Suppression of collision velocities in particle-laden protoplanetary disk turbulence

Understanding how the collision of dust grains is regulated by the turbulence generated in protoplanetary disks is key to understanding the growth of particles from a few nm to cm sized objects. But while particle clustering in gas-dominated turbulence is well understood, the clustering behavior in turbulence with significant dust-loading has not been investigated. Here we perform turbulence simulations using dust to gas ratios between 10^-2 and 10 and for a range of particle stopping times, showing that the particle collision velocities are reduced significantly when increasing the dust-to gas ratio of the simulations. Also, we find that increasing the dust-to-gas ratio changes the particle clustering at small scales, shifting the radial distribution function to mimic the behavior of particles with smaller particle stopping times. This means that significantly larger particles are able to grow through sticking collisions within regions with high particle concentration (such as zonal flows and vortices), affecting the maximum size of grains visible to (sub-)mm observations and the growth of the streaming instability in those regions.