ARR - Annual Report

In the previous year, the influence of different grinding aid types and dosages on different process aspects, such as the residence time and material hold-up within the grinding chamber was studied. Furthermore, the impact of the various grinding aids used throughout this project (Diethylene glycol, Heptanoic acid, Hexanol) on the powder flowability within the progress of grinding process was investigated. For this purpose, a wide range of particle sizes with different grinding aid types and dosages were measured and a relation between the flowability index obtained from the ring shear tester and BET specific surface area was found, that can be used to describe this impact.

In the current year the research is focusing on studying the influence of grinding aids on various bulk material properties and how these properties can be used to describe and explain the impact of grinding aids on the various process aspects of dry grinding in tumbling ball mills. For material characterization, different techniques, like the rotating drum (dynamic angle of repose), static angle of repose, a ring shear tester, a powder rheometer, bulk density, and tap density were utilized. Material characteristics obtained from the static angle of repose and rotating drum do not deliver for the range of investigation reliable values for describing the influence of grinding aids. The results of the bulk densities and tap densities measured for various particle sizes as well as grinding aid types and dosages showed that grinding aids causing high powder flowabilities also lead to high bulk and tap densities, as particles can easily move against other particles, e.g. fine particles can slip between bigger particles creating dense particle bed. In order to account for the impact of grinding aids on the bulk density by measuring the flowability with the ring shear tester, the flowability index weighted with the bulk density results in more pronounced and reliable values of grinding aid dosages. The powder rheometer as a modern technique for material characterization was used to study the influence of grinding aids on material properties. For the application of dry grinding, the measurement of the so-called cohesion strength was found to deliver values that can be used to describe the influence of grinding aids on the process

Furthermore, it was found that measuring material under conditions similar to the conditions in the mills delivers much more reliable values. In this context the cohesion strength values measured with an aerated powder bed showed much better results for describing the dry grinding process.

For modeling the internal material transport, the axial dispersion model could deliver good approximation of the experimental data. The dimensionless equation of this model contains only one unknown, namely the Peclet Number. Comparing the resulted Peclet Numbers from the tracer tests with the cohesion strength values measured with the powder rheometer under aeration revealed a good relation between both values that can be used to estimate the influence of grinding aids on the material transport.

It could be shown in the last year that the grinding aids do not influence the cut size of the air classifier but rather the bypass as the grinding aids improve the dispersion behavior of the particles. For this reason, the Whiten model has been implemented in the flow sheet simulation tool Dyssol to describe the efficiency curve of the air classifier.

The project ″Modeling Porosity Development during Drying of Liquids and Slurries″ aims to build a mechanistic, three-dimensional model that predicts how internal morphology (e.g., porosity) evolves as gas-saturated slurries dry. Started in September 2023 and updated as of May 2025, the project combines pore-scale simulations and microfluidic experiments to elucidate bubble formation, transport, and interaction in slurries containing dissolved gas in the course of drying.

The model, at its core, captures the dominant physical mechanisms driving drying kinetics: bubble nucleation, growth, coalescence, collapse, gas diffusion, surface evaporation, and capillary effects. These processes were formulated as discrete rules and implemented within a three-dimensional pore-network model. To verify these rules, preliminary two-dimensional microfluidic experiments were performed to monitor bubble dynamics and interface evolution. The observations confirmed key model behaviors, including the transition from concave to convex interfaces and characteristic nucleation and coalescence patterns.

Simulation results showed that the presence of bubbles significantly accelerates drying. Compared to bubble-free systems, bubble networks experienced reduced total drying time, driven primarily by an extended constant rate period. Bubble growth induced internal liquid transport toward the drying surface, maintaining surface wetness and delaying the onset of the falling-rate period.

Spatially, simulations accounting for bubbles exhibited more uniform saturation profiles, whereas in simulations without bubbles pronounced gradients from the surface downward. These findings underscore the role of internal bubble dynamics in redistributing liquid and promoting homogeneous drying. Consistent results across cubic and spherical pore network geometries further demonstrate the robustness and generality of the model.

In the remaining time of the first funding period, the model will be extended to account for solid-particle displacement driven by capillary pressure, enabling full simulation of morphology evolution during drying of a single slurry droplet. Systematic simulations will then be performed across a range of material properties and process conditions. Once this extension is completed, the model will be able to generate the high-resolution datasets required to parameterize a continuum-scale single-droplet model. After model reduction, this continuum model will be integrated into a CFD framework to simulate foaming spray-drying behavior at the tower scale during the second funding period of the project.

This annual report summarizes progress during the current reporting period (Year 3, 2024-2025) on the development of low-power acoustic excitation as a non-thermal method to accelerate aging in wet dispersions and nanoemulsions while preserving natural destabilization pathways. In other words, the central premise is that aging can be made faster without changing how it occurs. Acoustic perturbations are intended to increase the frequency of microscopic rearrangement and barrier-crossing events that already govern real-time aging, rather than introducing new degradation mechanisms. The biggest goal that the team wanted to accomplish this past year was to increase the rate of acceleration and to obtain correlations between the destabilization rate with material parameters. We are pleased to report that this goal was indeed met, with many promising routes that the project can take in Phase II.

The acoustic acceleration concept was tested across three systems of increasing complexity: a model depletion-induced colloidal gel, a commercial solid particulate agrochemical formulation, and oil-in-water nanoemulsions. Across all systems studied, low-power acoustic excitation produced statistically significant acceleration of aging, with acceleration factors ranging from approximately 1.5× in weak colloidal gels to roughly 4-5× in nanoemulsions and complex formulations. Importantly, the sequence of destabilization events appeared to remain unchanged. We found that acoustic exposure reduced the macroscopic phase separation time and increased the sedimentation rate while preserving particle morphology and microstructural signatures consistent with natural aging.

Direct comparison with thermal aging underscores the importance of mechanism fidelity. Although heating produced faster macroscopic failure, it also led to dense plug formation, particle fusion, and other irreversible changes that were absent during natural aging. Acoustic aging, by contrast, accelerated failure while maintaining representative pathways. Overall, the results demonstrate that low-power acoustics can substantially reduce aging times while preserving the physical mechanisms that control real-time stability, supporting its potential as a predictive accelerated aging method for industrial dispersions.

Two Ph.D. students were partly supported by this project. Each of them was assigned a different dispersion class to investigate and we were able to come up with an acoustic acceleration setup that is compatible with the lab's confocal laser scanning microscope. In the future, we will focus on the mechanisms through which low power acoustic energy enhances phase separation, and consider designs that are compatible with rheology measurement tools in the group. Scalability of the technique will also be considered after additional validation measurements are successful. These future research thrusts are described more fully in the renewal proposal submitted for consideration at the 2026 IFPRI Winter Meeting.

First, this work provides a unified framework for designing tougher and more processable
colloidal gels by tuning particle surface chemistry, roughness, and shape. We developed
a reproducible thermoreversible model system using octadecyl-grafted silica particles in
tetradecane, enabling precise control of grafting density and gelation temperature. Modeling
and AFM measurements show that temperature-dependent interactions between
grafted chains, rather than van der Waals forces, govern reversible gel formation.
Using this platform, we demonstrate how aspect ratio and surface roughness jointly determine
network architecture and mechanics: roughness lowers the percolation threshold
and improves stability, while elongated particles strengthen elasticity and yield behavior.
Combining both yields homogeneous, shear-resistant networks. These insights offer
practical design rules for tailoring flow, stability, and recovery in next-generation soft
materials and formulations.

Second, We directly image the microscopic dynamics of colloidal gels under shear and
show that yielding is triggered by rare, collective, near-critical plastic rearrangements
at the particle scale. By isolating these events through a robust nonaffine-displacement
framework, we establish the microscopic origin of the elastic–plastic transition and create
a foundation for microstructure-based constitutive models for thixotropic soft solids.
Building on these insights, we developed and validated elasto-visco-plastic models for
simple yield-stress fluids. A scalar version, reducing to a modified Maxwell model, accurately
predicts complex protocols such as LAOS, while its tensorial extension captures
the coupling between 3D stress components. Both models reproduce complex deformation
behavior and the interplay of elastic and dissipative responses.

Third, new micro-mechanical tools (rheo-confocal imaging and optical tweezers on model
aggregates) directly link bond- and node-level mechanics to macroscopic properties. In
particular, surface roughness extends the linear deformation regime and alters yielding
modes.

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.