Anisotropic Wetting Behavior on Gradient Surface Structures Fabricated by Direct Laser Interference Patterning on Stainless Steel

In this work, anisotropic line-like textures with gradually increasing spatial periods from 2.0 to 4.8 µm are fabricated on stainless steel by two-beam Direct Laser Interference Patterning using a ps-laser source. Three manufacturing strategies are developed to fabricate these so-called gradient structures achieving different surface roughness Rz and overall texture uniformity. The influence of unidirectional texture gradients on anisotropic wetting properties is studied by depositing 1 µL water droplets. The results show that the optimum-designed gradient patterns induce in all cases the spontaneous motion of water droplets in the direction of increasing periodicity, or equivalently, increasing hydrophilicity. It is also observed a direct correlation between the gradient ratio and the traveling distance, which reaches a maximum of 0.99 mm for a period gradient of 2.24 µm mm-1. Furthermore, multidirectional gradient textures are produced by combining gradient areas with opposing gradient orientations. The anisotropic wetting characteristics on such complex textures are measured, showing a strong local dependency of the water contact angle and droplet displacement. The findings from this work provide new design rules for customizing gradient surfaces able to realize predictable, position-dependent wetting properties and directional movement of microliter water droplets.