Significance of interparticle forces to powder behaviour

van der Waals surface forces have a dominant effect on the behaviour of fine powders. A bed of very small particles, from 1 nm to .1 mm in size, behaves like a solid material when the surface forces are so large that every particle sticks adhesively to neighbouring grains. However, if the interfacial force between such particles is reduced to zero, for example by immersing the particles in a dispersing fluid which eliminates van der Waals forces, then the powder behaves as a gas and the powder particles diffuse and flow. These ‘fluid’ and ‘solid’ extremes of powder behaviour are contrasted to illustrate the large influence of interparticle forces, which can be measured in several ways; by measuring contact size between grains; or by measuring the Young’s modulus of the powder assembly. Then, the transition between fluid and solid behaviour is studied by considering the gas/solid powder transition which occurs when interparticle forces are suddenly switched on, for example by chemically changing the surfaces of grains to increase the work of adhesion. It is evident that a powder can exhibit either fluid or solid behaviour depending on the powder cohesion number ϕ W/ ρ Dc 2 where ϕ is the volume fraction of grains, W is work of adhesion, ρ is the density of a particle, D its diameter and c its velocity.