ARR - Annual Report

This research seeks to develop fundamental knowledge about cohesive particle segregation that will lead to an understanding of the key flow and particle parameters that influence segregation as well as insights into how to predict and control the segregation of cohesive particles. We are using computer simulations, validated by experiments, to develop a physical understanding of the flow and segregation of cohesive particles at the flow level. This IFPRI project leverages US National Science Foundation funds for a similar project. We consider cohesive particle segregation from three viewpoints:

1) Single fine particle interactions

When a small cohesive particle collides with a large one, four scenarios can occur: bouncing-detachment, sticking-attachment, sticking-rolling-attachment, and sticking-rolling-detachment. The specific scenario that occurs depends on the combination of Bond number (Bo), restitution (e), sliding friction (μ), rolling friction (μr) and collision velocity.

2) Percolation of fines

The percolation of fine particles through a static bed of large particles (no shear) demonstrates how different combinations of Bo, e, μ, and μr can result in similar levels of fine particle trapping, indicating an underlying simplicity of cohesive particle segregation.

3) Bounded heap flow

The global effects of particle cohesive properties and shear on segregation are easily measured in terms of the segregation flux in one-sided bounded heap flow. Increasing Bo for all particles decreases segregation, as expected, although cohesion with Bo ≤ 5 has little effect on segregation due to shear. When only small particles are cohesive, increasing cohesion decreases segregation due to small particle clumping which reduces the effective size ratio; when only large particles are cohesive, increasing cohesion increases segregation due to large particle clumping, which increases the effective size ratio. Importantly, the degree of segregation is insensitive to the details of the cohesion model, making computational studies generalizable.

In year two of our effort, simulation studies will continue for the percolation of fines, to understand the influence of cohesion on segregation with and without shear, and for bounded heap flow, to explore the impact of shear on cohesive particle segregation. In addition, we are exploring wax- and polyborosiloxane-coated glass particles as well as partially-wetted particles and hydrogel particles in bounded heap flow experiments to validate simulation results.

The focus of this project is to understand the physical mechanisms that lead to defect formation – pitting, cracking, and delamination – during pharmaceutical tableting. A leading hypothesis among IFPRI members is that trapped interstitial air leads to high pore pressures that tend to fracture adhered particle interfaces after removal of the confining pressure. The project objective is to explore this problem through coupled numerical methods including:

• (i) continuum mixture models
• (ii) the discrete element method (DEM) coupled with a fluid solver.

The primary barrier to using these methods is that fact that the behavior of cohesive powders is not well understood, with neither a generally accepted constitutive relation nor contact model in existence.

To address this, the project has emphasized developing a reliable cohesive powder contact model for usage in DEM. This is the natural progression, since a powder DEM model will be indispensable in determining a constitutive relation for continuum simulations. In particular, we have concentrated on creating a mechanically-derived contact model for adhesive elastic-perfectly plastic particles.

In year one of the project, the majority of the theoretical framework for the contact model (i.e., the MDR contact model) was developed, but a number of issues remained. The JKR-type adhesion of the contact model needed to be validated once significant plastic deformation had occurred and the scheme to respect plastic incompressibility required an overhaul. The contact model was limited to simple symmetric loadings of a single particle, necessitating adaptation to manyparticle interactions. Additionally, the model, initially coded in Matlab, needed implementation in an established software like LIGGGHTS or LAMMPS, with a reliable fluid-solid coupling strategy.

In year two, the theoretical framework was completed by validating the adhesive model within the fully-plastic regime and correcting the plastic incompressibility scheme. The completed contact model was published in the premier solid mechanics journal, Journal of Mechanics and Physics of Solids, as a two part series. E↵orts to extend the contact model to the many-interacting particle case were started, as this was a necessary step to allow simulation of full-scale industrial applications. An initial implementation into LIGGGHTS was carried out and preliminary simulations showed promise in replicating compaction simulator data. Although progress was made, there were problems that remained both from a modeling and computational perspective when attempting to extend to the the many-interacting particles case. On the modeling side, the rigid-flat placement scheme used to extend the MDR contact model formulation to the many-interacting particle case was realized to be inaccurate with increasing polydispersity. On the computational side, a switch from LIGGGHTS to LAMMPS was desirable for three primary reasons:

• (i) LAMMPS is and will remain fully open-source with continual upkeep from Sandia National Laboratories (SNL),
• (ii) Joel Clemmer and Dan Bolintineanu, two SNL research scientists and primary contributors to LAMMPS, volunteered to assist with the implementation of the MDR contact model into LAMMPS, and
• (iii) LAMMPS provides the advantage of allowing immediate coupling with a multi-particle collision dynamics fluid solver, capable of simulating a compressible gas phase.

In the third year, a fully-parallelized implementation of the many-interacting MDR contact model was integrated into LAMMPS. The implementation was tested using both simple configurations involving a small number of particles and large-scale tableting simulations with tens of thousands of particles. This rigorous testing process prompted an overhaul of the rigid flat placement scheme and the development of a new topological algorithm to prevent contact through material during large deformation DEM. The unique ability of the MDR contact model to reconstruct deformed particle shapes was validated by comparisons with FEM predictions. The industrially relevant problem of pharmaceutical tableting was simulated, with experimental data provided by Vertex Pharmaceuticals for the compaction of Avicel PH102 (microcrystalline cellulose) serving as a benchmark. Good agreement was observed between experiments and numerical simulations for axial and radial stress measurements as functions of axial strain. Notably, the simulation also accurately predicted residual radial stresses after the release of axial confining pressure and the ejection force, aligning with experimental results. Preliminary coupled simulations involving the new DEM implementation and a compressible gas phase were also conducted. These simulations demonstrated that fracture caused by entrapped air could occur under specific loading conditions, though these were outside typical operating ranges.

In summary, a robust and reliable powder DEM model has been developed, which is opensource and accessible to all IFPRI members and is currently being used by some such as Amir Esteghamatian from Merck. Its utility in understanding defect formation in pharmaceutical tableting has been qualitatively demonstrated, and we are now positioned to conduct a comprehensive numerical investigation into air-induced defects. The mechanically-derived nature of the contact model o↵ers two significant advantages:

• (i) it provides a solid foundation for the future development of a continuum constitutive relation, and
• (ii) it enables the simulation of a wide range of industrial powder compaction problems beyond tableting, provided the material properties are known.

The following report is split into four chapters. Chapter 1 gives a high level update of the project and details on progress made that is not included in the attached papers. Chapter 2 is the soon to be submitted to Powder Technology paper covering the major advances and results from extending the MDR contact model to the many-interacting particle case. Chapter 3 and 4 are both papers of the two part series published in the Journal of Mechanics and Physics of Solids. These papers contain detailed explanations regarding the theoretical background of the contact model in addition to validations made against finite element simulations.

Particulate gels formed by colloidal particles dispersed in a solvent under a range of conditions and in a broad variety of formulations often undergo syneresis, a phenomenon where the gel structure remains intact but shrinks expelling a small volume fraction of the particle dilute phase. The overall mechanical strength of the gel can delay or hinder particle flocculation, sedimentation, or separation. This can promote the mid- or long-term stability of products. The structural and, especially, the mechanical heterogeneities along with changes in the nature of particle contacts are ubiquitous in these materials, with these factors certainly playing an important role in the poorly understood phenomenon of syneresis.

This project investigates how changes in particle contacts and, in the forces acting on them, may translate into stress redistribution, triggering changes in the gel structures at larger scales inducing syneresis. The research approach is based on computer simulations of particle based models and statistical physics analysis of gel properties that emerge from the evolution of microscopic structures and dynamics of the particles.

During this first year, we have developed a new computational model for particulate gels which allows us to include, in the description of the gels, specific information not only of the particle effective interactions through the solvent, but also of the particle surface contacts, characterized in terms of adhesion, and sliding and rolling friction. The idea is to use contact models, previously introduced for grains, in a general computational framework for colloidal gels. We have focused on two formulations of this model, which correspond to respectively soft and adhesive or stiff and cohesive particles. Regular meetings with IFPRI industrial partners and discussions with IFPRI academic researchers (Vermant and Hsiao) have guided choices towards an experimentally relevant parameter space.

In an extensive simulation study, we have prepared gel structures for different particle volume fractions between 10% and 30% and for different adhesion strengths. Different preparation protocols have been systematically explored to identify conditions that guarantee reproducibility and mechanical stability of the gel structures. We have characterized all gels obtained in terms of the distribution of local pressures and distributions of particle contacts, which display clear differences with varying the contact mechanics, for the same particle volume fraction and preparation protocols. Preliminary results indicate a pronounced difference, in the tendency to contract, for gels formed with different distributions of local pressures and with different contact mechanics. These differences correspond to dramatic differences in the rheology of the gels, as explored in large amplitude shear tests, performed numerically. Finally, we have developed new simulations in which the gel is in contact with surfaces that describe containers walls to analyze how confinement and wall-gel interactions can modify syneretic behavior.

In this report, we summarize the main results achieved and discuss the plan for the coming year. The report is organized in sections that cover, respectively, the numerical model and computer simulation details, the discussion of the preparation protocols, the gel characterization in terms of local pressure and contact distributions, the preliminary results of rheological tests, and a proof of concept of gel formation under confinement.

Flexible Intermediate Bulk Containers (FIBCs), or “Bulk Bags,” are widely used across industries for the transport and handling of bulk materials. They provide clear advantages—lightweight construction, compact storage, dust-free discharge, antistatic options, and cost-effectiveness—making them an attractive alternative to traditional storage and handling systems. However, unlike rigid bins, silos, or stockpiles, which have been extensively studied, FIBCs present unique challenges. Predicting discharge behaviour and preventing blockages remain difficult due to their flexible walls, and existing bulk solid handling theories have not been entirely extended to this class of containers.

This report reflects on the second year of the project and details the progress made in understanding FIBC behaviour. Key advances include experimental testing with a scaled rig, comprehensive pressure measurements, and Discrete Element Method (DEM) modelling and validation for different material sets. Findings to date show that FIBCs behave fundamentally differently from conventional rigid containers, with distinct discharge mechanisms, flow regimes, and stress states. These insights underscore the need for new frameworks to better characterise and predict FIBC performance and provide a foundation for future experimental programs and modelling efforts as the project advances.

This annual report summarizes progress made during the first year of the renewal project period (September 2024 - August 2025). Building upon the significant accomplishments from the initial three-year project (FRR-107-03, 2021-2024), this renewal phase addresses critical gaps identified by industry liaisons and extends mechanistic understanding of flow aid processibility and coating quality across various processing devices and intensities.

Major accomplishments:

Model development: accounting for guest particle aggregation

• Extended Chen's multi-asperity contact model to account for flow aid aggregation rather than assuming ideal monolayer deposition
• Developed mechanistic framework to account for non-uniform flow aid distribution and its aggregation, distinguishing effects of aggregates of flow aid via fractal analysis
• Analyzed effects of aggregation via spherical versus fractal aggregate structures and their differing impacts on cohesion reduction
• Incorporated fractal dimension analysis to relate aggregate size, porosity, and number of primary particles
• Quantified how aggregation reduces effective surface area coverage (SAC) and diminishes cohesion reduction by up to one order of magnitude
• Established relationship between coating device intensity and how differing aggregation effects for different shear imparted by nature of device

Coating device performance evaluation

• Evaluated three coating devices across shear intensity (low to very high shear) and operating mode (batch vs continuous):
• V-blender (low intensity, batch)
• Comil (medium intensity, continuous)
• LabRAM (high intensity, batch)
• Identified best processing parameters for each device type and compared their performance
• Demonstrated that higher shear creates better particle dispersion while lower shear results in nonuniform dispersion and significant aggregation; however, there is an optimum of high shear as well
• Validated model predictions with experimental coating quality analysis via SEM

Pilot scale validation and investigating effects on downstream processibility

• Tested the scalability of dry coating to pilot scale using Comil-U10 (10-20X scale-up)
• Assessed the impact of coating quality on downstream processing operations:
• Feedability assessment showing a dramatic reduction in feeding variability
• Tabletability studies across three drug loading formulations
• Weight variability reduction due to improved feeding consistency
• Demonstrated that coating quality directly impacts product attributes (downstream processibility)

Expanded flow aid (metal oxide based) testing beyond nano-silica flow aids

• Initial validation of metal oxide flow aids (Al₂O₃ and TiO₂)
• Achieved comparable flow enhancements to nano-silica coatings
• Established material property database including size, density, surface energy, and surface chemistry for metal oxide alternatives

Critical advances in mechanistic understanding:

The project has successfully addressed key limitations in guest particle coating assumptions. Previous models assumed uniform, monolayer coating; however, experimental evidence clearly showed that processing device intensity significantly affects flow aid aggregation state. The developed models now account for:

1. Aggregate size effect (Primary): Larger aggregates reduce effective SAC and shift contact regime transitions
2. Aggregate morphology effect (Secondary): Compact spherical aggregates perform better than porous fractal structures
3. Device intensity relationship: Processing intensity determines fractal dimension and porosity of resulting aggregates

These advances enable mechanistic understanding of device performance on bulk property improvement while coating with flow aids, moving beyond benchmarking with LabRAM to predictive guidance for scalable devices like V-blender and Comil.

Industry relevance:

This work provides IFPRI members with:

• Mechanistic understanding for selecting appropriate coating devices based on desired coating quality
• Framework explaining performance variations across different processing equipment
• Validated pilot-scale demonstration of scalability and downstream processing benefits
• Expanded flow aid selection beyond silica to include metal oxides

Next steps:

The coming year will focus on:

1. Completing metal oxide flow aid characterization and coating validation
2. Investigating mixing synergy when flow aid-coated material is blended with uncoated components

In the current (renewed) term of the project, we have focused on understanding the flow of cohesive powders in feeders. To study the system experimentally, we synthesized model cohesive powders by combining small quantities of glycerol to dry glass beads. Our experiments showed that the feed rate has the same qwualitative dependence on the ratio of pitch to diameter of the screw as for dry powders. A lacuna in the current understanding of cohesive powders is that there is no reliable constitutive model. During the last year we have addressed this issue by conducting experiments and DEM simulations of flow in a cylindrical Couette cell to measure the velocity and stress fields in the non-inertial slow flow regime.

Our results show a systematic dependence of the velocity and stress fields on the Bond number Bo (a dimensionless measure of the magnitude of the cohesive force). Importantly, the form of the velocity profile is quite similar to that of a non-cohesive powder. The main influence of cohesion is to alter the wall slip and the sharpness with which the velocity decays with radial distance. Another key observation we have made in the past year is that cohesion fundamentally alters the rheology of the powder: for relatively loose powders, cohesion causes a transition from the rapid (inertial) to slow flow regimes, and for relatively dense powders, cohesion extends the range of shear rate of the slow flow regime.

In the previous years of this project, we had presented a non-local constitutive model for non-cohesive powders and demonstrated its efficacy in accurately predicting the velocity field in simple flows and a complex dilation-driven secondary flow. Over the past year, we have concentrated on extending the model to cohesive powders. We have used the results of our experiments and simulations to determine the influence of cohesion on the parameters in the nonlocal model. We now have data for the dependence of the decay length of the velocity on Bo. Our ongoing work is directed at determining the dependence of all the parameters of the model on Bo. By the end of the project, we hope to be able to give recommendations on operating conditions and strategies that will enhance the precision and reliability of screw feeding of powders.

Over the past year we have built on a previously developed mechanistic model of granuleswelling and disintegration behaviour, with the aim to create a practical, fast tool for predicting and designing wet-granulated product performance. This work integrates physics with machine learning so that routine formulation inputs can be turned into reliable performance curves in seconds.

Core Models

• Mechanistic Performance Models (Single-granule Swelling → PBM Disintegration). Coupled single-granule swelling with population balance disintegration model to predict: Rp(t)  (granule radius) and F(t) (released-mass/ particles-released fraction).
• Physics-Informed Neural Network (PINN). A neural network implementation of the same physics that preserves mechanistic meaning while enabling efficient learning across formulations.
• ANN-Based Parameter Mapper. A supervised neural network that ingests standard formulation descriptors (e.g., L/S ratio, %SSG, filler type, %HPMC, initial porosity, skeletal density) and predicts the mechanistic parameters required by the Mechanistic model.

The workflow (Figure A) has been implemented and delivers rapid, physics-based forward prediction by converting formulation descriptors into mechanistic parameters, then full disintegration performance curves. Following further experimental validation, it will be ready to incorporate inverse modelling. Once incorporated, this inverse model will optimize formulation inputs and granulation conditions to give a desired released fraction at specific times.

modelling. Once incorporated, this inverse model will optimize formulation inputs and granulation conditions to give a desired released fraction at specific times.

1. Executive Summary

Flexible Intermediate Bulk Containers (FIBCs), commonly known as 'Bulk Bags,' are essential for transporting bulk materials across a wide range of industries and applications. These bags offer significant advantages, such as lightweight construction, compact storage, dust-free discharge, antistatic properties, and cost-effectiveness, while also providing versatility. For processing plants, while FIBCs offer significant advantages in handling bulk materials, the lack of established methods to predict their discharge rates and prevent blockages presents unique challenges. Addressing these factors is crucial to fully realising the plant’s capacity to maximise uptime and throughput.

This project was initiated in response to the International Fine Particle Research Institute (IFPRI) and its industry members' need for a better understanding of FIBC discharge behaviour. The report marks the first phase of this project, which takes a hybrid approach by combining numerical work using Discrete Element Method (DEM) simulations with experimental and theoretical methods to study the geometry of flexible bins and their discharge regimes. A comprehensive literature review has been conducted, covering flow theories, material properties, consolidation loads, and bin stresses.

By incorporating fundamental bulk material theory and leveraging recent advancements in the study of bulk solid mechanics, the project aims to develop flow models for FIBCs. Experimental testing will play a critical role, as we examine the discharge patterns of specific powders and analyse their flow properties in our laboratory. The report concludes with an outline of the next steps required to complete the research.

Experimental Approach to Investigate the Rheology of Powders

We propose an experimental approach to investigate the rheology of powders and their behavior during compaction and aeration processes. The first step is to develop protocols to synthetize and characterize model cohesive granular materials. The aim is to synthetize particles with tailored properties (stiffness and adhesion) using two technics (micro-polymer particles, or polymer coated silica particles).

Steps Involved

1. The first step involves developing protocols to synthetize and characterize model cohesive granular materials.
2. The second step involves developing tools to characterize particle properties and their bulk rheology.
3. The third step will involve studying different flow configurations encountered in packaging processes.
4. The final step concerns the coupling with air.

The presence of inter-particle cohesion in powders and grains, such as capillary bridges due to moisture, can drastically change their mechanical properties. For instance, cohesion causes grains to agglomerate into clumps of sizes much larger than the size of the constituent particles. The agglomeration of granules makes the processing of such materials challenging, particularly during drying under agitation processes, where agglomerates can reduce the overall efficiency of processes. The methods of drying a mixture of particles and liquid affect the state of agglomeration of the final dried product, particularly through the influence of impacts and shear forces on the agglomerates.

Our research direction in this area is based on the development of model experiments to understand and model the mechanics, size distribution, and time evolution of particle agglomerates. By controlling cohesion and grain properties, as well as the input of energy in the system, we hope to shed some light on the mechanical behavior of agglomerates to develop models at that can closely connect cohesion to agglomerate mechanics. Then, thanks to the models that will be developed, we will be able to consider industrial powders, such as calcium carbonate or powders used in the food industry, among other examples.

During the first year of the project, we developed an oscillating system consisting of a mechanical shaker and a quasi-2D transparent box, which allowed us to observe the agglomerates being mechanically agitated. Experiments with glass beads and water have illustrated the role of acceleration and amplitude of oscillation in determining the agglomerate sizes. In parallel, a second prototype, relying on airflow through a deposited bed, was designed. These tools were described in our last annual report, ARR-51-16 (2022-2023).

Following in-depth interactions and suggestions from IFPRI members, we have since modified the airflow setup to produce a much higher shear in a small region of the total setup during Year 2 of the project. Using small-diameter nozzles now allows us to reach considerably larger flow rates and shear stresses than we studied in the first year. This new setup, its characterization and some results are presented in the following. In addition, some members mentioned their need to also consider lower shear, and we have thus finalized the construction and characterization of a rotating drum, which was in our initial proposal to study the effects of low and moderate amounts of shear on drying wetted grains.

During this year, we were also invited to write a review article for the Royal Society of Chemistry journal Soft Matter, discussing the current state-of-the-art experimental techniques to study model cohesion in granular systems and some perspectives on future research. Since this review may likely be of general interest to some IFPRI members and was also initially strongly motivated by the interactions with IFPRI members, we have included the peer-reviewed version of this manuscript at the beginning of this annual report.

Finally, since our understanding of the evolution of the inter-particle force during drying is very limited, we have further developed an apparatus to measure the temporal evolution of the forces between individual particles during drying in our laboratory. These measurements will also help us to design model agglomerates to better estimate the shear rate and the effect of agitation on the time-evolution of particle agglomerates.