Femtosecond laser-based phase-change heat transfer surfaces

Condensation heat transfer is often limited by condensate flooding and the accumulation of large, pinned droplets on textured surfaces. Here, we demonstrate a substrate-versatile femtosecond laser texturing strategy that directly creates hierarchical metallic condenser surfaces capable of sustaining enhanced dropwise condensation under elevated vapor pressure. By systematically varying laser power and scanning speed, we identify a transition from shallow LIPSS-like nanostructures to three-dimensional micro/nano-hierarchical textures composed of tapered microgrooves and pinecone-like protrusions decorated with nanoscale roughness. After low-surface-energy modification, the hierarchical surfaces exhibit high receding contact angles and reduced contact-angle hysteresis, enabling frequent coalescence-induced droplet removal and delaying flooding. The optimized texture, fabricated at 7 W and 5 mm s-1 was transferred from planar copper samples to cylindrical condenser tubes using a custom rotational laser-processing platform. These tubes maintained condensation heat transfer coefficients of ~70 kW m-2 K-1 at high vapor pressure, outperforming an HTMS-coated reference tube even after ideal jumping condensation diminished. Mechanistically, the pinecone-like regions suppress the growth and interconnection of flooded cavities, while the tapered microgrooves generate a Laplace-pressure imbalance that assists condensate depinning and removal. Similar hierarchical morphologies and enhanced dropwise behavior were also obtained on aluminum and SUS304, demonstrating the broader applicability of the approach to practical metallic heat exchangers.