The breakup of a liquid jet under aerial firefighting conditions

In recent years, wildfires have increased significantly in both frequency and intensity worldwide. Prolonged droughts and strong winds enhance these fires, destroying forests, homes, and infrastructure. Developing new or better strategies to combat wildfires is, therefore, a pressing need. Aerial firefighting (AFF) is a widely used technique in which water or fire-retardant agents (with non-Newtonian properties) are dropped from aircraft to suppress or slow the spread of fire. However, under flight conditions, the interaction between the released liquid jet and the surrounding airflow introduces complex hydrodynamic instability and fragmentation phenomena that remain poorly understood. Hence, this study investigates the atomization of non-Newtonian liquid jets in air crossflow, focusing on conditions characteristic of AFF. Using high-speed imaging, we tracked the surface evolution and fragmentation dynamics, identifying the conditions that promote and define the onset and type of surface instabilities. We quantified the characteristic droplet sizes resulting from the jet fragmenting, seeking to uncover the roles of aerodynamic shear, viscosity, rheology, and surface tension in governing the process while mapping their influence on the transition points between dominant breakup modes. Overall, our findings may offer a mechanistic understanding of liquid fragmentation in AFF, aiming to establish physics-informed deployment strategies and enhance their

effectiveness.