Preparation of Polymer Nanopillar Arrays with Wettability Gradients by Nanoimprinting Using Anodic Porous Alumina Molds

Polymer nanopillar arrays with continuously varying diameters were fabricated by nanoimprinting using anodic porous alumina molds. The obtained polymer nanopillar array exhibited wettability gradient properties, and when a water droplet formed on its surface, it moved spontaneously. A wettability gradient surface was fabricated using a nanopillar array structure, and its inverted structure, nanohole array, and the spontaneous transport distance of water droplets were compared. The spontaneous transport distance of water droplets on the nanopillar array surface was longer than that on the nanohole array surface. To confirm this, the relationships between surface porosity and contact angle were determined in both structures. Although no significant difference in statistical contact angle was found between the two structures, a significant difference was observed in their dynamic contact angles. The contact angle hysteresis of the nanopillar array structure was smaller than that of the nanohole array, and the adhesive force of the water droplets was smaller. This indicates that surfaces with the small contact angle hysteresis and low surface adhesion of water droplets effectively increase the distance over which water droplets move spontaneously, owing to the wettability gradient structure. In addition, a polymer nanopillar array with a partially gradient structure could be formed by nanoimprinting using anodic porous alumina molds. On surfaces with a patterned wettability gradient structure, moving water droplets in any direction is possible. The polymer nanopillar arrays obtained by this process can be used in various applications such as microfluidic devices, water harvesting, and heat transfer enhancement.