Spreading and bouncing dynamics of nanodroplets: Center-to-center impact on a solid surface

Multiple-droplet impact dynamics has attracted research attention due to its frequent occurrence and complexity in real-world engineering and applications. Elucidating its underlying physics is essential for optimizing performance in key applications, including high-resolution inkjet printing, self-cleaning coatings, and anti-frosting surfaces. Using molecular dynamics simulations, we explore the spreading and bouncing dynamics of nanodroplets undergoing center-to-center impact on a solid surface. For the condition of a nanodroplet impacting on a pre-sessile droplet, simulation data demonstrate that the maximum spreading radius Rmax scales linearly with the velocity of impact, and the two are positively correlated. For the study of binary droplet successive impacts, we divide the impact forms into three situations based on the state of the leading droplet at impact: (I) spreading, (II) retraction, and (III) bouncing, as determined by the Weber number and the interval time. Except in situation (II), the maximum spreading factor progressively decreases with increasing interval time. Furthermore, an empirical correlation is proposed to predict the maximum spreading factor when the interval time approaches zero, based on the maximum spreading factors from single droplets of two distinct volumes. Intriguingly, when contact time equals interval time, two distinct linear regimes emerge in their relationship. As the interval time approaches zero, the contact time for successive droplet impacts converges to approximately 1.609 ± 0.2 times that of a single droplet. Based on this observation, a model for the contact time in successive binary droplet impacts has been developed. Furthermore, a preliminary analysis of the energy conversion pathway during the binary droplet impacts process is provided, offering a tentative mechanistic interpretation of their impact dynamics. These insights provide a foundation for studying multiple nanodroplet impacts, prompting a discussion of whether the maximum spreading radius saturates with increasing droplet count and how this influences the bouncing and spreading possibilities.