New insights from studying extremely large droplet breakup*

The aerodynamic breakup of droplets is essential for many applications like fuel atomization, agricultural spraying, fire suppression, and crashing sea waves. While most studies focus on droplet sizes from microns up to the capillary length, we explore the breakup of extremely larger droplets. We designed a novel device that releases a large droplet onto an air jet combined with a moving plate mechanism that impulsively exposes the droplets. The breakup phenomenon is visualized by high-speed shadowgraphy from two angles. Qualitative comparisons reveal that the breakup of large droplets differs from smaller ones. Notably, we identify a new breakup regime termed Forward Bag, where bag formation occurs in the direction of the incoming air stream, contrasted with the backward bag regime observed in smaller droplets at similar Weber/Ohnesorge numbers. One key distinction between small and large droplet breakup mechanisms is that small droplets exhibit singular breakup regimes (e.g., vibrational, bag, or sheet thinning). In contrast, large droplet breakup occurs in a cascading manner, with atomization occurring through multiple mechanisms, including forward bag and multi-bag with stamen formations.