LAUSR

Abstract This paper studied interfacial phenomena associated with droplet array impacting on liquid film with aid of high-speed photography. Aside from liquid humps following droplet array coalescing with liquid film under low impact velocity, crowns and an uprising central liquid sheet can be observed under high velocity. This central sheet finally evolves into two liquid columns as its middle part descends in advance, and its lifetime is improved by increasing the pre-existing film thickness. The time delay between two droplets leads to the site of central sheet being close to the leading droplet impact point, and the central sheet is tilted toward the trailing droplet rather than evolving vertically. Droplet vertical or horizontal spacing and liquid film thickness have a non-monotonous effect on the crown height, while impact velocity poses a positive effect. The crown diameter has a weak dependence on droplet vertical spacing and film thickness. While horizontal spacing has a positive effect on the crown diameter, impact velocity presents a negative effect in the later stage, which can be attributed to the inward tilting of crown wall. The height of the central sheet increases monotonously with decreasing droplet vertical or horizontal spacing. Impact velocity and film thickness produce positive effects on the evolution of central sheet height. This study provides a fundamental understanding for applications involving droplets impact.