Random loose packing of cohesive granular materials

The volume fraction ϕ of a disordered packing of noncohesive hard spheres may vary between the random loose packing at the limit of zero gravitational stress (ϕ RLP ≃ 0.56) and the random close packing (ϕ RCP ≃ 0.64). When interparticle attractive forces are present it is known however that ϕ can be as low as ∼ 0.01 for nanoparticles. Experimental and numerical works show that ϕ decreases as the ratio of interparticle attractive force to particle weight ( Bo g ) is increased. We focus on the packing fraction of cohesive particles settled at the limit of zero gravity (jamming transition). From a model of particle agglomeration in suspensions we find the simple equation ϕ J ≃ ϕ J * Bo g ( D − 3)/( D + 2) , where ϕ J * is the volume fraction of particle agglomerates at jamming, which is close to ϕ RLP , and D their fractal dimension. Our experimental results on fine powders agree with this equation and show that agglomeration of particles in suspension is the relevant physical mechanism to understand the packing of cohesive particles.