In order to realize the excellent functional antifouling coating, the anti-adhesive property is requiring for the material surface. The photocatalyst material such as TiO₂ are conventionally used for degrading pollution [1,2]. The superhydrophobic bumps such as lotus leaf structure [3,4], or super slippery surface such as Nepenthes pitcher plant are also conventionally used for self-cleaning function [5,6]. These functional surfaces, however, have been applied in limited practical use due to their difficulties in processability, wearability, permeability and resupplying the lubricant.

This paper reports the first example of super slippery surface using superhydrophilic material, which is opposite approach to generally using superhydrophobic material as mentioned above. The newly discovered superhydrophilic slippery surface has been realized by nanotextured photocatalyst with surface reforming process. Without hydrophobicity or lubricant, it has been succeeded in sliding waterdrops smaller than 5 μL with sliding angle lower than 5° (Fig. 1).

The GaN substrate has been selected as photocatalyst in this study, and 100 nm in diameter SiO₂ nanoballs have been dip coated on the substrate in the shape of single layered colloidal crystal following surface modification of the substrate. The nanotexture has been formed by dry etching process with hexagonal closed packed structure SiO₂ nanoball etching mask [7–9]. Finally, the nanotextured GaN surface has been modified again to superhydrophilic then the super slippery surface has been finished up [10]. The obtained new phenomenon, i.e. super slips on superhydrophilic surface, is supposed to be affected by the fractal curve at the 3-phase (water, GaN and air) contact line, and by the super low friction coefficient between waterdrop and nanotexture that likes commonly used Cassie-Baxter model.

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[3] T. Y. Liu & C. Kim, Science 346, 1096–1100 (2014).

[4] A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella, G. C. Rutledge, G. H. McKinley & R. E. Cohen, Science 318, 1618–1622 (2007).

[5] T. S. Wong, S. H. Kang, S. K. Tang, E. J. Smythe, B. D. Hatton, A. Grinthal & J. Aizenberg, Nature 477, 443–447 (2011).

[6] A. R. Parker, C. R. Lawrence, Nature 414, 33–34 (2001).

[7] M. Fujikane, A. Inoue & T. Yokogawa, US patent 8852965 (2014).

[8] M. Fujikane, A. Inoue & T. Yokogawa, US patent 8841220 (2014).

[9] M. Fujikane, A. Inoue & T. Yokogawa, US patent 9209254 (2015).

[10] M. Fujikane, EU patent 17167854.3 (2017).

Figure 1 | Relation between the water volume and the sliding angle for different types of surfaces. The values in parentheses at the end of graph legends indicate static contact angle with 2 μL water probe.