Dynamic transition approaching jetting singularity during the collapse of drop-impact craters

Understanding the dynamical transition from capillary-driven to pure inertial focusing is important to revealing the mechanism underlying the most singular micro-jetting produced from drop-impact craters. Herein we study dimple dynamics from the collapse of these craters when the drop is of a different immiscible liquid than the pool. The parameter space is considerably more complex than for identical liquids, revealing intricate compound-dimple shapes. In contrast to the universal capillary-inertial drop pinch-off regime, where the neck radius scales as $R\sim t^{2/3}$, a purely inertial collapse of the air-dimple portrays $R \sim t^{1/2}$ and is sensitive to initial and boundary conditions. Capillary waves can therefore mold the dimple into different collapse shapes. We observe a clear cross-over in the nature of the dynamics from capillary-inertial to purely inertial immediately before the pinch-off. Notably, for singular jetting the cross-over time occurs much earlier, irrespective of the Weber number.