Dropwise condensation on superhydrophobic nanostructure surface, Part I: Long-term operation and nanostructure failure

•Condensation experiment on superhydrophobic nanograsses surface (NGS) is performed.•Fresh NGS behaves smaller drop but deteriorates heat transfer compared with polymer surface.•Collapse and breakage of nanograsses worsen heat transfer for long-term operation.•Three mechanisms to induce nanostructure failure are proposed.•There is a limit of decayed heat transfer after nanostructure failure. Dropwise condensation heat transfer (DWC) on superhydrophobic nanograsses surface (NGS) was investigated for long-term operation. For DWC of pure water-vapor on fresh NGS, two heat transfer regimes are identified: higher heat transfer coefficients with droplet jumping, and constant heat transfer coefficients with droplet rolling. The one-week operation not only deteriorates heat transfer performance, but also changes jumping or rolling mode to sliding mode. The condensation heat transfer coefficients are apparently decreased from first to third day, but they approach a limit value since the third day. In order to identify if the single-molecule-layer of polymer (SML) modified on nanograsses was destroyed, DWC on a smooth single-molecule-layer of polymer surface (SSML) was tested to display stable heat transfer with drop sliding for one-week operation, concluding no failure of the polymer layer. The collapse and breakage of nanograsses were observed to explain the decayed heat transfer versus time on NGS. Compared with SSML, the NGS has smaller droplet departure size but lower heat transfer coefficients, indicating positive and negative effects after introducing nanostructures. Three nanostructure failure mechanisms are proposed. This work suggests a new research field of the nanoscale fluid-wall interaction.