Unidirectional liquid transportation and selective permeation for oil/water separation on a gradient nanowire structured surface

Liquid transport and permeation on solid surfaces with gradient wettability had attracted tremendous research attention to solve some important problems of interface science and technology. On account of the existing problem of film pore blocking during the oily water separation process, however, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Here we demonstrate a strategy to achieve unidirectional motion and selective permeation of underwater oil droplet and water on the Cu(OH)₂ nanowires structured copper foil/mesh surface with a density and length gradient. The gradient Cu(OH)₂ nanowires fabricated by anodic oxidation method with anodization time gradient, shows wettability gradient from hydrophilic/oleophobic to superhydrophilic/superoleophobic and adhesion force gradient to underwater oil from high to low. As a result, the underwater oil droplet can only move unidirectionally from the superhydrophilic/superoleophobic scope to the hydrophilic/oleophobic area on the nanowires structured copper foil/mesh surface, meanwhile, water can only permeate on the superhydrophilic area, but not permeate on the hydrophobic area of the nanowires structured copper mesh. Therefore, a new way was presented to provide more contact area of oil/water mixture, owing to the unidirectional motion of underwater oil droplets and selective permeation of water, for rapid, continuous, high flux oil/water mixture separation with different interfacial tension in engineering field, which would also be promising to develop smart interface materials for microfluidic devices.