Powder Flow

In this review, the focus will be on what the author believes are (a) the most broadly important results in of (particulate) tribocharging, (b) the most important issues to the academic community since the last IFPRI review by Matsuyama and Yamamoto in 1998, (c) the intersection of these with the issues most relevant in industry, and (d) the results that the author considers to be the most durable. In contrast to a more typical review format, where a very large number of results are discussed very briefly, this review will go into detailed summary of select results.

Section 2

Section 2 will begin with a succinct summary of essential knowledge to anyone concerned with tribocharging: namely the work-function mechanism for metal-metal contacts as developed primarily by Harper and Lowell.

Section 3

In §3, the focus will be on charging between metals and insulating particles, in particular highlighting the work of Matsuyama and Yamamoto, to illustrate the (at the very least functional) inadequacy of the work-function model for this situation and highlight the importance of dielectric breakdown.

Section 4

Section 4 will switch focus from different material (i.e., metal-insulator) charging to same-material charging—i.e., what occurs between the particles in a particulate system. Here, we will recapitulate the essential results of the groups Lacks and Jaeger, who convincingly showed that charge separation based on size occurs.

Section 5

The focus will then naturally shift in §5 to the patch models for tribocharging, including the “trapped” electron model which can be considered a special case.

Section 6

Finally, §6 will focus on the latest generation of experiments, which repeatedly measure the charge exchange between the same two objects, and which challenge the most basic tenets of the patch-model framework and force us to reconsider the underlying cause of same-material tribocharging.

Due to finite time, energy, and space, but also due to the predilections, focus, and expertise of the author, many relevant results worth mentioning will surely be missed. It is the hope, however, that this review will serve as a “good starting point” for anyone in industry who would like to know about the topic’s most durable results, as well as the most cutting edge ones, to be able to think about their particulate charging issues in a more informed way.

Objective: This project aimed to advance the fundamental understanding of powder flows by developing model cohesive granular materials and characterizing their rheological behavior to improve powder manipulation in industrial processes. The primary focus was to tune adhesive forces, develop original experimental setups to characterize particle interactions, and design a shear cell capable of investigating inertial rheology at low pressure. The ultimate goal was to provide physical insights into flowability, with applications in processes such as aeration, compaction, and industrial handling.

Key Achievements:

• Development of Model Materials: Successfully created model cohesive particles with tunable adhesion, using polymer coated silica and functionalized polymer particles. Demonstrated the ability to control adhesive properties, enabling systematic studies of cohesion effects.
• Characterization of Particle Properties: Developed experimental techniques to measure interparticle adhesion and friction. Identified a lubrication transition in large particles, characterized through tribological measurements at the particle scale.
• Bulk Rheology Measurements: Designed and implemented a novel shear cell capable of measuring the rheology of _ne particles under low confinement stresses and in the inertial regime. Achieved the first measurements of constitutive laws under low confining stress, revealing unexpected behaviors such as shear weakening and pressure-dependent transitions.
• Study of Flow Configurations: Initiated investigations into ow configurations relevant to industrial processes, including compaction and drag/lift forces in cohesive materials. Observed that lift forces are signficantly more sensitive to cohesion than drag forces, suggesting potential new methods for powder characterization.

Challenges and Limitations:

The study of air-particle coupling, a critical aspect of powder dynamics, was not addressed due to time constraints and the complexity of designing appropriate experimental tools. Some objectives, such as the systematic study of ow configurations, remain preliminary and require further investigation.

Impact and Future Directions:

This project has laid the groundwork for a deeper understanding of cohesive granular flows, with implications for industrial processes involving powders. The development of model materials and advanced experimental techniques opens new avenues for studying the interplay between adhesion, friction, and ow dynamics. Future work will focus on refining measurements to reveal the control parameters of the powder rheology, and extending findings to a broader range of industrial applications.

Conclusion:

This project has made signficant progress in developing innovative tools to characterize cohesive granular materials, both at the particle scale and in terms of bulk properties. We have begun to gain valuable insights into the behavior of powders under low confinement and inertial conditions. While challenges remain, particularly in fully understanding the role of air-particle interactions and refining ow configurations, the findings provide a robust foundation for future research, with direct relevance to the industrial characterization and handling of powders.

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.

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.

The project aimed to develop a comprehensive set of models and decision tools for selecting and optimizing flow aids for the purpose of tackling the inherent challenges associated with the flowability of fine, cohesive powders. These powders often exhibit poor flow due to cohesion, which is influenced by factors such as particle size, shape, surface roughness, and environmental conditions. The project aimed to develop predictive models and decision tools that help select and optimize flow aids to improve powder processing. Over the past three years, significant progress has been made toward this goal, with several key accomplishments. Critical gaps have also been identified that will be addressed in the renewed project.

Major Accomplishments:

• Model guided dry coating (flow aid amount, size, and surface area coverage):
• Single and multi-asperity mechanistic contact models were surveyed and assessed for their ability to guide flow aid selection based on their effectiveness.
• A multi-asperity model, developed earlier in our group, was identified for its ability to explain how dry coated nano-flow aids work and result in improved powder flow as they lower the inter-particle cohesion by one to two orders of magnitude. This model, which uses a surface area coverage-based approach, accounts for factors such as the amount of silica, particles surface energy, particle surface roughness, and surface roughness distribution.
• Comprehensive validation of contact mechanics-based predictive models was conducted, focusing on selecting type and amount of silica for best flowability enhancements.
• An interactive mixture model provided a framework for assessing host-guest compatibility based on surface energy considerations.
• Material Characterization and Database:
• A database of industry-relevant materials was established to support informed decision-making for flow aid selection and suitability of dry coating.
• The functionality of different silicas (hydrophilic and hydrophobic) with varied sizes was examined to understand their impact on reducing cohesion in powders.
• Processibility Guidance:
• Ideal processing conditions using bench-marking dry coating devices like LabRAM were identified for the purpose of demonstrating that high-intensity mixing can significantly improve the use and efficacy of flow aids. This is expected to help explore the use of conventional mixing devices which will be further addressed in the renewal project.
• Synergistic effects in the flowability of blend were having a minor dry-coated component discovered, demonstrating improved blend flowability compared to that of its constituents.
• The effect of blend mixing time on this synergy when a dry-coated component is blended with other uncoated constituents was examined.
• Predictive Approaches:
• A dimensionless parameter, the granular Bond Number, based predictability of powder bulk properties was incorporated so that the same mechanistic model could be used for predicting the properties of uncoated as well as dry-coated powders.
• A size class-dependent Bond number approach was introduced to account for variations in cohesion for fine powders having broad and/or bimodal particle size distributions.
• Comprehensive approach for predicting flow aid, e.g., silica, performance in reducing cohesion was proposed by combining multiple models:
  1. Chen’s multi-asperity contact model: Explains the effect of host-guest sizes, guest surface energy, and host surface roughness on cohesion reduction.
  2. Deng’s stick and bounce model: Describes the aggregation tendency of nanoflow aids (e.g., silica) based on process intensity and guest-host particle sizes.
  3. Interactive mixture model: Based on host-guest total surface energy differential, predicts host-guest compatibility and coating efficacy.

These models provide a more nuanced understanding of how different flow aids, e.g., silica, perform under varying conditions, particularly concerning process intensity, nano flow aid particle size, and its surface energy.

• Industry Relevance:
• The findings underscore the importance of dry coating as a critical tool for enhancing powder flowability in industrial settings.
• Insights into powder blend design and processing have significant implications for improving end-product quality.
• A key highlight of this project is the successful evaluation and benchmarking of dry coating methods at both laboratory and pilot scales, achieved through collaborations with industry partners, including Merck, and support from NSF. Pilot-scale testing using scalable devices such as the COMIL-U10 validated the processability, scalability, and practicality of dry coating methods at pilot scale.

Industry liaisons provided important feedback and support during the project period including the quarterly project meetings. Consequently, several critical gaps have been identified that will be addressed in the renewal project.

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.

During the first term of the project, our investigation on screw feeder performance had three components. The first was to formulate two mechanics-based models for the kinematics and mechanics of non-cohesive powders: a simple model that relies on several simplifying approximations, and a more detailed continuum model that predicts the variations of velocity, packing fraction and stress within the feeder. Both models make the interesting prediction that the feed rate is maximum for a specific value of p/(2R!), the ratio of the screw pitch to barrel diameter of the feeder. The second component of our work was to conduct DEM simulations to validate the models and guide experimental efforts. The third component was to conduct experiments on a custom-built feeder assembly to test the model predictions and to extend/refine the model. Overall, we find good agreement between the experimental data, DEM simulations, and model predictions for non-cohesive powders (such as glass beads). In particular, the simulations and experiments verify the model predictions of the feed rate being maximum at a certain ratio of pitch to diameter of the screw. The experiments also identify the free surface at the feeder exit as being responsible for feed rate fluctuations for dry powders, and show that the fluctuations may be mitigated by appropriate end-cap design. The combination of theoretical analysis, DEM simulations, and experiments yielded substantial insight.

In the current (renewed) term of the project, we have focused on understanding the flow of cohesive powders in feeders. To quantify the effect of cohesion, we first created powders of controlled cohesion by combining a small quantity of glycerol to dry glass beads. Interestingly, the experiments show that the feed rate as a function of p/(2R!) shows the same qualitative trend as for dry powders. However, the feed rate fluctuations for cohesive powders are quite different from those seen in dry powders. A lacuna in the current understanding of cohesive powders is that there is no reliable constitutive model. To address this, we have initiated a study to determine the flow rule, a relationship between the strain rate and stress, for cohesive powders. Our initial studies on horizontal rotating drums point to the formation of clusters or agglomerates that crucially affect the flow of cohesive powders. On the modelling front, we have shown that the non-local model correctly predicts a complex dilatancy-driven secondary flow seen in experiments and DEM simulations, thereby increasing confidence on the model.

In ongoing work, we are conducting DEM simulations and experiments to quantify the formation and persistence of clusters in cohesive powders and determine how they influence their flowability.