Tunable anisotropic wettability of rice leaf-like wavy surfaces

Rice leaves can directionally shed water droplets along the longitudinal direction of the leaf. Inspired by the hierarchical structures of rice leaf surfaces, synthetic rice leaf-like wavy surfaces are fabricated that display a tunable anisotropic wettability by using electrostatic layer-by-layer assembly on anisotropic microwrinkled substrates. The nanoscale roughness of the rice leaf-like surfaces is controlled to yield tunable anisotropic wettability and hydrophobic properties that transitioned between the anisotropic/pinned, anisotropic/rollable, and isotropic/rollable water droplet behavior states. These remarkable changes result from discontinuities in the three-phase (solid-liquid-gas) contact line due to the presence of air trapped beneath the liquid, which is controlled by the surface roughness of the hierarchical nanostructures. The mechanism underlying the directional water-rolling properties of the rice leaf-like surfaces provides insight into the development of a range of innovative applications that require control over directional flow. Rice leaf-like wavy surfaces with tunable anisotropic wettability are fabricated through the combination of electrostatic layer-by-layer assembly and surface wrinkling. The hierarchically structured surfaces, with a surface roughness that can be tuned by changing the number of polyelectrolyte and nanoparticle deposition cycles, yields controlled anisotropic/pinned, anisotropic/rollable, or isotropic/rollable dynamic water droplet behavior states.