Probability that the relative speed between two solid particles lies below the sticking threshold v_crit ~ 0.03 cm/s (Güttler et al. 2010). The particle size is τ_f = 0.003 and Z = Σ_solid/ Σ_total = 0.074 in all the simulations. Given a set of particle pairs with separations less than D, one can fit a Maxwellian distribution (Figs. 10 and 12) determine the fraction of speeds below v_crit. Extrapolating to D → 0 gives an estimate of the fraction of particle collisions that result in sticking. The region in green corresponds to the 1σ confidence region marked in Fig. 10.