Effect of coalescence on the propagation of water droplets from a jet

The distance traveled by drops in a spray is of paramount importance for many applications of spraying, from aerosol virus transmission to pesticide spraying on crops. Commonly used models to describe droplet trajectories in sprays often neglect interactions between droplets such as coalescence and air drag. We study the behavior of monodisperse and polydisperse droplet trains produced by Rayleigh breakup of a single liquid jet exiting a nozzle. We show that coalescence increasingly affects the droplet size distribution away from the nozzle. It was previously reported (but not explained) that droplet trains travel farther than single drops of the same size. Here, we show that this is due to the coalescence; as large droplets have a larger inertia, they are less affected by air drag. Comparing a polydisperse droplet train from regular Rayleigh breakup with a monodisperse one, we find that monodispersity does not prevent coalescence, but does delay it in the first part of the droplet train's trajectory, leading to droplets up to 50% bigger and faster along the trajectory for polydisperse trains. Thus, droplet size variations due to coalescence should be taken into account in most applications of sprays.