Bouncing and coalescence dynamics during the impact of a falling drop with a sessile drop on different solid surfaces

The dynamic of an impacting drop with a sessile drop of the same liquid on different solid surfaces is investigated experimentally and theoretically. By controlling the surface wettability and the impact velocity, different regimes were observed, such as complete rebounding, direct coalescence, and coalescence during the retraction phase or the spreading phase. It is observed that the complete rebounding phenomenon has widely occurred during the impact on hydrophilic surfaces. In addition, both the maximum and minimum limits of coalescence/complete rebounding thresholds were determined. During the complete rebounding phenomena, and based on the energy balance, the maximum spreading diameter of the falling drop diameter on the sessile drop was proposed. In addition, the restitution coefficient and the contact time between the falling and sessile drops were studied experimentally, and theoretically, the models show a good agreement with the experimental work.