Experimental study on the impact forces of a water droplet colliding with a surface carrying a deposited droplet

In this study, the process of water droplet collision with a horizontal surface carrying a deposited droplet and the impact forces caused by the Rayleigh jet in the retraction stage of collision were experimentally investigated. Such collisions are common in various industrial applications. The droplet impact force on the surface was measured using a piezoelectric sensor and the corresponding instantaneous droplet morphology was obtained using high-speed photography. For comparison, the characteristics of Rayleigh jets and their corresponding impact forces were studied. When droplets collide with a surface carrying a deposited droplet, the Rayleigh jet may be accompanied by a peak impact force that is much greater than that in the spreading stage of the collision. This impact force exhibits a decaying oscillatory pattern. The appearance of decaying oscillatory impact forces is closely related to the mass ratio of deposited droplets to falling droplets, as well as the Weber number and Froude number of falling droplets. The reaction force of the upward Rayleigh jet on the surface in the case where decaying oscillatory impact force does not appear leads to a relatively small peak impact force. A scaling relationship for the reaction force peak is derived.