Kinetic modelling of droplet ripening in a single-component liquid–vapour system

Droplet ripening is conventionally modelled by macroscopic interfacial theories, which inherently neglect the intrinsic microscopic structures of the liquid–vapour system, specifically the finite diffuse interface and the adjacent Knudsen layer. Resolving these structures is critical, as they govern mass transfer rates in regimes where interfacial transport kinetics dominate over bulk diffusion, particularly in micro- and nanodroplet interactions. To elucidate the underlying fundamental physical mechanisms, we investigate droplet ripening in a single-component liquid–vapour system using the Enskog–Vlasov kinetic model. This approach naturally captures the full spatial structure of both the diffuse interface and the Knudsen layer. We first focus on two-droplet interactions, deriving a theoretical growth rate based on the intrinsic imbalance of molecular fluxes at the interface. Numerical simulations quantitatively validate this prediction, confirming that the droplet-radius change rate scales linearly with the thermodynamic driving force. However, a distinct deviation from this linear behaviour emerges in the late stage of ripening. We attribute this breakdown to the overlap of diffuse interfaces, which reduces the liquid density and induces thermodynamic destabilisation. Finally, our simulations of three-droplet systems confirm the universality of the proposed theoretical model. These findings provide a microscopic understanding of ripening and help bridge nanoscale dynamics with macroscopic coarsening theories.