Influence of Weber Number on Crown Morphology during an Oblique Droplet Impact on a Thin Wall Film

Spray impacts can be found in several technical applications and consist of many single droplets, which impact under different trajectories on wetted walls. This study investigates the asymmetric crown morphology resulting from an oblique impact (alpha= 60(degrees)) of a single droplet on a horizontal and quiescent wall film of the same liquid. A droplet generator with an accelerated needle releases the droplets (D= 1.5 mm) in a controlled trajectory on a thin film (h(f)/D= 0.2). The impact process is recorded from two perspectives with two synchronized high-speed cameras. Varying the Weber number within the splashing regime reveals distinct crown morphologies, which are described in detail. For We< 500, a single central finger develops at the front of the crown, with subsequent detachments of secondary droplets. At higher We (>500), a collision of the crown with the wall film shortly after impact introduces disturbances into the rim, leading to two fingers in the middle of the front crown. A further increase in We (>600) intensifies the crown-film interaction, resulting in an early ejection of tiny droplets and a complete breakup of the front rim. The influence of We on the crown morphology during an oblique impact is also compared to the normal impact (90(degrees)). This study paves the way for a classification of impact regimes and a comprehensive picture of the oblique impact process, which deserve more investigation.