Discrete motion modes of eccentric droplet impact on sessile droplet at low Ohnesorge and Weber numbers

This paper presents an experimental study on the discrete motion characteristics of droplets colliding with sessile droplets at low Weber and Ohnesorge numbers. Droplets generated by a micro-injection pump impact sessile droplets on a stainless-steel flat plate, with the process recorded by a high-speed camera and data collected using a micro-force sensor. The results show three distinct modes of droplet behavior upon collision with the sessile droplet: rebound and coalescence, rebound and separation, and slide and coalescence. Apart from these modes, direct coalescence is the dominant outcome under most experimental conditions, while this study specifically focuses on the less frequent discrete behaviors to explore the dynamics of such motions. As the eccentric distance between the impacting droplet and the sessile droplet increases, these three discrete modes transition sequentially. Changes in the volume ratio between the sessile droplet and the impacting droplet shift the boundaries of these modes along the eccentric distance. The effect of eccentric distance on contact-line spacing transitions from promoting to suppressing. An increase in the volume ratio leads to greater kinetic energy dissipation of the impacting droplet and reduces its rebound height. During the coalescence and rebound events, two pressure peaks are observed on the wall surface, but the origins and temporal intervals of these peaks differ. The volume ratio significantly affects the peak impact pressure and contact time. This study contributes to a deeper understanding of the process of continuous droplet impact on wall surfaces.