Mean free path of the gas, λ, multiplied by 2.25 after 100 and 300 kyr of PSN evolution. Particles smaller than 9λ/4 are in the Epstein regime, while larger particles fall into the Stokes regime. Blue and white lines represent the median trajectories of 500 simulated 1 cm particles released at 80 K and 20 K, respectively. The gray hatching highlights the region where 1 cm and smaller particles transition from the Epstein regime to the Stokes regime.