The Landscape of Newtonian Droplet Breakup under Impulsive Acceleration

We examine the complete landscape of parameters which affect secondary breakup of a Newtonian droplet under impulsive acceleration. A Buckingham-Pi analysis reveals that the critical Weber number We_cr for a non-vibrational breakup depends on the density ratio (ρ), the drop (Oh_d) and the ambient (Oh_o) Ohnesorge numbers. Volume of fluid (VOF) based numerical simulations are performed using Basilisk to conduct a reasonably complete parametric sweep of the non-dimensional parameters involved. It is found that, contrary to current consensus, even for Oh_d ≤ 0.1, a decrease in Oh_d has a substantial impact on the breakup morphology, motivating plume formation, and in turn affecting We_cr. It is observed that in addition to ρ (which previous studies had observed), Oh_o affects whether local inertia or aerodynamic drag is the dominant force acting on the droplet. This behavior manifests in simulations through the observed pancake shapes and ultimately the breakup morphology (forward or backward bag). We summarize these results and present physical explanations of the observed behavior.