Top panel: evolution of the particle density (normalized to the background dust-to-gas ratio ϵ₀), for three different resolutions in 2D simulations and a 3D simulation (run3d.4, gray). Bottom panel: evolution of the instability based on the horizontal gas dispersion, , (as an alternative tracer to the maximal particle density, which is an intrinsically noisy variable). We used a similar color coding as for the top panel. Note for the runs with ϵ₀ = 4, we displaced the dashed curves for clarity with a factor 10 downwards.