Femtosecond laser patterned superhydrophobic surface with anisotropic sliding for droplet manipulation

Patterned superhydrophobic surfaces have a wide range of applications in terms of microfluidic devices. In this work, a simple and robust method is proposed to fabricate patterned superhydrophobic surfaces by two combined steps including femtosecond laser microfabrication and immersion treatment. The obtained surface exhibits contact angles of 170.0 & DEG;, 167.6 & DEG;, and 169.5 & DEG;, and sliding angles of 5.8 & DEG;, 6.0 & DEG;, and 3.9 & DEG; for 12 & mu;L, 18 & mu;L, and 30 & mu;L water droplets, respectively. The high-adhesion hydrophobic linear pattern combined with the surrounding low-adhesion superhydrophobic surface exhibits controllable wettability and pronounced liquid-sliding anisotropy. Since the contact angle and the adhesion force can be controlled by adjusting the width of the linear pattern, the droplet motion can be directionally modulated. As the line width increases, the sliding angle in parallel direction can be regulated from 18.9 & DEG; to 90 & DEG;, from 15.3 & DEG; to 47.6 & DEG;, and from 8.6 & DEG; to 25.0 & DEG; for 12 & mu;L, 18 & mu;L, and 30 & mu;L water droplets, respectively. Based on the modified Furmidge equation, the relationship between adhesion and liquid-solid interface width can be reasonably explained. Furthermore, the potential applications of the patterned surfaces for droplet transport and mixing are explored. The wettability of the superhydrophobic surfaces to liquids of different types and pH is also investigated. The work presented here has the potential to advance femtosecond laser scanning as a simple, precise, and controllable method for fabricating patterned superhydrophobic surfaces, and to accelerate their applications in parallel reactors, chemical micro-assays, drug delivery systems and so on.