Picosecond Laser-Induced Ratchet-Valley Composite Microstructures for Directional Transport of Nucleate Boiling Droplets

Directional transport of droplets on high-temperature surfaces offers distinct advantages, including operation without external driving forces and precise controllability, with potential applications in self-cleaning, spray cooling, and microfluidic self-propulsion. Although most existing studies focus on Leidenfrost droplet transport, systematic investigations of directional transport and dynamic manipulation under the nucleate boiling regime remain limited, restricting practical engineering implementation. In this work, a facile ratchet-valley composite microstructure (R-VCM) is fabricated via picosecond laser ablation to enable directional droplet transport. The effects of R-VCM spacing, valley depth, surface temperature, Weber number (We), and droplet volume on the directional transport of nucleate boiling droplets are systematically investigated. The results demonstrate that increasing valley depth significantly lowers the threshold temperature for directional transport, reducing R-VCM spacing enhances both transport distance and velocity, and We has a negligible influence on directional transport behavior. Finally, the R-VCM demonstrates practical potential for high-temperature surface self-cleaning and antigravity droplet transport. These findings clarify how key parameters control the directional transport of nucleate boiling droplets and provide theoretical insights and design guidelines for thermal management and directional microfluidics.