Quantized vortex nucleation in collisions of superfluid nanoscopic helium droplets at zero temperature

We address the collision of two superfluid He-4 droplets at non-zero initial relative velocities and impact parameters within the framework of liquid He-4 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.