(75a) Micromechanics of Particle Adhesion

The rapid increasing production of cohesive to very cohesive ultrafine powders, e.g. very ad-hering pigment particles, micro-carriers in biotechnology or medicine, auxiliary materials in catalysis or chromatography, make technical problems much serious like undesired adhesion in particle conversion and processing, powder handling and transportation, and desired, in ag-glomeration or coating. Thus, it is very essential to understand the fundamentals of particle adhesion with respect to product quality assessment and process performance in powder tech-nology. The state of art in modeling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single, normal loaded, isotropic, smooth contact of two spheres is briefly discussed. Than the new comprehensive models for force-displacement behaviors of elastic-plastic and viscoplastic-adhesion contacts are shown by dia-grams. The decreasing contact stiffness with decreasing particle diameter is the major reason for adhe-sion effects at nanoscale. Using the model ?stiff particles with soft contacts?, the combined influence of elastic and elastic-plastic repulsions in a characteristic particle contact is shown. The attractive particle adhesion term is described by a sphere-sphere model for van der Waals forces FH0 without any contact deformation. A plate-plate model is presented to describe the micro-contact flattening or overlap. Various contact deformation paths for loading, unloading, reloading and contact detachment are discussed. Thus, the varying adhesion forces between particles depend directly on this ?frozen? irreversi-ble deformation, the so-called contact pre-consolidation history. With respect to the influence of these effects on consolidation history of particle contacts by a normal force FN, adhesion force based model FH(FN) is presented that describe the load dependent adhesion of particle contacts. The contribution of this history dependent adhesion on the tangential force in an elas-tic-plastic frictional contact FT(FN, FH(FN)) is derived. With this increasing pre-consolidation, normal and tangential contact stiffness, energy absorption, Coulomb friction limit and friction work increase. These constitutive models are generally applicable for solid micro- or nanocontacts but have been shown here for an ultrafine limestone powder (d50 = 1.2 µm).