Development of particle swarms by a drafting instability. Displayed is the evolution of the particle density in the two-dimensional simulation run2 (see Table 1). The left-most panel illustrates the initial conditions: a stratified gas column in the vertical directions with particles sedimenting at terminal velocity, placed randomly throughout the simulation domain (1 l_f wide and 20 l_f high, note that the figure aspect ratio is enlarged in the x-direction). In the other panels (time t = 10,15 t_f), regions marginally overdense with particles locally break the stratification equilibrium and accelerate downwards while in particle-poor regions, a deceleration from terminal velocity occurs. This leads to continued particle pileups through drafting, originating from gas being dragged by the particles. The particle and fluid components unmix and remain in this state. At the end of the simulation, when most particles have sedimented out of the simulation domain, the maximal particle density has increased by a factor 10.