The dynamics of head-on collision between two identical droplets was experimentally and computationally investigated, with emphasis on the transitions from merging to bouncing and to merging again, as the collision Weber number was increased. Experimentally the stroboscopically illuminated microphotographic images of two colliding droplet streams, generated through the ink-jet printing technique, were acquired with adequate temporal resolution of the collision event such that the instant at which the droplets merged for both soft and hard collisions was identified. Using this empirical information as an input, the simulated collision images were found to agree well with the experimental observations and allowed investigation of the collision flow field including the energy budget and the fundamental differences between the soft and hard collisions that lead to merging. It is further shown that the merging instant can be computationally assessed through the use of an augmented van der Waals force to effect merging through rupturing of the surfaces, with the associated Hamaker constant empirically but consistently extracted from the experimental observations.

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