GoKawiilNatural and commercial materials
Tradescantia spathacea leaves were obtained from potted plants in our office. PVC foam boards (PVC-Schaumplatte Fixmaß, Bauhaus AG, 250 × 500 × 3 mm3) and transparent PS glass (Hobbyglas Owocor, Bauhaus AG, 250 × 500 × 2 mm3) were purchased from a local hardware store. Fluorinated ethylene propylene (FEP) film (Nenull, 140 × 200 × 0.15 mm3), copper foil tape (3M 1181 Kupferband leitend, 0.04 mm, 50 mm × 16.5 m), super-hydrophobic spray (Glaco Mirror Coat Zero, 40 ml) and PS sheets (Evergreen Scale Models, 130 μm) were purchased from an online shop.
Sample preparation
Sixteen sample types were prepared:
(1) PFOTS layers on quartz. Quartz plates (75 × 25 × 1 mm3, proQuarz GmbH) were treated with O 2 plasma at 300 W for 10 min (Femto low-pressure plasma system, Diener electronic). Using CVD, 1H,1H,2H,2H-PFOTS (97%; Sigma-Aldrich) was then coated on the surface of the quartz plate. The cleaned quartz plates were subsequently placed in a vacuum glass container together with a vial containing 0.5 ml PFOTS. Then we evacuated the container to a pressure below 100 mbar, closed the pump and let it react for about 30 min. Finally, the quartz plates were rinsed with ethanol to remove any unbound silane molecules. (2) PFOTS layers on ITO glass. ITO glass (75 × 25 × 1.1 mm3, surface resistivity 30–60 Ω sq−1, Sigma-Aldrich) were treated with O 2 plasma at 300 W for 10 min (Femto low-pressure plasma system, Diener electronic). Using CVD, 1H,1H,2H,2H-PFOTS (97%; Sigma-Aldrich) was then coated on the surface of the ITO glass. The cleaned ITO glass was subsequently placed in a vacuum glass container together with a vial containing 0.5 ml PFOTS. Then we evacuated the container to a pressure below 100 mbar, closed the pump and let it react for about 30 min. Finally, the ITO glass was rinsed with ethanol to remove any unbound silane molecules. (3) 60-nm Teflon-coated copper. To obtain flat copper substrates, a 35-nm-thick copper layer was sputtered onto a quartz plate. A 60-nm-thick Teflon film was then coated onto the copper by dip-coating with a pulling speed of 10 mm min−1 from 1 wt% Teflon AF 1600 (ε r = 1.9, Sigma-Aldrich) in FC-75 (97%, Fisher Scientific). Finally, the Teflon-coated copper samples were heated in an oven at 160 °C under vacuum for 24 h. The film thickness was measured by a profiler (P-7 stylus profiler, KLA-Tencor). (4) A 60-nm-thick PS film was coated onto the copper by dip-coating with a pulling speed of 60 mm min−1 from 2 wt% PS (molecular weight 192 kg mol−1, ε r = 2.6; Sigma-Aldrich) in toluene (99.8%, Sigma-Aldrich). The sample was then heated in an oven at 120 °C under vacuum for 24 h. (5) A 200-nm-thick PS film was coated onto copper by dip-coating with a pulling speed of 40 mm min−1 from 4 wt% PS in toluene. The samples were then annealed (120 °C under vacuum for 24 h). (6) A 1-μm-thick PS film was coated onto copper by dip-coating with a pulling speed of 20 mm min−1 from 10 wt% PS in toluene. The samples were then annealed (120 °C under vacuum for 24 h). (7) A 5-μm-thick PS film was coated onto copper by dip-coating with a pulling speed of 10 mm min−1 from 20 wt% PS in toluene. The samples were then annealed (120 °C under vacuum for 24 h). (8) 200-nm PS-coated gold samples. A 35-nm gold layer was sputtered onto quartz plates. A 200-nm PS film was then coated onto the gold layer by dip-coating, as described previously. (9) 60-nm SiO 2 -coated gold samples. First, a 35-nm gold layer was sputtered onto quartz plates. Then a 60-nm SiO 2 layer was sputtered onto a gold layer. (10) 60-nm Teflon-coated aluminium sample. First, a 100-nm aluminium layer was deposited onto glass slides (76.2 × 25.4 × 1 mm3, Sail Brand) by vacuum thermal evaporation. Then a 60-nm Teflon film was coated on the aluminium layer by dip-coating with a pulling speed of 10 mm min−1 from a solution of 1 wt% Teflon AF 1600 (ε r = 1.9, Sigma-Aldrich) in FC-75 (97%, Fisher Scientific). Finally, the samples were annealed (160 °C under vacuum for 24 h). (11) PFOTS layers on aluminium. First, a 100-nm aluminium layer was deposited onto glass slides. The sample was then treated with O 2 plasma at 300 W for 10 min and PFOTS was deposited by CVD, as described previously. (12) 60-nm Teflon-coated quartz–copper samples (for sliding experiments). We used a rectangular mask to cover two-thirds of the quartz plate (75 × 25 × 1 mm3, proQuarz GmbH) and 35 nm of copper was sputtered onto the uncovered area. After removing the mask, we obtained a plate with one-third covered by Cu and two-thirds comprising the quartz surface. The boundary between the quartz and copper areas formed a sharp straight line. Finally, a 60-nm Teflon film was coated on this plate by dip-coating with a pulling speed of 10 mm min−1 from 1 wt% Teflon AF 1600 (ε r = 1.9, Sigma-Aldrich) in FC-75 (97%, Fisher Scientific). Finally, the samples were annealed (160 °C under vacuum for 24 h). (13) 200-nm PS-coated quartz–gold samples (for sliding experiments). We used the same method as above to make a substrate of two-thirds quartz and one-third 35-nm gold. A 200-nm PS film was then coated on this plate by dip-coating with a pulling speed of 40 mm min−1 from 4 wt% PS in toluene. Then the samples were annealed (120 °C under vacuum for 24 h). (14) 60-nm Teflon-coated commercial copper foil samples. A 40-μm-thick single-sided copper foil tape (3M 1181 Kupferband leitend, 0.04 mm, 50 mm × 16.5 m) was cut into dimensions of 50 mm × 25 mm, stacked face to face on the adhesive side and cleaned with ethanol. A 60-nm-thick Teflon film was then coated onto the copper by dip-coating with a pulling speed of 10 mm min−1 from 1 wt% Teflon AF 1600 (ε r = 1.9, Sigma-Aldrich) in FC-75 (97%, Fisher Scientific). Finally, the samples were heated in an oven at 160 °C under vacuum for 24 h. (15) 500-nm super-hydrophobic coating commercial Copper foil samples. A 40-μm-thick single-sided copper foil tape (3M 1181 Kupferband leitend, 0.04 mm, 50 mm × 16.5 m) was cut into dimensions of 50 mm × 25 mm, stacked face to face on the adhesive side and cleaned with ethanol. A commercial super-hydrophobic spray (Glaco Mirror Coat Zero) was applied to copper foil from a distance of approximately 0.5 m for 3–4 s. The sample was then left undisturbed for 30 min to allow the ethanol in the solution to evaporate. (16) A 12-μm-thick PS film was coated onto copper by dip-coating with a pulling speed of 10 mm min−1 from 30 wt% PS in toluene. The samples were then annealed (120 °C under vacuum for 24 h).
Water drop impact experiments
We generated water drops with a volume of 35 μl water (<1 μS cm−1; Gibco, Thermo Fisher Scientific) containing 1 mM NaCl (diluted from 1 M NaCl solution, Carl Roth) through a grounded syringe needle (electrically neutral). The needle was connected to a peristaltic pump (MINIPULS 3, Gilson). Water drops were released from a height of about 4 cm from the sample surface at intervals of 12 s. More than 1,000 water drops were continuously released in each experiment. The sample was tilted by 10° so that, after impact, the water drop rolled off the surface. Each water drop always showed the same behaviour (Supplementary Fig. 19).
In the experiments of the spontaneously charged water drops that induce corrosion, the water drops first fell onto the top of a 50° tilted surface (Tradescantia spathacea leaves, PVC foam board, PS glass, FEP film or PFOTS-on-quartz plate) at intervals of 12 s from a release height of about 5 mm. Driven by gravity, water drops slid down roughly 4 cm and then left the tilted surface. During this process, the water drops undergo slide electrification, causing them to accumulate charge and deposit opposite charge onto the tilted surface. To achieve defined electrical conditions, the tilted surfaces were placed on a grounded metal plate. After the water drops left the tilted surface, they carried the electrical charge and, after falling about 5 mm, hit the Teflon-coated copper or other samples. A similar discharge cone was observed at different falling heights (Supplementary Fig. 20). As well as the 1 mM NaCl, there were more salt solutions (10 mM NaCl (Carl Roth), 100 mM NaCl (Carl Roth), 10 mM KCl (Carl Roth), 10 mM NaBr (Carl Roth), 10 mM KNO 3 (Carl Roth), 10 mM ZnSO 4 (Fluka) and also deionized water (<1 μS cm−1; Gibco, Thermo Fisher Scientific) and rainwater collected from the Mainz area.
The charge deposited by the drops on the tilted surface reduced the slide electrification effect of subsequent drops. Because it takes several minutes or more for the surface to naturally return to electrical neutrality, we needed to speed up the experimental process. To do this, we used an ionizing air blower (Aerostat PC ionizing air blower, Simco-Ion), which continuously neutralized the charge on the tilted surfaces during the experiment. This ensured that each subsequent water drop had the same slide electrification effect. In practice, tilted surfaces were neutralized in the 12 s between subsequent drops. Similarly, more than 1,000 water drops were continuously released in each experiment. Surface neutralization created by the ionized air took several seconds and was negligible while drops were sliding (about 100 ms) but effective in the intervals between drops. We also tested the case without the ionizing air blower and still observed surface corrosion (Supplementary Fig. 21).
For both experiments, we used a side-view high-speed camera (Photron, FASTCAM MINI UX100, 25,000 fps, resolution 1,280 × 200, with 1× SilverTL Telecentric Lens, Edmund Optics) to observe the behaviour of water drops when they hit the Teflon-coated copper surface. For the experiment observing the evolution of surface corrosion patterns from the bottom in situ, we used a non-high-speed camera (FLIR Blackfly S) for recording.
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