Inhibition of condensation-induced droplet wetting by nano-hierarchical surfaces

Superhydrophobic nanostructured surfaces can enhance water condensation efficiency by facilitating droplet departure via coalescence-induced jumping. However, condensed droplets tend to transit from a mobile jumping mode to a highly pinned state at high condensation heat flux because excessive water nucleates within the nanostructures and anchors the condensed droplets. The large pinned droplets act as a thermal barrier and insulate the cooling surface, thus severely degrading its heat transfer efficiency. This work developed a nanohierarchical structured surface by growing branched TiO2 nanorod arrays to prevent condensation-induced droplet pinning. After hydrophobization, the nano-hierarchical structure can spontaneously push the water out of nanostructures with an outward Laplace capillary pressure gradient when the droplet size is only at the nanoscale level. This effective de-wetting process maintains the high droplet mobility on the nano-hierarchical surface over a wide subcooling range, resulting in an up to similar to 90 % increase in heat transfer coefficient at a high heat flux of 132 kW center dot m(-2) compared to the single-tier nanorod surface. Our investigation of how the nanohierarchical structures fundamentally suppress the condensation-induced wetting on superhydrophobic surfaces represents a significant advance in understanding multiphase wetting phenomena and paves the way for the rational design of cooling surfaces.