  We address the collision of two superfluid 4 He droplets at non-zero initial
relative velocities and impact parameters within the framework of liquid 4 He
time-dependent density functional theory at zero temperature. In spite of the
small size of these droplets (1000 He atoms in the merged droplet) imposed by
computational limitations, we have found that quantized vortices may be readily
nucleated for reasonable collision parameters. At variance with head-on
collisions, where only vortex rings are produced, collisions with non-zero
impact parameter produce linear vortices which are nucleated at indentations
appearing on the surface of the deformed merged droplet. Whereas for equal-size
droplets vortices are produced in pairs, an odd number of vortices can appear
when the colliding droplet sizes are different. In all cases vortices coexist
with surface capillary waves. The possibility for collisions to be at the
origin of vortex nucleation in experiments involving very large droplets is
discussed. An additional surprising result is the observation of the drops
coalescence even for grazing and distal collisions at relative velocities as
high as 80 m/s and 40 m/s, respectively, induced by the long-range Van der
Waals attraction between the droplets.
