The rapid production of ultrafine, and therefore cohesive to very cohesive powders (d<10 µm, e.g. very adhering pigment particles) in biotechnology, pharmacuticals, auxiliary materials in catalysis or chromatography, generates a number of serious technical problems and issues due to inadequate apparatus and reliability of processing plants. These problems include particle adhesion or sticking during processing (generation or conversion) as well as issues with powder handling and transportation, and particle agglomeration, formulation or coating at the point of use. The understanding of particle adhesion fundamentals is therefore essential to improve product quality and process performance in powder technology.
These fundamentals of cohesive powder compression, consolidation and flow behaviour can be explained using a reasonable combination of particle and continuum mechanics.
Comprehensive models are shown that describe the elastic-plastic force-displacement, viscous damping and frictional moment-angle behaviour of adhesive particle contacts. Using the stiff particles with soft contacts model, a sphere-sphere interaction of van der Waals forces without any contact deformation describes the stiff attractive term. The soft micro-contact response generates a flattened contact, i.e. plate-plate interaction, and increasing adhesion.
These increasing adhesion forces between particles directly depend on this frozen irreversible deformation. Thus, the adhesion force is found to be load dependent. It essentially contributes to the tangential forces in an elastic-plastic frictional contact with partially sticking or microslip within the contact plane. The load dependent rolling resistance and torque of mobilized frictional contact rotation (spin) are also shown.
With this as the physical basis, incipient powder consolidation, yield and cohesive steady-state flow, consolidation and compression functions, compression and pre-shear works are explained. These constitutive models are used to design process apparatuses and to ensure reliable powder flow. Finally, conclusions are drawn concerning particle stressing, powder handling behaviour and product quality assessment in processing industries.
