Single drop impact onto liquid films: Neck distortion, jetting, tiny bubble entrainment, and crown formation

Single drop impact onto liquid films is simulated numerically. Surface tension and gravity are taken into account, whereas viscosity and compressibility are neglected. This permits recourse to a boundary-integral method, based on an integral equation for a scalar velocity potential. Calculations are performed for normal impacts resulting in axisymmetric flows. For times that are small compared to the characteristic time of impact 2R/w₀ (R being the drop radius, w₀ its initial velocity towards the liquid film), it is found that a disk-like jet forms at the neck between the drop and the pre-existing liquid film, if the impact Weber number is high enough. This jet can pinch off a torus-shaped liquid volume at its tip or reconnect with the pre-existing liquid film, thus entraining a torus-shaped bubble. In reality, both the torus-shaped bubble and liquid torus will decay according to Rayleigh's capillary instability, thus breaking the cylindrical symmetry. This mechanism of bubble entrainment differs from those described in literature. For times that are comparable to or larger than the characteristic time of impact, capillary waves on the film, or the well-known crowns, are obtained again according to whether the impact Weber number is low or high enough.