Collision factor f_coll as a function of planetesimal radius R for a constant internal density ρ_• = 1 g cm^-3. Crosses indicate numerical data, while solid lines are fits. The color scale ranges from dark to light blue for increasing particle radius. a) Results obtained with the (inviscid) potential solution (Eq. (7)) at 1 AU orbital distance from the star. For a small planetesimal size, the collision factor is approximately geometrical (f_coll ≈ 1). However, for small pebble sizes, aerodynamic deflection results in a significant drop of f_coll (see Table 1 for the minimum collision efficiency, f_coll,min). b) Results with the (highly viscid) Stokes flow solution (Eq. (8)). Settling occurs at smaller sizes due to the increase in encounter time. c) Results at 3 AU orbital distance show a shallow barrier only for the lowest particle size. d) At 10 AU a barrier is no longer present because of the reduced gas density.