Directional shedding-off of water on natural/bio-mimetic taper-ratchet array surfaces

Surfaces that control fluids are important in self-cleaning, liquid-transport and cell-directing. They are significantly observed on biological surfaces that control wettability and adhesion by means of micro-/nanostructures, and have aroused interest in foundational and biomimetic research. Here, we report a novel taper-ratchet array on ryegrass leaf. It integrates a gradient of retention at solid-liquid interfaces in contrasting directions to reversibly generate the release or the pinning of solid-liquid contact lines, and accordingly, achieves effective directional water shedding-off properties. By mimicking taper-ratchets from ryegrass leaf, the polymer surfaces are fabricated successfully. They display a robust property of directional water shedding-off. When external vibrations are executed on polymer surfaces, the drops achieve a unidirectional self-shedding along the oriented direction of tips of taper-ratchets, because asymmetric retention forces are formed in the contrasting oriented directions. This investigation will be helpful to design a novel fluid-controlling surface that can be extended to applications such as self-cleaning, liquid-transport and cell-directed projects.