Evolution of the radial concentration of the particle layer for two 3D models. The colors show the azimuthal average of the column density of the particles Σ_p as a function of radial position x and time t, where Σ_p, x, and t are normalized by the initial gas column density Σ_g,0, the vertical scale height of the gas H, and the local orbital period P, respectively. The left panel has particles of dimensionless stopping time τ_s = 10^-2 and a solid abundance of Z = 0.02, while the right panel has τ_s = 10^-3 and Z = 0.04. Both models have the same computational domain of 0.2H on each side and resolution of 640H^-1.