Optimizing surface design via nanodroplet dynamics on inclined wettability gradients

The process of droplet impact is prevalent in nature and engineering applications. At the nanoscale, the scale effects dominated by viscous forces give rise to unique dynamic behaviors. This study systematically investigates the impact behavior of nanodroplets on inclined surfaces with varying wettability through molecular dynamics simulations. Innovative theoretical methods for analyzing the maximum spreading factor and contact time are proposed, revealing their coupling mechanisms with surface properties. Results indicate that the maximum spreading factor exhibits distinct characteristics with changes in the Weber number and wettability, with moderate contact angles optimizing spreading behavior. Contact time follows a three-phase variation, with the cavity rebound mode significantly prolonging it, underscoring the role of droplet morphology evolution in dynamics. A two-dimensional phase diagram of impact outcomes is constructed, categorizing modes such as regular rebound, cavity rebound, and splashing rebound, and their formation mechanisms and critical conditions are theoretically elucidated. Furthermore, analysis shows that sliding distance reaches a maximum under high Weber numbers but slightly decreases due to cavity rebound and asymmetric retraction. This study not only provides new theoretical perspectives for understanding nanodroplet dynamics but also guides the optimization of surface engineering design and related applications.