This review may contribute to improve process engineer’s understanding of rheo-mechanical properties of wet-mass powders by means of a suitable combination of molecular mechanics, particle contact models and continuum mechanics.
If a non-drained paste is sheared then the shear stress will increase with applied normal stress (Coulomb friction between particle contacts) for a certain water content below the pore saturation. During this shearing, the packing is consolidated and the porosity de- creases. At the point of pore saturation a sharp transition is often assumed to change immediately the previous dominant particle friction into viscous shearing of the liquid. Now, the shear stress remains constant and the paste flow state is achieved without any influence of external pressure. But by means of intensive pre-stressing, large shear strain, spatial drainage with pore liquid flow, liquid pressure drop and comparatively small moisture contents the reversal transition between initial viscous paste flow to Coulomb friction is obtained. The wet-mass powder is expanding, pore saturation is reduced, the capillary pressure increases and consequently the particle contacts are loaded and more and more de- formed. This produces an undesired crumbly structure of the mass, like green pellets. An increased normal stress is the consequence to press the mass through moulds and matrices. Considering the stiffening by capillary pressure distribution as an attractive internal pressure and, consequently, remarkable soft particle contact deformation within the shear zone plus softening by a hydrodynamic repulsion force, flow of pore liquid relative to the particle shear velocity, long-range particle-particle interactions and lubrication effects between particle surface asperities, one may expect a certain metastable range of transition in a compressed wet-mass powder or paste. This term “metastability” may be used to describe serious transition problems as follows: Either the wet-mass powder is exposed dominant Coulomb friction during “supersaturation” of pore liquid instead of expected viscous flow, or the paste is sheared with dominant viscous flow during “supersaturation” of particles instead of expected Coulomb friction. This may be equivalent to jamming in rheology as well as caking, or contrary, avalanching and flooding effects in powder mechanics.
Nevertheless the fundamentals of this metastable transition range are not completely understood. A basic research in detail seems to be necessary to understand what is really happened in the packing during these transitions of dominant Coulomb friction ←→ viscous paste flow. These facts lead to the necessity of closing the huge gaps between particle mechanics, powder mechanics and paste rheology. This complex combination of theoretical work, computer simulations, experimental evaluation and full-scale practical application may be sufficient to ensure the success of this joint research and development project. Un- fortunately the reality of technological conversion processes in powder technology is com- plicated enough to discover the adequate constitutive model which requires an amount of practical experience, sure instinct, theoretical knowledge, experimental skills, cleverness and sometimes fortune as well.