Abstract
When a drop impacts on a deep pool at moderate velocity it forms a hemispheric crater which subsequently rebounds to the original free-surface level, often forming Worthington jets, which rise vertically out of the crater centre. Under certain impact conditions the crater collapse forms a dimple at its bottom, which pinches off a bubble and is also known to be associated with the formation of a very fast thin jet. Herein we use two ultra-high-speed video cameras to observe simultaneously the dimple collapse and the speed of the resulting jet. The fastest fine jets are observed at speeds of approximately
$50~\text{m}~\text{s}^{-1}$
and emerge when the dimple forms a cylinder which retracts without pinching off a bubble. We also identify what appears to be micro-bubbles at the bottom of this cylinder, which we propose are caused by local cavitation from extensional stress in the flow entering the jet. The radial collapse of the dimple does not follow capillary-inertial power laws nor is its bottom driven by a curvature singularity, as has been proposed in some earlier studies. The fastest jets are produced by pure inertial focusing and emerge at finite dimple size, bypassing the pinch-off singularity. These jets emerge from the liquid contained originally in the drop. Finally, we measure directly the compression of the central bubble following the pinch-off and the subsequent large volume oscillation, which occurs at frequencies slightly above the audible range at approximately 23 kHz.