Dust-to-gas density ratio ρ_d/ρ_g in the mid-plane as a function of time after the dust particle initialisation t – t_d,init. The mid-plane density ratio is computed as the mean over r = 55 au to 65 au and the rolling average over 100 yr. Simulations of the streaming instability only as well as the scenarios SIwhileVSI and SIafterVSI with initial dust-to-gas surface density ratios of 1% (dashed lines) or 2% (solid lines) and a Stokes number of 0.1 are shown. The mid-plane density ratio is similar in the simulations of the streaming instability and the scenario SIwhileVSI, and lower in the scenario SIafterVSI. While the ratio fluctuates throughout the latter simulation, it initially attains a maximum before evolving into a quasi-steady state in the former two. In all three cases, doubling the initial surface density ratio entails an increase by about a factor of two also in the mid-plane density ratio.