ABSTRACT
Granular materials play a major role in a large number of technological applications. The size of grains used depends on the application, ranging from large particles with diameters of several centimeters down to nanoparticles. However, the physical properties of powders change drastically when reaching the size of nanoparticles. While in ordinary dry granular materials contacts between the particles cannot sustain tensile loads, in the case of ultra-fine powders van der Waals forces lead to aggregation which is essentially irreversible. In contrast to volume forces such as gravity, which decrease with decreasing particle radius a like a3, van der Waals forces are proportional to a and thus dominate all other forces in the case of sufficiently small particles. On the other hand, van derWaals forces are only short ranged, diverging like h−2 on very short distances and decaying like h−7 with increasing distance h between the surface of particles (van de Ven 1989). Thus, the van der Waals force can be approximated as an irreversible sticking force, which has no influence on particles separated from each other, but causes them to stick together irreversibly, as soon as they touch each other.
