Effect of surface tension and viscosity on the collision behavior between an upward flying droplet and a sessile droplet in an acoustic droplet ejection system

The stable coalescence of upward flying droplets on the target substrate is a fundamental requirement during the application process of the acoustic droplet ejection system. However, the liquid properties significantly affect the droplet collision dynamics behavior during high-throughput liquid transfer. This study investigated the impact mechanisms of surface tension and viscosity on the collision behavior of upward flying droplets. The results show that four different outcomes occur as the droplet collision velocity increases: coalescence after minor deformation, complete rebound, coalescence accompanied by conglutination, and direct coalescence. Moreover, as the surface tension decreases to a certain extent, it will lead to partial rebound with conglutination. A theoretical model was developed to calculate the maximum spreading diameter based on the law of conservation of energy, which allowed for an examination of how liquid properties affect the dimensionless parameters associated with direct coalescence. During the calculation of the maximum diameter, it was found that the higher the surface tension or the lower the viscosity of the liquid, the less viscous dissipation energy occurs in the droplet coalescence process. The decrease in energy dissipation increases the probability of direct coalescence taking place. By combining theoretical analysis and experimental observations of the binary droplet collision behavior, we have initially established a connection between the ideal droplet collision outcomes, particularly direct coalescence, and input power (or velocity). It can provide a reliable method that can be referenced for achieving efficient coalescence across various liquid types under given experimental conditions.