Droplet-jet collision following the monodispersedly dripping of coaxial binary droplets above a pool surface

Abstract

In the present study, the collision between a falling droplet and a rising Worthington jet was experimentally studied. The event is followed by the monodispersedly dripping of coaxial binary droplets into a quiescent pool of glycerol solution. Different concentrations of the solution are considered. Unique droplet-jet collision characteristics are observed when the dripping flow rate is manipulated to release binary droplets. When the first droplet impacts the pool, a significant disturbance is imposed onto the pool, forming a deep crater followed by a Worthington jet. The second droplet is timed to collide with the rising jet to create a unique mushroom-shaped droplet-jet collision. Two jet pinch-off modes (tip pinch-off and no pinch-off) and four distinct collision regimes (partial rebounding, end-pinching, elongated, and clotted central jet collision) are recognized. Liquid viscosity and jetting mode significantly influence the collision dynamics and splattering characteristics. To achieve partial rebounding collision at low Weber number, a high-impact coefficient incorporating characteristic dimensions of the droplets and the Worthington jet is required, whereas a low-impact coefficient is required at high Weber number to attain clotted jet collision. The overall end-pinching phenomenon occurs due to the interaction between liquid flow toward the jet tip and the retraction of the tip, which causes the jet neck diameter to decrease on a capillary timescale. As the impact parameter decreases, the Worthington jet is inhibited, and the mushroom-shaped collision splash spreading is suppressed.