Fluid transportation by droplets impacting wettability-controlled surfaces at the nanoscale: a molecular dynamics simulation study

This work investigates nanodroplets impacting wettability-controlled surfaces via MD simulations. In comparison with fluid transportation of droplets at the macroscale, which is effective on hydrophilic/superhydrophobic surfaces, the one at the nanoscale is only achieved for nanodroplets on hydrophobic/hydrophobic or hydrophobic/superhydrophobic surfaces. This difference lies in the different transportation mechanism that breakup can take place for the former with the part on the superhydrophobic side being tossed, whereas, the droplets in the latter process always forms as an intact one owing to the enhanced viscous force at the nanoscale, indicating the transportation by bouncing instead of tossing. More quantitatively, the normalized transportation velocity, V* = V-t/V-0, is extracted from MD simulation results, where V-t is the transportation velocity and V-0 is the impact velocity. This feature parameter is controlled by the difference of contact angles ( increment theta) and impact velocity and our simulations show two kinds of conditions are found to have a good performance of fluid transportation at the nanoscale, i.e., high V-0 with high increment theta and low V-0 with increment theta. Finally, a simple energy conversion model with a fitting parameter is established for quantifying the relationship between V* and increment theta, attesting that the mechanism of fluid transportation at the nanoscale stems from the difference surface energy of the liquid-gas interface on wettability-controlled surfaces.

Automated summary

This study investigates the impact of nanodroplets on wettability-controlled surfaces through MD simulations. It is found that the transportation mechanism for nanodroplets differs from macro-scale droplets, as nanodroplets on hydrophobic/hydrophobic or hydrophobic/superhydrophobic surfaces undergo bouncing instead of breakup. The normalized transportation velocity, V*, is extracted from the MD simulation results and controlled by the difference of contact angles and impact velocity. The study establishes an energy conversion model to quantify the relationship between V* and the contact angle increment.