  The ejection of liquid droplets from a nozzle is highly important for physics
of fluid. The Weber number describes how much kinetic energy is needed to
overcome the surface tension and create a free-flying droplet. According to
literature Weber numbers above 12 assure the creation and safe break up of a
liquid droplet. However, even when this number goes down below 8, it is still
possible to observe droplet break-up but sometimes with particular effects. We
present here a fluid dynamics video showing experimental results and CFD
simulations for droplet break-up at low Weber number where the droplet is
generated with negative momentum. Such droplet generation is characterized by
the droplet breaking up and then returning back into the nozzle. This is due to
the fact that during the droplet formation the surface tension begins to slow
down the flow velocity inside the droplet and then finally inverts the flow
direction, while the droplet tail still breaks off from the nozzle. Thus after
the break up the droplet momentum is oriented toward the nozzle. It is
therefore possible to observe the droplet returning into the bulk fluid.
High-speed images of this particular phenomenon are shown and simulation
results are presented to illustrate the break up dynamics and the local
velocities in the droplet.
