Satellite Droplet Formation and Elimination in Binary Droplet Collision *

In drop-drop collisions with large Weber number (We), separations frequently occur, featuring the fragmentation after a tentative coalescence. Our experiments showed that, in both head-on and off-center collisions of two identical drops, at least one satellite droplet can be formed after pinching of the coalesced drop. The simulations of head-on collision demonstrate the asymmetrical pinching necks of the temporarily coalesced drop, in agreement with the universal features of pinching free-surface flow and the scaling laws. This allows two pinched free-surfaces with mirrored asymmetrical profiles, permitting the satellite droplet formation upon breakup (Huang, Pan & Josserand, Phys. Rev. Lett., 2019). For the simulations of off-center collisions, when the neck of the tentatively coalesced drop forms at the drop center, the maximum pressure in the neck makes the maximum velocities to locate at two sides of the center. This makes the local radius along the drop to decrease fastest at two sides close to the primary drops, leading to two pinched points and thus the satellite droplet formation. Via slight breaking of the symmetry, no satellite droplet could be observed in head-on collision, thus providing a possible implication for controlling undesirable drop formation.