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Design of robust superhydraphobic surfaces, Dehui Wang, Zuankai Wang, Robin H.A. Ras, Xu Deng, et al., Nature, 2020

Superhydrophobic surfaces have promising applications in various medical and engineering applications; however, these surfaces, either via physical and/or chemical methods, suffer from one major flaw- extremely vulnerable to mechanical damage, so lose superhydrophobicity easily in practice.

This work presents a simple, effective and thus ingenious strategy for robust, hydrophobic surfaces. It proposes a design that integrates robustness and superhydrophobicity at micro- and nano-scales, respectively, so that the overall structure show both properties, which have been hardly achieved by previous research work.

Key of how: Microstructure: interconnected frame, prevents the nanostructure from being removed and enhances mechanical robustness. Nanostructure: within the microstructure, chemically and physically provides sustained superhydrophobicity.

Manuscript major points:

1stly, design strategy

2ndly, optimize the microstructure framework (length scale, shape, geometry) considering the mechanical robustness (interconnectness, microscale, preventing larger scale abrasion), superhydrophobicity (change of the liquid-solid contact fraction), and mechanical stability (FEA-third principle stress and microindentation, compare with pyramid, pillar microstructures). The microstructure framework of microscale inverted-pyramidal cavities were fabricated using photolithography on silicon substrates, and can be generalized to other curved substrates using roll-to-plate printing technology. I personally think that the following figure from their supplementary material provides clearer information of the structure than that in the main text.

3rdly, integrate nanostructures (fluorinated fractal nanoclusters of silica) into the framework, then obtain sustained superhydrophobicity after scrapping and abrasion (through systematic, quantitative evaluations of the contact angle, liquid-solid contact fractions/ pull-off forces w.r.t. surfaces before and after abrasion).

4thly, demonstrate the generality, by fabricating inverted-triangular-pyramidal (tripyramidal) and inverted-hexagonal-pyramidal (hexpyramidal) structures, and obtained similar properties.

 

5thly, demonstrate the long-term mechanical durability of the superhydrophobic surfaces (contact angles, critical fracture force to destroy superhydrophobicity, maximum number of abrasion cycles, tape-peeling tests, thermal stability in 100oC, chemical corrosion, impact of water jet).

 

This concept of ‘an armored structure’ secures mechanical robustness and durability through microscale framework and superhydrophobicity via nanoscale structures. It considers and fabricates novel surfaces that simultaneously possess long-term mechanical robustness, non-wettability and optical transparency. The effectiveness, generality, and scaling-up potential of the design and fabrication shows a great potential in moving superhydrophobic surfaces to practical applications.

Here is the link of the full text: https://www.nature.com/articles/s41586-020-2331-8

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