Von K\'arm\'an Vortex Street within an Impacting Drop
  The splashing of a drop impacting onto a liquid pool produces a range of different sized micro-droplets. At high impact velocities, the most significant source of these droplets is a thin liquid jet emerging at the start of the impact from the neck that connects the drop to the pool. We use ultra-high-speed video imaging in combination with high-resolution numerical simulations to show how the ejecta gives way to irregular splashing. At higher Reynolds number, its base becomes unstable, shedding vortex rings into the liquid from the free surface in an axisymmetric von K\'arm\'an vortex street, thus breaking the ejecta sheet as it forms. 
Marie-Jean Thoraval, Kohsei Takehara, Takeharu Goji Etoh, Stephane   Popinet, Pascal Ray, Christophe Josserand, Stephane Zal...
physics.flu-dyn