Filefish-inspired surface design for anisotropic underwater oleophobicity

Surfaces with anisotropic wettability, widely found in nature, have inspired the development of one-dimensional water control on surfaces relying on the well-arranged surface features. Controlling the wetting behavior of organic liquids, especially the motion of oil fluid on surfaces, is of great importance for a broad range of applications including oil transportation, oil-repellent coatings, and water/oil separation. However, anisotropic oil-wetting surfaces remain unexplored. Here, the unique skin of a filefish Navodon septentrionalis shows anisotropic oleophobicity under water. On the rough skin of N. septentrionalis, oil droplets tend to roll off in a head-to-tail direction, but pin in the opposite direction. This pronounced wetting anisotropy results from the oriented hook-like spines arrayed on the fish skin. It inspires further exploration of the artificial anisotropic underwater oleophobic surfaces: By mimicking the oriented hook-like microstructure on a polydimethylsiloxane layer via soft lithography and subsequent oxygen-plasma treatment to make the PDMS hydrophilic, artificial fish skin is fabricated which has similar anisotropic underwater oleophobicity. Drawn from the processing of artificial fish skin, a simple principle is proposed to achieve anisotropic underwater oleophobicity by adjusting the hydrophilicity of surface composition and the anisotropic microtextures. This principle can guide the simple mass manufacturing of various inexpensive high surface-energy materials, and the principle is demonstrated on commercial cloth corduroy. This study will profit broad applications involving low-energy, low-expense oil transportation, underwater oil collection, and oil-repellant coatings on ship hulls and oil pipelines. Unique anisotropic underwater oleophobicity of the skin of filefish Navodon septentrionalis is revealed, which results from its oriented hook-like spines. Inspired by this, an anisotropic underwater oleophobic surface is fabricated on an oxygen plasma-treated PDMS layer. Anisotropic microfeatures on the surfaces and hydrophilicity are proposed as two main factors in achieving anisotropic underwater oleophobicity.