  The breakup of water droplets when exposed to high-speed gas flows is
investigated using both high-magnification shadowgraphy experiments as well as
fully three-dimensional numerical simulations, which account for viscous as
well as capillary effects. After thorough validation of the simulations with
respect to the experiments, we elucidate the ligament formation process and the
effect of surface tension. By Fourier decomposition of the flow field, we
observe the development of specific azimuthal modes, which destabilize the
liquid sheet surrounding the droplet. Eventually, the liquid sheet is ruptured,
which leads to the formation of ligaments. We further observe the ligament
formation and shedding to be a recurrent process. While the first ligament
shedding weakly depends on the Weber number, subsequent shedding processes seem
to be driven primarily by inertia and the vortex shedding in the wake of the
deformed droplet.
