Contours of the logarithm of the hydrodynamical factor log ₁₀(f_hydro) (see Eq. (105)) as a function of dust size (in cm) and planetesimal size (in km), at 1 AU in the MMSN. The physical density of dust and planetesimals is set to 1 and 3 g cm^-3, respectively. The contours are for f_hydro = 10^-7, 10^-6, 10^-5, 10^-0.5, 10^-0.1. The transition from the hydrodynamic flow regime to the boundary regime (dark to light blue) takes place over a narrow range of dust sizes for planetesimals smaller than 100 km in radius. For larger planetesimal sizes, the transition between the hydrodynamic and settling regimes is discontinuous by construction. A proper modeling of this transition will require more work.