Self-shedding and sweeping of condensate on composite nano-surface under external force field: enhancement mechanism for dropwise and filmwise condensation modes

In this work, we propose the concept to use the hydrophilic or neutral surface for condensation heat transfer and to use the superhydrophobic surface for enhancement by self-shedding and sweeping of condensate. Molecular dynamics simulation results show that no matter the vapor condenses on the solid surface in dropwise or filmwise mode, the grown-up condensate self-sheds and falls off the superhydrophobic surface, sweeping the growing condensate on the condensing surface downstream. We characterize the dynamics of condensate that the continuous self-shedding and sweeping effectively remove the droplets from the solid surface in dropwise mode or thin the condensate film on the solid surface in filmwise mode, which significantly enhances the condensation heat transfer. We reveal that the mechanism for self-shedding is two-fold: (1) that the external force on condensate bulk defeats the adhesive force between the condensate and the solid surface triggers the self-shedding; (2) the release of the surface free energy of condensate promotes the self-shedding. We also reveal that the mechanism of heat transfer enhancement is essentially due to the timely suppression over the growing condensate bulk on the condensing surface through the self-shedding and sweeping. Finally, we discuss the possible applications.