Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation.

Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 μL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.