Contact angle evolution during nanodroplet nucleation

Vapor condensation is widely adopted in thermal management technology and condenser of energy power plant, benefiting from its high heat transfer coefficient. Contact angle is the key factor to regulate vapor condensation rate, leading to the rapid development of surfaces with various wettability fabricated by micro/nanoengineering. However, the nucleated nanodroplets (1–100 nm) at the beginning of condensation exhibit different wetting dynamics from macroscopic droplets, the mechanisms of which are not well understood. In this work, we perform molecular dynamic (MD) simulations to study the evolution of contact angle during nanodroplet nucleation processes. The results show the nucleated contact angle of a nanodroplet is smaller than the contact angle predicted by the classical Young equation on a hydrophobic surface, while it is opposite for nanodroplet nucleating on hydrophilic surface. Moreover, we have calculated line tension for the nucleated nanodroplet on the surfaces with different wettability to explain this discovery, and found positive line tension on hydrophobic surface, while negative line tension for nucleated nanodroplet on hydrophilic surface. Furthermore, the calculated line tension, which is at the order magnitude of 10⁻¹¹ J/m, aligns well with the data documented in the literature.