Impact regimes of nanodroplets impacting nanopillared surfaces

In this study, a phase diagram is constructed for the impact of nanodroplets on nanopillared surfaces via molecular dynamics simulations. Four impact regimes, i.e., first nonbouncing (1NB), bouncing, second nonbouncing (2NB), and sticky, are discovered at various pillar heights and Weber numbers. The impact regimes are compared with those at the macroscale, and several significant differences are distinguished. The differences are attributed to the significantly enhanced viscous effect, the modified viscous dissipation mechanism, and the altered wetting transition mechanism. The impact regimes are found to strongly depend on the properties of surfaces. On monostable Wenzel surfaces (small pillar heights), the sticky regime is the only regime; on metastable coexisting Cassie-Wenzel surfaces (moderate pillar heights), the 1NB and sticky regimes successively take place with increasing Weber numbers; and on monostable Cassie surfaces (large pillar heights), the sticky regime disappears, but two new regimes, i.e., the bouncing and 2NB regimes, appear. The boundaries between the 1NB and sticky regimes, between the 1NB and 2NB regimes, and between the bouncing and 2NB regimes are all related to the formation of a partial wetting state, in which the central gaps beneath the nanodroplet are intruded and completely wetted. The wetting transition at the nanoscale does not follow the macroscale depinning mechanism. Therefore, a theoretical model is developed to understand the wetting transition mechanism at the nanoscale and thus predicting the critical Weber number for triggering the wetting transition, yielding Wecr1/2∼-h∗cosθi, which well describes the boundaries mentioned above. © 2022 American Physical Society.