In particle processing and product handling of fine (d < 100 mu m), ultrafine (d < 10 mu m) and nanosized particles (d < 0.1 mu m), the well-known flow problems of dry cohesive powders in process apparatuses or storage and transportation containers include bridging, channelling, widely spread residence time distribution associated with time consolidation or caking effects, chemical conversions and deterioration of bioparticles. Avalanching effects and oscillating mass flow rates in conveyors lead to feeding and dosing problems. Finally, insufficient apparatus and system reliability of powder processing plants are also related to these flow problems. Thus, it is very essential to understand the fundamentals of particle adhesion with respect to product quality assessment and process performance in particle technology. The state-of-the-art in constitutive modelling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single isotropic contact of two smooth spheres is briefly discussed. Then the new models are shown that describe the elastic-plastic force-displacement and moment-angle behaviour of adhesive and frictional contacts. Using the model "stiff particles with soft contacts", a sphere-sphere interaction of van der Waals forces without any contact deformation describes the "stiff" attractive term. A plate-plate model is used to calculate the "soft" micro-contact flattening and adhesion. Various contact deformation paths for loading, unloading and contact detachment are discussed. Thus, the varying adhesion forces between particles depend directly on this "frozen" irreversible deformation. Thus, the adhesion force is found to be load dependent. Their essential contribution on the tangential force in an elastic-plastic frictional contact with partially sticking within the contact plane and microslip, the rolling resistance and the torque of mobilized frictional contact rotation is shown. (c) 2007 Elsevier Ltd. All rights reserved.