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). The top plot is in the suspension regime (particle size τ_f = 0.001) and has solid concentration Z = ⟨ Σ_solid ⟩ / ⟨ Σ_total ⟩ = 0.095. In this regime, there is very little clumping even at high Z. The middle plot is the beginning of the streaming regime (τ_f = 0.003) and Z = 0.095. In this regime, clumps form and are stable. The bottom plot is in the radial drift regime (τ_f = 3) with Z = 0.05. In this regime particles form large over-densities that experience rapid radial drift.