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

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

30 January 2024 | Bohong Li, Lan Jiang, Xiaowei Li, Zhipeng Wang and Peng Yi
This study presents a method for achieving directional self-propulsion of Leidenfrost droplets using a surface with a periodic hydrophobicity gradient (SPHG) fabricated by femtosecond laser. The SPHG surface is created by shaping femtosecond laser pulses and subsequent chemical treatment, resulting in a surface with varying hydrophobicity. The SPHG enables directional propulsion of Leidenfrost droplets by modulating the vapor layer between the droplets and the hot surface, which in turn drives the droplet movement through viscous forces between the gas and liquid. The SPHG surface was fabricated on a silicon wafer using femtosecond laser processing, which allows for precise control of surface topography and hydrophobicity. The surface was divided into four regions with distinct micro/nanostructures, each with different hydrophobicity levels. The contact angles of these regions were measured, showing that increasing laser power leads to higher contact angles, indicating increased hydrophobicity. The study also investigated the effect of temperature on the motion of Leidenfrost droplets on the SPHG surface. At temperatures below the Leidenfrost point, droplets remain fixed on the surface. As the temperature increases, the droplets bounce and eventually transition into continuous bouncing. The critical temperature for continuous bouncing varies depending on the hydrophobicity of the surface, with lower hydrophobicity surfaces having higher critical temperatures. The SPHG surface was also shown to enable self-propelled movement of Leidenfrost droplets. When the sample was tilted, the droplets moved in the direction of the hydrophobicity gradient. High-speed imaging confirmed that the droplets accelerated and reached a steady velocity on the SPHG surface. The velocity of the droplets was found to increase with temperature, as the vapor layer between the droplet and the surface becomes thinner, reducing viscous forces. The study also examined the effect of droplet volume and drop height on self-propulsion. The maximum velocity of the droplets was found to be nearly independent of volume, but decreased with increasing drop height. The trajectory of the droplets was found to be positively correlated with drop height, indicating that longer contact times with the surface result in higher maximum velocities. The results of this study demonstrate that femtosecond laser fabrication of SPHG surfaces can be used to control the movement of Leidenfrost droplets, offering a new method for droplet manipulation and expanding the scope of applications in microfluidics and other fields.This study presents a method for achieving directional self-propulsion of Leidenfrost droplets using a surface with a periodic hydrophobicity gradient (SPHG) fabricated by femtosecond laser. The SPHG surface is created by shaping femtosecond laser pulses and subsequent chemical treatment, resulting in a surface with varying hydrophobicity. The SPHG enables directional propulsion of Leidenfrost droplets by modulating the vapor layer between the droplets and the hot surface, which in turn drives the droplet movement through viscous forces between the gas and liquid. The SPHG surface was fabricated on a silicon wafer using femtosecond laser processing, which allows for precise control of surface topography and hydrophobicity. The surface was divided into four regions with distinct micro/nanostructures, each with different hydrophobicity levels. The contact angles of these regions were measured, showing that increasing laser power leads to higher contact angles, indicating increased hydrophobicity. The study also investigated the effect of temperature on the motion of Leidenfrost droplets on the SPHG surface. At temperatures below the Leidenfrost point, droplets remain fixed on the surface. As the temperature increases, the droplets bounce and eventually transition into continuous bouncing. The critical temperature for continuous bouncing varies depending on the hydrophobicity of the surface, with lower hydrophobicity surfaces having higher critical temperatures. The SPHG surface was also shown to enable self-propelled movement of Leidenfrost droplets. When the sample was tilted, the droplets moved in the direction of the hydrophobicity gradient. High-speed imaging confirmed that the droplets accelerated and reached a steady velocity on the SPHG surface. The velocity of the droplets was found to increase with temperature, as the vapor layer between the droplet and the surface becomes thinner, reducing viscous forces. The study also examined the effect of droplet volume and drop height on self-propulsion. The maximum velocity of the droplets was found to be nearly independent of volume, but decreased with increasing drop height. The trajectory of the droplets was found to be positively correlated with drop height, indicating that longer contact times with the surface result in higher maximum velocities. The results of this study demonstrate that femtosecond laser fabrication of SPHG surfaces can be used to control the movement of Leidenfrost droplets, offering a new method for droplet manipulation and expanding the scope of applications in microfluidics and other fields.
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[slides and audio] Self-propelled Leidenfrost droplets on femtosecond-laser-induced surface with periodic hydrophobicity gradient