Designable and unidirectional motion of Leidenfrost droplets on heated asymmetric microgrooves written by femtosecond laser

Abstract

Unidirectional droplet motion is realized on heated asymmetric microgroove arrays prepared by femtosecond laser direct writing. The plasma expansion under laser ablation compresses the two sides of the induced microgroove differently, resulting in the formation of asymmetrical microgrooves. The asymmetry of the microgrooves can rectify the water vapor that ejects from the Leidenfrost droplet and generate a viscous shear force at the bottom of the droplet, causing the droplet to move in a certain direction (where the laser scanning line is added) when the substrate temperature is higher than a certain critical value (the transition temperature of disordered motion and unidirectional motion). The velocity of droplets can exceed 318 mm/s, and the droplets can even climb surfaces that are tilted 14°. With the advantages of femtosecond lasers in the flexible design of surface microstructures and patterns, this unidirectional droplet motion can support a variety of complex droplet-manipulation applications, such as droplet movement along designed trajectories, droplet accelerator devices, fixed-point capture of droplets, and fixed-point cooling of hot solid surfaces. Compared with traditional macroscopic ratchets, laser-written asymmetrical microgrooves make the Leidenfrost droplet motion more designable and controllable.