Femtosecond laser asymmetric microgrooving for the ultrastrong directional propulsion of Leidenfrost droplets

In femtosecond laser precision manufacturing, unfavorable factors such as nonperpendicular laser incidence, noncircular focused laser spots, and asymmetric energy distribution in the focal spot severely affect the accuracy and morphology of laser-induced microstructures. Typically, significant efforts are made to avoid these adverse effects. However, by leveraging these unfavorable factors, asymmetric microgrooves can be easily fabricated on solid surfaces through femtosecond laser direct writing. On the overheated grooved surface, Leidenfrost droplets can be unidirectionally propelled due to the rectification effect of the vapor flow beneath the Leidenfrost droplets by the laser-induced asymmetric microgrooves. The maximum driving velocity of the droplets can reach ~400 mm·s⁻¹, and the droplets can even climb an 18° inclined slope against gravity. This Leidenfrost droplet propulsion method is effective for a wide range of volatile liquids (e.g., water, ethanediol, ethanol, cyclohexane, n-hexane, and acetone) and is applicable to various substrate materials (e.g., aluminum, stainless steel, Ti alloy, Mg alloy, silicon, and ceramics). In particular, Leidenfrost droplets moving along curved or arced trajectories have been realized on laser-designed double-layered crisscrossing microgroove arrays. This achievement provides a new approach for designing and flexibly controlling the motion paths of Leidenfrost droplets, whereas previously reported strategies are typically limited to driving droplets in straight lines. The current simple and effective approach for Leidenfrost droplet transport has significant application potential in fundamental physics, energy conversion, mechanical engineering, and chemical engineering.