Spreading and retraction kinetics for impact of nanodroplets on hydrophobic surfaces

Impact dynamics of nanodroplets has recently gained extensive attention because of its potential applications in nanoscale inkjet printing, nanodroplet spray cooling, and nanocoating. In this study, a nanodroplet impacting unheated, flat, smooth, and hydrophobic surfaces is investigated via molecular dynamics simulations. The emphasis is placed on spreading and retraction kinetics, i.e., time-dependent wetting radius or r-tau relation, where r and tau are the normalized wetting radius and time. On the basis of an energy conservation approach, an analytical model of r-tau kinetics is developed for impacting nanodroplets. Hypotheses of cylinder droplet and extensional flow are employed to calculate the transient kinetic energy and viscous dissipation rate, which are found to be the most appropriate for impacting nanodroplets. The model is tested in a range of Weber numbers from We = 15 to 60, Reynolds numbers from Re = 11.07 to 22.19, and surface wettability theta (0) = 105 degrees and 125 degrees. The tests show that the mean relative deviation ranges from 2.22% to 5.47%, and hence, the developed model captures the spreading and retraction kinetics of a nanodroplet impacting hydrophobic surfaces with satisfactory accuracy. Furthermore, it is found that the model can also be extended to predict the retraction kinetics of nanodroplets on hydrophilic surfaces for high Weber numbers.