Retraction kinetics of impacting nanodroplets on hydrophobic surfaces: A molecular dynamics simulation study

In recent years, due to the huge application potential of nanodroplets in nanotechnology, the kinetics of impacting nanodroplets has attracted a great deal of research interest. Although the retraction process of nanodroplets is crucial for many industrial applications, it receives less attention compared to the spreading process. In this work, we perform molecular dynamics simulations to investigate the retraction kinetics of impacting nanodroplets on hydrophobic surfaces. Our simulations indicate that in the retraction process, the normalized spreading radius of nanodroplets with different Weber numbers cannot collapse into a universal curve, which is quite different from the experimental results of macrodroplets. We calculate the retraction rate of impacting nanodroplets with various Weber numbers on surfaces with different hydrophobicity for further investigation. The results show that the retraction rate of nanodroplets increases with Weber number and surface hydrophobicity. By comparison with the simulation results, three existing models for macrodroplets are proved inapplicable to the retraction rate of nanodroplets. A new theoretical model for the retraction rate of nanodroplets is proposed based on the conservation of energy. The present model provides a more accurate prediction for the retraction rate of nanodroplets in different conditions. (C) 2021 Elsevier B.V. All rights reserved.