Powder computing

At first glance, a powder is a powder. But, look more closely and the picture is not so simple. How does a powder behave when it flows, what happens if the particles from which it is composed stick to each other, and what effect does particle size and shape have on the powder behaviour?

What if the particles are different sizes? What are the effects of friction between touching particle surfaces? The answers to such questions could provide researchers with important insights into the behaviour of granular substances with particular relevance to industrial processes involving powders. Such fundamental understanding could help manufacturers improve product quality and lower energy consumption and costs.

With EPSRC support and in collaboration with industry, Professor Kevin Kendall of The University of Birmingham department of chemical engineering has teamed up with Daresbury Laboratory’s molecular simulations experts Dr Bill Smith and Dr Chin Yong, and chemist Professor Les Woodcock of UMIST, to find a way to model powders at the atomic level using a computer.

In the extreme, scientists regard powders at the atomic level as obeying the statistical principles of kinetic theory. Each atom moves according to these principles but each atom also influences its neighbours through attractive and repulsive electrostatic forces.

The way particles in a powder behave also depends on how they stick together, or aggregate to form clumps. Aggregation is a complex process which requires particles to make atomic contact while on the move but thermal jostling can break these contacts.

‘The purpose of our project was to understand this idea of atomic contact and fracture using a computer statistical model, with only the known atom interaction potentials to work from,’ explained Professor Kendall.

The researchers believe this behaviour could explain how custard powder flows and what happens when tablets are formed from a powdered pharmaceutical.

The team developed POWMOD, a software package built on the powerful molecular dynamics program DL_POLY that was developed at the Daresbury Laboratory by Dr Smith and his colleagues.

DL_POLY is used by chemists to model the movements of the hundreds, sometimes thousands, of atoms in macromolecules such as proteins and polymers. The software simulates the position of each atom depending on its chemical characteristics and how it is linked to its neighbours. The researchers reasoned that they might also use this approach to model particles in a powder and how they interact.

‘Now that larger computers are available, it is possible to simulate not just molecules but small particles in the machine, making this project possible,’ adds Professor Kendall.

POWMOD extends the modelling prowess of DL_POLY enabling the researchers to simulate the behaviour of the bulk powder in industrially important processes. The first version focused specifically on ionic powders, such as sodium chloride and oxides, says Professor Kendall, but the researchers continually refine it to better simulate the interactions on a particle-to-particle basis.

The model takes into account shape and size, surface roughness, deformability and surface contamination with water and other molecules. The model reveals particles coming into contact and also tearing apart as they jostle around.

‘Existing experimental techniques such as Atomic Force Microscopy (AFM) are very useful in studying phenomena at the atomic scale but complications make characterising atomic surface contacts difficult. The exact contact geometry and the presence of contaminants on the surface often make interpretation of the results difficult,’ explained Dr Yong.

Computer simulations, however, can probe the underlying atomic mechanisms and relate them to the experimental observations.

According to the team, POWMOD has established links between the fundamental physical properties of powders, testing of particles in the Atomic Force Microscope and the processes to which they are subjected during manufacturing.

The ultimate goal of powder simulations is to reduce, or perhaps eliminate, the requirements for tedious, costly, and (in the case of radioactive powders) hazardous, trials to predict how powders will behave in real processes.

‘The next step is to use the model in realistic industry situations,’ adds Professor Kendall.

This article has been reproduced by permission of the EPSRC from its magazine Newsline.