Morphology and Impact Forces of a Droplet Impacting on a Droplet-Carrying Solid Surface

The phenomenon of liquid droplets colliding with wetted solid surfaces is widely found in nature and in industrial applications. In this study, we experimentally investigated the phenomenon of a droplet colliding with a droplet-carrying solid surface. The impact force on the solid surface was measured with a high-precision piezo-electric sensor and the morphological changes during droplet collisions were captured with a high-speed camera. Experimental results showed that in such collision, the spreading factor of the droplet and the impact force on the surface are substantially influenced by droplet Reynolds number and are slightly affected by droplet Weber number. At a high Reynolds number, an annular jet appears between the falling droplet and the deposited droplet during the initial stage of collision, and rapid droplet spreading and a multipeaked impact force curve were measured. On the contrary, at a low Reynolds number, the collision only results in a single force peak with the disappearance of the annular jet. Additionally, a considerable reduction in both the spreading factor and velocity is observed. This alteration is due to the quite different viscous forces in the two types of droplet collision. Detailed analysis suggests that the droplet spreading process at high Reynolds numbers can be classified into three distinct stages: the buffering stage, the high-speed spreading stage, and the maximum spreading stage. Furthermore, the volumetric ratio of the deposited droplets to the falling droplets has only a slight influence on the impact process.