Continuum mechanical models and appropriate measuring methods to determine the materials parameters are available to describe the flow behaviour of cohesive powders. These methods are successfully applied to design process equipment as silos. In addition, "microscopic" studies on the particle mechanics can give a better physical understanding of essential "macroscopic" constitutive functions describing a powder "continuum". At present, by means of the discrete element method (DEM), a tool is available that allows one to consider repulsive and frictional as well as attractive adhesion forces in detail. Within the framework of Newton's equations of motion, each particle in the system is tracked, and reacts to the forces acting. The knowledge of the interaction forces between particles is thus a prerequisite for understanding (via DEM) the stability and flow of particulate systems and other phenomena. In this study, macroscopic cohesion and friction are related to their microscopic counterparts, adhesion and contact-friction. The macroscopic cohesion is found to be proportional to the maximal microscopic adhesion force, and the macroscopic friction coefficient is a non-linear function of the contact friction, dependent (or independent) on the preparation procedure for yielding (or steady-state flow). One of the few methods available for the direct measurement of surface-and contact-forces is the atomic force microscope (AFM) and, related to it, the so-called particle interaction apparatus (PIA). A contact model for ultrafine cohesive particles (average radius d 50 ≈ 1 µm) is introduced, based on such experiments. Plugged into DEM, consolidation, incipient yielding, and steady-state flow of the model powders are studied. Also the dynamic formation of the shear zone is examined and compared with experimental observations. Eventually, the shear experiments with volumetric strain measurements in a translational shear cell are used for validation.

The cohesion and anisotropy in dense, cohesive powders was studied by using a discrete element method for the simulation of a biaxial box deformation. The approach includes normal (repulsive/cohesive) as well as tangential forces. Reorganization of the contact network first leads to anisotropy and later to shear band formation and softening behavior from the macroscopic point of view. As an important result of this work it was found that the macroscopic cohesion of the material can be related to the maximal microscopic attractive adhesion force.

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