Regions in which the collection of dust grains becomes very inefficient (defined as a collision cross section lower than 1% of the geometric one – see text) as a function of orbital distance and planetesimal radius for various dust sizes (labeled) and for two values of the turbulence: α = 10^-2 (top panel) and α = 10^-4 (bottom panel). Large particles are collected relatively efficiently everywhere except in the innermost regions of the disk where the gas density is assumed to be high. Small particles are inefficiently collected everywhere except at the largest orbital distances, at least in the case of a disk characterized by the MMSN gas density distribution.