The physical response of a particle reacting to the gas flow is set by the friction time t_f. A particle of given size R has a dimensionless friction time τ_f = Ω_Kt_f that depends on the orbital distance r. The red dash-dotted line marks the distance at which the particle size equals 9/4 of the mean free path λ of molecular hydrogen in the MMSN. Particles with R > (9/4)λ are located in the dashed region, and experience Stokes drag as opposed to Epstein drag. For the curves in the Stokes regime, we have ignored the transition into the non-linear Stokes regime, applicable for large particles close to the host star.