Maximum particle density, relative to the mid-plane gas density, versus time for a series of 64³ simulations (top panel) and 128³ simulations (bottom panel) of turbulence driven by the streaming instability with different treatment of collisions. The maximum particle density increases by a factor approximately 2 when doubling the resolution, but the maximum density peaks are consistently 50% lower when including particle collisions. Note the different scale of the axes in the two plots.