Particle density distribution for three particle sizes. Particle size is measured in the stopping time τ_f, where τ_f = 0.001, 0.003 and 3 correspond to the suspension regime, streaming regime and radial drift regime respectively. Row a) is the Surface density of solids Σ_solid as a function of the radial coordinate x, evaluated at two points in time 25 orbits apart (first blue, then green). Row b) averages Σ_solid over the 25-orbit interval ⟨ Σ_solid ⟩ _t. Row c) shows ⟨ Σ_solid ⟩ _t with the grid cells sorted from highest to lowest density. A steep curve indicates stable particle clumps. Row d) shows the cumulative distribution of ⟨ Σ_solid ⟩ _t in row c) (solid red curve) along with the cumulative distribution of a perfectly uniform particle distribution (dashed blue line). The distance between these two curves serves as a measure of particle clumping. In these plots the particle concentrations Z = ⟨ Σ_solid ⟩ / ⟨ Σ_total ⟩ are (left to right) 0.095, 0.095 and 0.05.