(38b) Quantifying Cohesive, Frictional and Interlocking Effects for Universal Powder Flow Characterisation

The infinite combinations of powder processing and handling conditions make it difficult for a single powder flow characterisation instrument or testing methodology to accurately simulate how a material will perform in any flow pattern in any combination of energy, shear-rate and stress regimes. Traditional and newer rheological testing methodologies can facilitate a quantitative and sensitive differentiation of powder samples, but the root morphological cause of behavioural differences between the samples is often unknown, and the various testing methodologies can lead to conflicting results.
We explore a new framework of interpretation of a minimal number of rheological tests for powder flow characterisation based on the quantitative description of the mechanisms that oppose the motion of particles past each other: cohesion, friction and mechanical interlocking. The degree to which each mechanism contributes to ‘flowability’ scales differently with the normal force between particles and with the shear rate induced, and the different mechanisms lead to different flow-related issues: while a highly cohesive powder will tend to form clumps and agglomerates and loosely packed structures, a highly frictional and interlocking powder will form strong force chains than can arch and jam at openings. The proposed methodology offers numerical values that inform directly on the effects of morphological and environmental conditions, and that relate directly to powder performance in a wide variety of powder processing and handling scenarios.
We propose quantifying the relevance of cohesive forces on powder flow through the ratio of inter-particle cohesive forces to the average particle weight, i.e. the granular Bond number, Bo. This number then defines the degree to which cohesion prevents particles from fitting into gaps and flowing past each other due to gravity, leading to a poor packing efficiency and a likelihood of clumping. We propose the most direct method for measuring Bo can be carried out through a recently developed Fluidization test, where the fluidization behaviour and uniformity of a powder depends on the balance of air drag, particle weight and cohesive forces – by measuring the excess pressure drop across the bed necessary to fluidise a powder, relative to the pressure drop once it is fluidised, we calculate how much larger the net interparticle cohesive force is than the average particle weight, and so Bo. The difficulty of layers of powder to shear over each other is quantified through the coefficient of friction obtained by shearing layers of powder under different applied loads in a Shear Cell test, while mechanical interlocking effects due to particle shape irregularity are calculated based on the powder packing efficiency and on how this changes under compressing stresses.
By quantifying the intrinsic strength of cohesive bonds, the effective coefficient of friction and the packing fraction under different consolidating loads, the degree to which each mechanism contributes to ‘flowability’ can be predicted in terms of process parameters. This information can complement other rheological testing by rationalising differences in sensitivity and ‘flowability’-ranking obtained through other methodologies, it can support a more accurate understanding and prediction of how the powder will behave in different conditions, and it can inform on the best course of action for solving flow-related issues. The metrics used are all dimensionless, allowing for the first time, a universal direct comparison of flowability across completely different materials, and a common language for powder flow characterisation across materials, processes and industries.