The Forces Acting Between Dry Powder Particles

Publication Reference: 
Author Last Name: 
H M Pollock
Report Type: 
Research Area: 
Powder Flow
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Publication Month: 
United Kingdom

The purpose of this state-of-the-art review is to survey the literature on inter-particle forces in dry powder systems - in particular their origin and their known influence on powder processing operations - and current research that indicates to what extent these forces can be measured directly.

Chapter headings include: Introduction and background; Types of force acting between solid surfaces; Contact mechanics (deformation associated with adhesive forces and with friction); Surface-electrical parameters and forces; and Recommendations for future lans (the potential most fruitful research objectives). The .results of current research lead to a number of relevant conclusions:

(i) while macroscopic concepts (plasticity, viscoelastic deformation, etc.) appear in general to hold at the level of a single interaction between two particles, it will become clear that there is an urgent need for more direct force measurements to be made with individual particles, where the variable effects of surface condition and topography are especially hard to predict.

(ii) Using proximal-probe technology, it is possible to derive useful quantitative information that relates to contact area and the nano-mechanical and adhesive properties of model particles (atomic force microscope tips) or actual particles.

(iii) Relatively little detail is known about changes to the van der Waals interaction that are induced by the presence of an adsorbed layer of permittivity different to that of the material of the underlying particle.

(iv) Examples of processes in which inter-particle forces are strongly influenced by surface-electrical properties include electrostatic precipitation, powder coating, fibre filtration, xero raphy, particle collection, and separation by means of triboelectrification. Within the general area of contact electrification, there remain many other complications, unsolved problems, and recent intruiging findings. For instance, van der Waals forces (in experiments with model particles of high curvature) are masked by a longer-range attraction. A “patch charge” model gives the most romising ex lanation and the best fit to the data, but analysis of patch charge effects arising from inhomogeneities in work function is in its infancy. An explosion of activity in proximal probe work on surface-electrical properties of surfaces in general at the nanometre level is taking place. So far, there have been few attempts to extend these techniques to work on actual particles.This now calls out to be done.

In recommending possible forward plans, a final chapter identifies two broad fundamental research objectives as being potentially the most fruitful, and involving measurements at the level of an individual particle-particle contact, but geared to the requirements of powder technology. The two topics recommended are: (i) study of energy-dissipative contact processes (as measured, for example, by force curve hysteresis) that underlie dynamic and frictional recesses, (ii) a proper characterisation of surface- electrical roperties of particles and the factors that influence them. For example, what are the individual sites on the surface of a particle of photoreceptor material, that allow it to retain positive or negative charge? How may polymer powders be designed specifically for electrostatic deposition, with a view to improving the proportion of sufficently charged particles in a particular process? There is always hysteresis in the loading/unloading cycle of an individual particle-particle contact - what is the role of this hysteresis in determining the dynamic and frictional behaviour of particles?

These are just three of the many situations in which selective application of the “individual particle” experimental methods would help to answer specific questions of importance in particulate technology.