Micromechanics of Particle Adhesion
The rapid increasing production of cohesive to very cohesive ultra- fine powders (d < 10 µm), e.g. very adhering pigment particles, micro-carriers in medicine, auxiliary materials in catalysis make technical problems much serious like undesired adhesion in particle processing, powder handling, and desired, in agglomeration 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 technology. The state of arts in modelling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single, normal loaded, isotropic contact of two smooth spheres is briefly discussed. Then the force-displacement behaviour of elastic-plastic and adhesive contacts is shown. Using the model “stiff particles with soft contacts”, the combined influence of elastic and elastic-plastic repulsions in a characteristic particle contact is demonstrated. A sphere-sphere model for van der Waals forces F H0 without any contact deformation describes the “stiff” attractive particle adhesion term. A plate-plate model is used to calculate 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” irreversible deformation, the so-called contact pre-consolidation history. The adhesion force is found to be load dependent F H(F N). The contribution of this history dependent adhesion on the tangential force in an elastic-plastic frictional contact F T (F N, F H(F N)), the rolling resistance F R(F N, F H(F N)) and the torque of mobilized frictional contact rotation M to(F N, F H(F N)) are shown. With this increasing load, 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 (d 50 = 1.2µm).
KeywordsTorque Catalysis Lime Compaction Agglomeration
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