Droplet breakup and size distribution in an airstream - effect of inertia*

We experimentally investigate the morphology and breakup of a droplet descending from various heights into an airstream. Using high-speed shadowgraphy and in-line holography, we analyze the size distributions of the resulting child droplets. Droplets exhibit shape oscillations due to the interplay between inertia and surface tension, leading to variations in radial deformation and breakup dynamics under identical airstream conditions. Droplets undergo vibrational breakup when released just above the air nozzle. As the release height increases with a constant Weber number, breakup modes transition through vibrational, retracting bag, bag, bag-stamen, and retracting bag-stamen, before returning to vibrational. Retracting bag breakup results in a bimodal size distribution, while bag and bag-stamen breakups yield a tri-modal distribution. A theoretical model incorporating the effective Weber number predicts the size distributions from these breakup modes accurately.