Bouncing dynamics of impacting unequal-sized sessile droplets: An atom-level study

The dynamic behavior of nanodroplets impacting unequal-sized sessile droplets has a wide range of applications in engineering, including anti-erosion, anti-icing, and self-cleaning. This study employs molecular dynamics simulation to investigate the dynamic behavior of a nanodroplet impacting another unequal-sized sessile droplet. The time evolutions of the spreading factors are used to analyze the particular dynamic behaviors and elucidate the underlying physics. A new extended scaling law β_max ∼ ηWe^1/2Re^1/5 characterizes the relationship between the maximum spreading radius β_max and the ratio of droplet radius η, Weber number We, and Reynolds number Re. The ratio of the contact time of unequal-sized droplet impact to that of a single droplet impact decreases with the increase in η, with a slope of −1/2 when η < 1 and −20/7 when η > 1. The two conditions intersect at 1.414 when η = 1, representing equal-sized droplet-on-droplet impact, which agrees with previous macro-scale droplet impact studies. Two boundaries are identified to discriminate among three outcomes: deposition, bouncing, and splashing, which are highly dependent on both the impacting velocities and the ratio of droplet radius. The conclusion of this article can lay a theoretical foundation for effectively manipulating nanoscale droplets. The research findings can provide theoretical guidance for engineering fields such as anti-icing coating design, micro-droplet printing, and spray cooling. © 2026 Author(s).