Do drop-impact craters produce singular jets?*

Drop-impact craters rebound and usually produce prominent Worthington jets.  For a limited range of drop-impact Weber numbers, capillary waves travel down the crater surface and a dimple forms at the bottom of the crater.  This dimple can pinch off and entrap a bubble into the pool, in a process call regular bubble entrapment.  This bubble oscillates and emits sound.  This parameter range is also associated with the appearance of high-speed fine jets.  It has been suggested that the fastest jets are driven by a singularity in the surface curvature at the bottom of the dimple.  Herein we use two ultra-high-speed video cameras to simultaneously image the jetting and the dimple evolution*.  Our imaging at micro-second resolution never shows any curvature singularity at the bottom of the dimple.  Furthermore, the fastest jets emerge when the inertial focusing drives the jet without pinch-off of a bubble.  In contrast the bottom dimple air-cylinder is pushed up at high velocity when its diameter is of the order of 25 microns, producing jets which emerge at speeds as fast as 50 m/s.

*Thoroddsen et al., J. Fluid Mech., 848, R3 (2018).