Using sedimentation to obtain precisely controlled packings of noncohesive
spheres, we find that the volume fraction ϕRLP of the loosest
mechanically stable packing is in an operational sense well defined by a limit
process. This random loose packing volume fraction decreases with decreasing
pressure p and increasing interparticle friction coefficient μ. Using
X-ray tomography to correct for a container boundary effect that depends on
particle size, we find for rough particles in the limit p→0 a new lower
bound, ϕRLP=0.550±0.001.Comment: significantly revised, published versio