Impact dynamics of nanodroplets on pillared surfaces

This work investigates impacting nanodroplets on pillared surfaces via molecular dynamics (MD) simulations, especially to understand the intrusion effect of liquid in pillar gaps at the nanoscale, by comprehensively revealing outcome regimes and modeling the maximum spreading factor $\left({\beta }_{\max }\right)$. A total of six outcomes, including first sticky (1S), second sticky (2S), first nonbouncing (1NB), second nonbouncing (2NB), first bouncing (1B), and second bouncing (2B), are identified. The 1S, 2S, and 2B regimes take place on monostable Wenzel surfaces with the Wenzel-to-Cassie dewetting transition and bouncing boundaries separating them; the 1NB, 2NB, 1B, and 2B regimes occur on monostable Cassie surfaces, distinguished by the Cassie-to-Wenzel wetting transition and bouncing boundaries. By establishing criteria of all boundaries, a universal phase diagram of impacting nanodroplets on pillared surfaces is constructed. Besides, to understand the altered spreading dynamics by the liquid intrusion effect, ${\beta }_{\max }$ is modeled. The bulk droplet above pillared surfaces is found to have the same spreading dynamics as a nanodroplet on flat surfaces, which decouples the effects of the bulk droplet and the liquid intruding into pillar gaps. Subsequently, two intrusion regimes are classified based on different intrusion morphology of the liquid front, and the scalings for intrusion volume in different intrusion regimes are obtained with the corresponding transition criterion being proposed. Eventually, scaling laws of ${\beta }_{\max }$ for impacting nanodroplets on pillared surfaces are established by incorporating the volume term of the bulk droplet, and are in good agreement with all available MD data of ${\beta }_{\max }$, showing their strong robustness and universality.