Secondary Atomization of Droplets at Extreme Conditions

Droplets, which are ubiquitous in nature, are formed through intriguing processes, and one such route is air-assisted atomization or aerobreakup. This review focuses on secondary atomization, particularly the breakup of an individual droplet subjected to high-speed flows. This process involves complex interfacial dynamics with multiscale deformations, ranging from global flattening to local unstable waves. The deformations occur at progressively smaller scales while interacting with the surrounding gas phase, forming a nonlinear cascade. Each local undulation serves as a precursor to a self-similar evolution or subsecondary breakup process that ends with a ligament-mediated mechanism. In practical scenarios, droplets often encounter nonuniform, unsteady, impulsive, or compressible flows, like shock waves, which pose extreme conditions. The spatiotemporal scales of the nonuniformity or unsteadiness of the external flow must be comparable with the drop deformation scales at either global or local levels to influence aerobreakup that cascades across hierarchical deformation scales. The compressible effects at high Mach numbers are interestingly shown to suppress the tendency toward breakup.