The flowability of a bulk solid depends on the adhesive forces between individual particles.
Different mechanisms create adhesive forces [5].
With fine-grained,
dry bulk solids, adhesive forces due to van der Waals interactions play the essential role.
With moist bulk solids, liquid bridges between the particles usually are most important.
Liquid bridges are formed by small regions of liquid in the contact area of particles, in which due to surface tension effects a low capillary pressure prevails.