Vertically integrated dust density distribution at different times of evolution. Case of a surface density with a gap created by 1 M_Jup located in a fixed orbit at 20 AU in a flared disk and α = 10^-2 (top panel), α = 10^-3 (middle panel). Case with no planet and α = 10^-3 (bottom panel). The solid white line represents the particle size corresponding to St = 1, while the blue line corresponds to the fragmentation barrier according to Eq. (4) (Brauer et al. 2008; Birnstiel et al. 2010a). The dashed line corresponds to the size for which particles drift faster than those dragged by the gas. Particles above the dashed line are perfectly trapped.