Self-propelled Leidenfrost droplets on femtosecond-laser-induced surface with periodic hydrophobicity gradient

A surface with periodic hydrophobicity gradient (SPHG) is fabricated by shaped femtosecond laser.The directional self-propulsion of the Leidenfrost droplets is realized.The viscous gradient force between gas and liquid is used to drive the droplet to move.A brand-new method for controlling the movement of droplets is provided. The controllable transfer of droplets on the surface of objects has a wide application prospect in the fields of microfluidic devices, fog collection and so on. The Leidenfrost effect can be utilized to significantly reduce motion resistance. However, the use of 3D structures limits the widespread application of self-propulsion based on Leidenfrost droplets in microelectromechanical system. To manipulate Leidenfrost droplets, it is necessary to create 2D or quasi-2D geometries. In this study, femtosecond laser is applied to fabricate a surface with periodic hydrophobicity gradient (SPHG), enabling directional self-propulsion of Leidenfrost droplets. Flow field analysis within the Leidenfrost droplets reveals that the vapor layer between the droplets and the hot surface can be modulated by the SPHG, resulting in directional propulsion of the inner gas. The viscous force between the gas and liquid then drives the droplet to move.