Uni-directional liquid spreading control on a bio-inspired surface from the peristome of Nepenthes alata

Uni-directional liquid spreading without energy input has gained much attention due to its potential application in various areas such as microfluidic devices and energy fields. Recently, continuous uni-directional liquid spreading with fast speed was discovered on the peristome of Nepenthes alata, which possesses superhydrophilic hierarchical microgrooves and duck-billed microcavities with arc-shaped edges and gradient wedge corners. Inspired by the surface structure of the peristome, a novel bio-inspired uni-directional liquid spreading surface with various arc curvatures and wedge angles was built via two-step inclined UV exposure photolithography. The effects of the surface wettability and structural features, i.e. the arc-shaped outlines and wedge corners of microcavities, on the anisotropy of liquid spreading were investigated. The underlying mechanisms were made clear by comparing the effects of surface wettability and structural features of microcavities on both liquid spreading ability and liquid pinning ability. Finally, the controlling of anisotropic liquid spreading and thorough uni-directional liquid spreading were realized. This study provides inspiration to design novel uni-directional liquid spreading surfaces without energy input, and can further expand their application in areas such as non-powered delivery systems, microfluidic devices and self-lubrication in mechanical engineering.