Enhanced passive anti-frosting on superhydrophobic Millimeter-scale structured surfaces through coupled vapor-transport and droplet-evolution regulation

Frost formation on cold surfaces degrades heat-transfer performance and increases energy consumption in thermal systems operating under humid conditions. In this work, a passive anti-frosting strategy based on superhydrophobic copper surfaces with millimeter-scale protrusions is investigated through frosting experiments and vapor-diffusion simulations. The coupled effects of surface wettability and protrusion geometry on condensation, freezing, ice-bridge propagation, and frost-free-zone formation are systematically examined. The experimental results show that the superhydrophobic millimeter-scale surface delayed the onset of freezing by a factor of 1.9 and increased the average frost-free-zone width by 34% compared with the bare copper surface. For multi-protrusion surfaces, optimizing the protrusion height, spacing, and width further increased the frost-free-zone coverage to approximately 93.98%. The simulations reveal that millimeter-scale protrusions redistribute the local vapor flux, promoting preferential condensation and frosting on the protrusion tops while reducing vapor supply in the inter-protrusion regions. Meanwhile, superhydrophobicity decreases droplet coverage and enhances droplet discreteness, thereby weakening inter-droplet ice bridging and retarding frost propagation. In addition to frost-free-zone coverage, frost thickness, frost mass, and frost density are evaluated to provide a more comprehensive assessment of anti-frosting performance. These results clarify the coupled role of wettability-mediated droplet evolution and geometry-induced vapor transport in frost suppression, providing design guidance for passive anti-frosting surfaces.