Directional Leidenfrost droplet propulsion on femtosecond-laser-engineered dual heterogeneous surfaces with hybrid boiling states

Liquid propulsion on overheated surfaces plays an important role in numerous engineering applications.However, most reported methods are limited to homogeneous asymmetric structures such as ratchet-shaped substrates.In this study, a dual heterogeneous structure is designed on an aluminium surface via femtosecond laser direct writing.The substrate surface is alternately covered with non-ablated smooth strips and laser-structured regions composed of periodic ripple microstructures.The droplets on the heated sheet exhibit a hybrid boiling state: film boiling in the smooth regions and intermittent transition boiling in the ripple microstructures.This hybrid state enables the droplets to remain in the Leidenfrost regime and combines the advantages of both the film-and transition-boiling states.That is, droplets on hot surfaces combine the long lifetime characteristic of the film-boiling state with the efficient substrate-liquid heat exchange of the contact-boiling state, achieving a balance between extended lifespan and effective heat transfer.The film-boiling regions facilitate the formation of a stable vapour cushion beneath the droplet, whereas the intermittent asymmetric contact boiling on the ripple microstructures produces a directional driving force and unidirectionally propels the Leidenfrost droplets.In particular, the droplets consistently move along the laser-scanning lines and opposite the laser processing direction.By ingeniously designing laser processing paths, diverse functions and applications of Leidenfrost droplet propulsion, including curved-path droplet transport, droplet expulsion, droplet trapping, targeted cooling, and droplet rotors, can be realised.