powder bed. The bonding formed between the first layer and the second layer particles is crucial for the mechanical strength of the bilayer tablet. The bonding and the contact forces between particles of the first layer and second layer are affected by the deformation and rearrangement of particles due to the compaction pressures. Our model takes into consideration the elastic and plastic deformations of the first layer particles due to the first layer compaction pressure, in addition to the mechanical and physical properties of the particles. Using this model, bilayer tablets with layers of the same material and different materials, which are commonly used pharmaceutical powders, are tested. The simulations show that the strength of the layer interface becomes weaker than the strength of the two layers as the first layer compaction pressure is increased. The reduction of strength at the layer interface is related to reduction of the first layer surface roughness. The reduced roughness decreases the available bonding area and hence reduces the mechanical strength at the interface. In addition, the simulations show that at higher first layer compaction pressure the bonding area is significantly less than the total contact area at the layer interface. At the interface itself, there is a non-monotonic relationship between the bonding area and first layer force. The bonding area at the interface first increases and then decreases as the first layer pressure is increased. These results are in agreement with findings of previous experimental studies.

Scaling of heat transfer and temperature distribution in granular flows in rotating drums.

Accurate prediction of the time required to heat up granular materials to a target temperature is crucial for several processes. However, we do not have quantitative models to predict the average temperature or the temperature distribution of the particles. Here, we computationally investigate the scaling of heat transfer in granular flows in rotating drums. Based on our simulations, which include a wide range of system and material properties, we identify the appropriate characteristic time that is used to derive equations that predict the particles' average temperature and the particles' temperature distribution.

Evolution of the microstructure during the process of consolidation and bonding in soft granular solids.

The evolution of microstructure during powder compaction process was investigated using a discrete particle modeling, which accounts for particle size distribution and material properties, such as plasticity, elasticity, and inter-particle bonding. The material properties were calibrated based on powder compaction experiments and validated based on tensile strength test experiments for lactose monohydrate and microcrystalline cellulose, which are commonly used excipient in pharmaceutical industry. The probability distribution function and the orientation of contact forces were used to study the evolution of the microstructure during the application of compaction pressure, unloading, and ejection of the compact from the die. The probability distribution function reveals that the compression contact forces increase as the compaction force increases (or the relative density increases), while the maximum value of the tensile contact forces remains the same. During unloading of the compaction pressure, the distribution approaches a normal distribution with a mean value of zero. As the contact forces evolve, the anisotropy of the powder bed also changes. Particularly, during loading, the compression contact forces are aligned along the direction of the compaction pressure, whereas the tensile contact forces are oriented perpendicular to direction of the compaction pressure. After ejection, the contact forces become isotropic.

Modeling and simulation of compact strength due to particle bonding using a hybrid discrete-continuum approach.

The compaction of powder beds into solid bodies occurs by virtue of the formation of inter-particle bonds. The mechanical strength of the compact depends on the type of bonding interaction, as well as, the inter-particle contact area created in the compression process. A hybrid quasi-continuum computational approach has been implemented to study the bonding occurring in compressed granular assemblies. The approach resolves the powder bed on the particle level, allowing for the tracking of contact area generated by particle deformation and the computation of history-dependent inter-particle bonding forces. The magnitude of the bonding force is calculated using a synthetically constructed potential aiming to mimic the shape of typical molecular-type interactions. The uniaxial compaction and subsequent relaxation of powder beds, representative of pharmaceutical excipients have been simulated. Due to the bonding occurring between the individual particles the compressed beds acquire tensile strength. Post-relaxation tensile loading of the compacts is used to quantify the magnitude of this tensile strength. To validate the computational results, tablets are prepared using a compaction simulator under conditions closely resembling the simulated scenarios, where subsequently the tablets are subjected to tensile loads until failure. The predicted values for the tablet strength utilizing the present methodology capture the general trends exhibited by the experimental record.

Two-phase densification of cohesive granular aggregates.

When poured into a container, cohesive granular materials form low-density, open granular aggregates. If pressure is applied to these aggregates, they densify by particle rearrangement. Here we introduce experimental and computational results suggesting that densification by particle rearrangement occurs in the form of a phase transition between two configurational phases of the aggregate. Then we show that the energy landscape associated with particle rearrangement is nonconvex and therefore consistent with our interpretation of the experimental and computational results. Our conclusions are relevant to many technological processes and natural phenomena.

Finite element analysis of vertebral body mechanics with a nonlinear microstructural model for the trabecular core.

In this study, a finite element model of a vertebral body was used to study the load-bearing role of the two components (shell and core) under compression. The model of the vertebral body has the characteristic kidney shape transverse cross section with concave lateral surfaces and flat superior and inferior surfaces. A nonlinear unit cell based foam model was used for the trabecular core, where nonlinearity was introduced as coupled elastoplastic beam behavior of individual trabeculae. The advantage of the foam model is that architecture and material properties are separated, thus facilitating studies of the effects of architecture on the apparent behavior. Age-related changes in the trabecular architecture were considered in order to address the effects of osteoporosis on the load-sharing behavior. Stiffness changes with age (architecture and porosity changes) for the trabecular bone model were shown to follow trends in published experimental results. Elastic analyses showed that the relative contribution of the shell to the load-bearing ability of the vertebra decreases with increasing age and lateral wall curvature. Elasto-plastic (non-linear) analyses showed that failure regions were concentrated in the upper posterior region of the vertebra in both the shell and core components. The ultimate load of the vertebral body model varied from 2800 N to 5600 N, depending on age (architecture and porosity of the trabecular core) and shell thickness. The model predictions lie within the range of experimental results. The results provide an understanding of the relative role of the core and shell in vertebral body mechanics and shed light on the yield and post-yield behavior of the vertebral body.