Dynamics of Droplet-Droplet and Droplet-Film Collision

摘要

The physical phenomena of droplet-droplet and droplet-film collision in the head-on orientation were studied experimentally and computationally, with emphasis on the transition between bouncing and merging of the liquid surfaces. Experimentally, the droplets (approximately 300 m diameter) were generated using the ink-jet printing technique, and imaged using stroboscopy and high-speed cine-photography for the droplet-droplet and droplet-film collision events respectively. Computationally, the collision event was simulated using the front-tracking technique. For the study of droplet-droplet collision, the instant of merging was experimentally determined and then used as an input in the computational simulation of the entire collision event. The simulation identified the differences between collision and merging at small and large Weber numbers, and satisfactorily described the dynamics of the interdroplet gap including the role of the van der Waals force in effecting surface rupture. For the study of droplet-film collision, extensive experimental mapping showed that the collision dynamics is primarily affected by the droplet Weber number (We) and the film thickness scaled by the droplet radius (H), that while droplet absorption by the film is facilitated with increasing droplet Weber number, the boundary of transition is punctuated by an absorption peninsula, in the We-H space, within which absorption is facilitated for smaller Weber numbers. Results from computation simulation revealed the essential dependence of the collision dynamics on the restraining nature of the solid surface, the energy exchange between the droplet and the film, and the coherent motion of the gas-liquid interfaces. Partial absorption with the emission of a secondary droplet of smaller size was also observed and explained.