Self-propelled water droplet movement on a laser-etched radial gradient copper surface

Abstract

This paper reports the behavior of water droplets on a topography-based, radial gradient on a metallic surface. The radial gradient was designed to concentrate water in the center region through the spontaneous motion and coalescence of individual droplets on the gradient pattern. In this work, parallel microchannels having a fixed width (~25 μm) and depth (100 μm) but variable spacing were used to impart a net surface tension force to the droplet for actuation. The diameter of the radial gradient (i.e. circular test region) was approximately 18.9 mm (or, ~0.75 in.). The surfaces were fabricated using a 355 nm YVO4 laser system and then characterized using droplet injection (5–38 μL) and spray testing techniques. Injected droplets were observed to move spontaneously away from the hydrophobic outer ring region of the circle towards the hydrophilic center as designed by the gradient. Droplet travel distances as large as 4 mm were measured depending on the droplet volume and injection site location. In application, such a design could be useful for promoting droplet coalescence on a surface and subsequently removal by gravity through the formation of sufficiently large droplets from two or more otherwise small droplets.