Superhydrophobicity is a remarkable evolutionary adaption manifested by several natural surfaces. Artificial superhydrophobic coatings with good mechanical robustness, substrate adhesion and chemical robustness have been achieved separately. However, a simultaneous demonstration of these features along with resistance to liquid impalement via high-speed drop/jet impact is challenging. Here, we describe all-organic, flexible superhydrophobic nanocomposite coatings that demonstrate strong mechanical robustness under cyclic tape peels and Taber abrasion, sustain exposure to highly corrosive media, namely aqua regia and sodium hydroxide solutions, and can be applied to surfaces through scalable techniques such as spraying and brushing. In addition, the mechanical flexibility of our coatings enables impalement resistance to high-speed drops and turbulent jets at least up to ~35 m s−1 and a Weber number of ~43,000. With multifaceted robustness and scalability, these coatings should find potential usage in harsh chemical engineering as well as infrastructure, transport vehicles and communication equipment.
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The work was partially supported by M.K.T.’s EPSRC First Grant (EP/N006577/1) and from the European Research Council (ERC) under the European Uninon's Horizon 2020 research and innovation programme under grant agreement no. 714712. The authors also thank D. Cripps from Blade Dynamic Company (UK) for supplying the carbon fibre fabrics and epoxy resin. We also acknowledge helpful discussions with F. Fang, S. Zhang and P. Kelly in setting up the wettability experiments; J. Davy for scanning electron microscopy and P. Hayes for Fourier transform infrared spectroscopy measurements.
Supplementary Movies 1–8 legends, Supplementary Methods, Supplementary Figures 1–14, Supplementary Notes 1–8, Supplementary References.
Water droplets bouncing off the PKFE coating at different velocities. At higher speed, the water droplets atomize upon impact and spend much less time on the substrate compared to the drops impacting at lower speed
Fine water jets (diameter ~0.25 mm) impacting on the PKFE coating vertically with different speeds. The videos show the corresponding jet velocities, and the Weber numbers for liquid (Wel=ρlV2d/γLG) and gas (Weg=ρgV2d/γLG). The jets are indicated as laminar, transitional and turbulent jets based on standard jet atomization thresholds1. At low speed we observe a liquid accumulation at the point of impact, without any impalement. At high speeds (> 10 m s–1) the jets atomize upon impacting the substrate
Thick water jets (diameter ~2.5 mm) impacting on the PKFE coating vertically with different speeds. The videos show the corresponding jet velocities, and the Weber numbers for liquid (Wel=ρlV2d/γLG) and gas (Weg=ρgV2d/γLG). The jets are indicated as laminar, transitional and turbulent jets based on standard jet atomization thresholds1. These thick jets do not show atomization at the point of substrate impact, rather a stagnation point flow characterised by axisymmetric bending of incoming jet is observed. However, the liquid did not impale into the coating texture (tested by drop contact and sliding angle measurements at the point of impact) right after jet impact tests
Fine water jets (diameter ~0.25 mm) impacting at different speeds on the PKFE coating inclined at 45°
Thick water jets (diameter ~2.5 mm) impacting at different speeds on the PKFE coating inclined at 45°
A turbulent water jet impacting on the PKFE coating with ~35 m s–1, corresponding to a Wel ~43,000. The video demonstrates the excellent impalement resistance of the nanocomposite coating and its ability to sustain high speed liquid impact. After jet impact test, the left over water droplets from the nozzle bounced or rolled right off from the impact spot. This substantiates the fact that the PKFE coating retains superhydrophobicity after high speed jet impact
Demonstration of good adhesion and mechanical flexibility of the PKFE coatings. PKFE coating on A4 paper maintains superhydrophobicity after rolling, folding and crumpling randomly. Minimum bending radius was less than 2 mm
Water droplets roll-off much faster on the PKFE coating than on the Krytox oil infused PKFE nanocomposite. The mechanical flexibility and low water adhesion are key novel features of our PKFE coatings underpinning their excellent water impalement resistance during high speed impacts