Impact of a nanodroplet on liquid surfaces

The present study investigates impact of nanodroplets on liquid films with varied film thicknesses over hydrophilic to hydrophobic surfaces via molecular dynamics simulations. A new crown pattern, hollow crown, is observed on medium liquid films for nanodroplets, whereas it is not reported for millimeter-sized droplets. Holes (or hollows) are attributed to capillary vibration of a nanometer-thick liquid film. The occurrence of holes is inhibited effectively for deep liquid films because the cavity thickness is much larger than the amplitude of capillary vibration. Surface wettability has a significant influence on thin and medium liquid films for nanodroplets, whereas it only affects thin liquid films for macrodroplets because the range of solid-liquid intermolecular forces is comparable to the size of droplets when the size of droplets reduces to the nanoscale. Subsequently, the accuracies of the existing macroscale models for predicting the time-dependent crown radius of nanodroplets are tested by simulated results. The results show that the macroscale models cannot capture the time-dependent crown radius of nanodroplets. On the basis of energetics analysis, we prove that the failure of macroscale models is attributed to the distinct difference in viscous dissipation mechanisms between macroscale and nanoscale droplets.