Control of Water Droplet Transport Using Anodic Porous Alumina with a Wettability Gradient.

Water droplets can spontaneously move on surfaces with a wettability gradient, and such surfaces can be used in various functional devices. In this study, surfaces with a wettability gradient were prepared from anodic porous alumina with different pore arrangement regularities. The evaluation of the water droplet transport distance on the surfaces of the prepared samples showed that the pore arrangement regularity of the anodic porous alumina had no effect on the droplet transport distance. It was also shown that the droplet transport distance was affected by the droplet size and the width of the wettability gradient structure. When two water droplets were formed at both ends of the anodic porous alumina with an increasing gradient of pore diameter from the center to both ends, they moved from the ends toward the center and merged. The method reported in this paper shows that by controlling the pore size of anodic porous alumina, one can fabricate a substrate on which droplets can move in any direction. The anodic porous alumina with a wettability gradient as obtained in this study has potential use for various applications such as microfluidic devices and analytical chips.