Underwater Anisotropic 3D Superoleophobic Tracks Applied for the Directional Movement of Oil Droplets and the Microdroplets Reaction

Abstract

DOI: 10.1002/admi.201900067 microfluidics.[15–18] In nature, rice leaf has three-level geometrical structures with anisotropic superhydrophobicity.[19,20] On the rice leaf surface, microscale papillae and nanostructures directionally arrange on the surface of the sub-millimeterscale grooves.[21] The superhydrophobicity is attributed to the micro/nanoscale hierarchical rough structures and low surface energy. The macrogrooves endow the rice leaf with the anisotropic sliding property for water droplets, so the dewdrops more easily roll to the root of rice leaf and help rice to well survive in an arid environment. The ridging nanostripes and flexible nanotips directionally cover on the butterfly wing, which exhibits superhydrophobicity and unidirectional water adhesion.[14] This unique structure allows rain droplets to easily roll along the radial-outward direction on the butterfly wing, reducing the flight resistance in a rainy day.[22] The filefish skin shows superoleophobicity and unidirectional oil adhesion in water. Many hook-like spines distribute on the filefish skin. Due to the directional arrangement of the spines, underwater oil droplets can easily roll off the filefish skin along the head-to-tail direction but tend to be pinned in the opposite direction, enabling the fish to swim in the oil-polluted water.[23,24] Inspired by natural surfaces, Yong et al. constructed directional patterns on poly(dimethylsiloxane) (PDMS) surface by femtosecond laser ablation. The prepared surface showed anisotropic superhydrophobicity.[25] Wu et al. prepared the three-level (macro/micro/nanoscale) anisotropic microstructures by the combination of photolithography and imprint lithography.[20] Water droplets on the sample surface were more inclined to roll along the macroscale grooves. Brueck and co-worker used multibeam-laser interference lithography to fabricate a 1D nano-patterned surface and observed the anisotropic wetting behavior on such a surface.[26] Until now, a large number of anisotropic superhydrophobic surfaces have been fabricated, and these surfaces are successfully applied to manipulate water droplets, e.g., the directional movement of water droplets.[27–31] However, the directional and no-loss movement of oil droplets in a water medium has still rarely been reported. The lossless manipulation of oil droplets is of great importance in various miniature systems for both chemical and biological applications, such as high-throughput In this paper, a simple way to fabricate underwater anisotropic superoleophobic tracks is reported for manipulating underwater oil droplets by the femtosecond laser etching and the oxygen plasma treatment. Laser ablation is able to generate micro/nanoscale hierarchical structures on the poly (dimethylsiloxane) (PDMS) surface. The textured PDMS surface is further turned from superhydrophobicity to hydrophilicity and underwater superoleophobicity by subsequent oxygen plasma irradiation. Underwater anisotropic 3D superoleophobic tracks can also be fabricated on the PDMS surface by the laser etching method. The width of the tracks depends on the laser-treated area, while the depth of the tracks increases with increasing the laser power and the scanning number and with decreasing the laser scanning speed/space. The underwater superoleophobic 3D tracks show ultralow adhesion and anisotropic sliding property to oil droplets, thereby allow the underwater oil droplets to move just along the track. A microdroplets reaction is proposed based on the underwater superoleophobic tracks. There is no loss of the reactants during the whole reaction process. The anisotropic sliding property of underwater organic droplets will potentially have enormous applications in droplet manipulation, microfluidics system, surface lab-chip devices, and chemical engineering.