Contour plot of f_coll (Eq. (21)) forpotential flow as a function of Safronov number (x-axis) and dimensionless stopping time τ_X = t_s/t_X (y-axis). The former is a proxy for the planetesimal size, whereas τ_X is a measure of the particle aerodynamical size. The dashed curve gives the transition between ballistic and settling interactions. The dotted lines Θ = 1 and St = 1 separate the geometric regime (where f_coll ≈ 1) from the Safronov (where Θ > 1) and aerodynamic deflection (A.D., where St < 1) regimes. At small Stokes number, aerodynamic deflection suppresses accretion because these particles couple too well to the gas for ballistic interactions to be effective. Particles of τ_X ≲ 1 experience aerodynamic deflection before entering the settling regime, while τ_X ≳ 1 particles experience Safronov focusing before entering the pebble accretion regime. (⋆: the conversion of τ_X,Θ to physical parameters has been conducted for the default parameters at 1 AU.)