Characteristic collision speed â₀ between solid particles at different points in our simulations, with 1σ error bars, for particle size τ_f = 0.003. The solid concentration (Z = Σ_solid/ Σ_total) has little effect on â₀. The value â₀< 0.1 cm/s (red dashed line) is a robust upper bound on â₀. This corresponds to coagulation timescale of t_stick< 10⁵ years, which is significantly less than the disk lifetime (10⁶−10⁷ years). Note that the fitting method can give negative values because it is a simple linear extrapolation (see main text).