Numerical investigation for primary breakup study of unperturbed air filament embedded in a steady environment

The dynamics of a quiescent finite-size air filament embedded in a denser medium with a range of viscosity are studied numerically. Surface tension causes the two edges of the filament to retract back. It is shown that for a small aspect ratio, the filament recoils in a single bubble while when the aspect ratio is large, it ruptures at both ends. In between these limits, the filament can avoid end pinch-off by reopening the neck and/or by undergoing a breakup in the middle region. The pinch-off of the air filament is followed by the formation of a thin thread of air, which later breaks up into microbubbles. Viscous effects are shown to not significantly impacting the fate of the air filament but can alter the shape of the end bulge and the neck region. For Ohnesorge numbers smaller than 0.01, the bulge adopts a prolate shape, while for Ohnesorge numbers larger than 1 an oblate shape is observed. Between these two limits, a spherical shape is obtained. Close to breakup, the neck radius is shown to decrease as a power-law function of the time with an exponent of 0.5 for small Ohnesorge numbers and 1 for large Ohnesorge numbers.