Morphological Transitions in Droplet Breakup Under Shock-Induced Flows

This work presents a controlled parametric study of shock-induced droplet aerobreakup to examine how Mach number (Weber number) and droplet temperature influence breakup morphology and Helvin-Helmholtz (KH) instability development. An upgraded shock-tube facility with automated pressure control, synchronized droplet injection, and high-speed imaging was used to generate a range of normal shocks. Diffuse background illumination (DBI) and shadowgraphy were compared to determine their effectiveness in resolving transient deformation and fragmentation. Across the Mach sweep, vibrational, bag, sheet-thinning, and catastrophic breakup modes were observed, with trends that aligned with Weber number regime boundaries. DBI consistently provided clearer visualization of droplet structures, while shadowgraphy emphasized density gradients that frequently obscured ligament formation. KH wavelengths extracted from ambient and heated droplets showed the expected inverse relationship with Weber number, but rapid cooling of the heated droplets minimized the differences between conditions. Overall, the study demonstrates that Mach number strongly governs breakup severity, DBI is better suited for resolving unobscured small-scale morphology, and thermal effects on KH instabilities require strict temperature control to produce meaningful deviations from ambient behavior.