Spontaneously Charged Water Droplets Accelerate Corrosion

Natural and Commercial Materials for Surface Testing

Tradescantia spathacea leaves were collected from potted office plants. Commercial PVC foam boards (PVC-Schaumplatte Fixmaß, Bauhaus AG, 250 × 500 × 3 mm³) and transparent polystyrene (PS) glass sheets (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), conductive copper foil tape (3M 1181 Kupferband leitend, 0.04 mm, 50 mm × 16.5 m), superhydrophobic spray (Glaco Mirror Coat Zero, 40 ml) and PS sheets (Evergreen Scale Models, 130 μm) were purchased online.

Preparation of Experimental Samples

Sixteen sample types were prepared for the surface electrification, water-drop impact and corrosion experiments:

1. (1)

   PFOTS-coated quartz: Quartz plates (75 × 25 × 1 mm³, proQuarz GmbH) were treated with O₂ plasma at 300 W for 10 min using a Femto low-pressure plasma system (Diener electronic). The quartz surfaces were then coated by chemical vapour deposition (CVD) with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTS; 97%; Sigma-Aldrich). Cleaned quartz plates were placed in a vacuum glass container with a vial containing 0.5 ml PFOTS. The container was evacuated to below 100 mbar, isolated from the pump and left to react for approximately 30 min. The plates were rinsed with ethanol to remove unbound silane molecules.

2. (2)

   PFOTS-coated ITO glass: ITO glass plates (75 × 25 × 1.1 mm³; surface resistivity 30–60 Ω sq⁻¹; Sigma-Aldrich) were treated with O₂ plasma at 300 W for 10 min. PFOTS was deposited by CVD using the same procedure described for quartz. The coated ITO glass was rinsed with ethanol to remove unbound silane.

3. (3)

   60-nm Teflon-coated copper: A 35-nm copper layer was sputtered onto a quartz plate to create a flat copper substrate. A 60-nm Teflon film was deposited by dip-coating at a pulling speed of 10 mm min⁻¹ from a 1 wt% solution of Teflon AF 1600 (ε_r = 1.9; Sigma-Aldrich) in FC-75 (97%; Fisher Scientific). The coated samples were heated at 160 °C under vacuum for 24 h. Film thickness was measured with a P-7 stylus profiler (KLA-Tencor).

4. (4)

   60-nm PS-coated copper: A 60-nm PS film was deposited onto copper by dip-coating at 60 mm min⁻¹ from a 2 wt% PS solution in toluene. The PS had a molecular weight of 192 kg mol⁻¹ and a relative permittivity of ε_r = 2.6 (Sigma-Aldrich). Samples were annealed at 120 °C under vacuum for 24 h.

5. (5)

   200-nm PS-coated copper: A 200-nm PS film was deposited by dip-coating at 40 mm min⁻¹ from a 4 wt% PS solution in toluene. The samples were annealed at 120 °C under vacuum for 24 h.

6. (6)

   1-μm PS-coated copper: A 1-μm-thick PS film was applied by dip-coating at 20 mm min⁻¹ from a 10 wt% PS solution in toluene. The samples were annealed at 120 °C under vacuum for 24 h.

7. (7)

   5-μm PS-coated copper: A 5-μm-thick PS film was deposited by dip-coating at 10 mm min⁻¹ from a 20 wt% PS solution in toluene. The samples were annealed at 120 °C under vacuum for 24 h.

8. (8)

   200-nm PS-coated gold: A 35-nm gold layer was sputtered onto quartz plates. A 200-nm PS film was then deposited onto the gold by dip-coating using the procedure described above.

9. (9)

   60-nm SiO₂-coated gold: A 35-nm gold layer was first sputtered onto quartz plates, followed by deposition of a 60-nm SiO₂ layer onto the gold surface.

10. (10)

    60-nm Teflon-coated aluminium: A 100-nm aluminium layer was deposited onto glass slides (76.2 × 25.4 × 1 mm³, Sail Brand) by vacuum thermal evaporation. A 60-nm Teflon film was applied by dip-coating at 10 mm min⁻¹ from a 1 wt% Teflon AF 1600 solution in FC-75. The samples were annealed at 160 °C under vacuum for 24 h.

11. (11)

    PFOTS-coated aluminium: A 100-nm aluminium layer was deposited onto glass slides. The samples were treated with O₂ plasma at 300 W for 10 min, after which PFOTS was deposited by CVD using the procedure described previously.

12. (12)

    60-nm Teflon-coated quartz–copper samples for sliding tests: A rectangular mask covered two-thirds of each quartz plate (75 × 25 × 1 mm³, proQuarz GmbH), while 35 nm of copper was sputtered onto the exposed one-third. Removing the mask produced a substrate with one-third copper and two-thirds quartz, separated by a sharp, straight boundary. A 60-nm Teflon film was applied by dip-coating at 10 mm min⁻¹ from a 1 wt% Teflon AF 1600 solution in FC-75. The samples were annealed at 160 °C under vacuum for 24 h.

13. (13)

    200-nm PS-coated quartz–gold samples for sliding tests: A substrate consisting of two-thirds quartz and one-third 35-nm gold was prepared using the same masking method. A 200-nm PS film was deposited by dip-coating at 40 mm min⁻¹ from a 4 wt% PS solution in toluene. The samples were annealed at 120 °C under vacuum for 24 h.

14. (14)

    60-nm Teflon-coated commercial copper foil: Single-sided copper foil tape, 40 μm thick (3M 1181 Kupferband leitend, 0.04 mm, 50 mm × 16.5 m), was cut into 50 mm × 25 mm pieces. The pieces were stacked face to face on the adhesive side and cleaned with ethanol. A 60-nm Teflon film was applied by dip-coating at 10 mm min⁻¹ from a 1 wt% Teflon AF 1600 solution in FC-75. The samples were annealed at 160 °C under vacuum for 24 h.

15. (15)

    500-nm superhydrophobic commercial copper foil: Copper foil tape was prepared as described above and cleaned with ethanol. Glaco Mirror Coat Zero superhydrophobic spray was applied from approximately 0.5 m for 3–4 s. The coated foil was left undisturbed for 30 min to allow the ethanol solvent to evaporate.

16. (16)

    12-μm PS-coated copper: A 12-μm PS film was deposited by dip-coating at 10 mm min⁻¹ from a 30 wt% PS solution in toluene. The samples were annealed at 120 °C under vacuum for 24 h.

Water-Drop Impact Experiments

Water drops with a volume of 35 μl were prepared using ultrapure water (<1 μS cm⁻¹; Gibco, Thermo Fisher Scientific) containing 1 mM NaCl, diluted from a 1 M NaCl stock solution (Carl Roth). Drops were generated through a grounded, electrically neutral syringe needle connected to a MINIPULS 3 peristaltic pump (Gilson). They were released from approximately 4 cm above the sample at 12-s intervals. More than 1,000 drops were delivered continuously during each experiment. Samples were tilted by 10° to allow impacted drops to roll off the surface. Each drop displayed consistent behaviour (Supplementary Fig. 19).

To study corrosion caused by spontaneously charged water drops, drops first landed on the upper edge of a surface tilted at 50°. The tested surfaces included Tradescantia spathacea leaves, PVC foam board, PS glass, FEP film and PFOTS-coated quartz. Drops were released from approximately 5 mm at 12-s intervals and slid approximately 4 cm under gravity before leaving the surface. During sliding, the drops underwent contact electrification, accumulating charge while depositing an opposite charge on the surface. The tilted samples were placed on a grounded metal plate to establish defined electrical conditions. After leaving the tilted surface, the charged drops fell approximately 5 mm before striking Teflon-coated copper or other test substrates. Similar discharge cones were observed at different drop heights (Supplementary Fig. 20).

In addition to 1 mM NaCl, the tested liquids included 10 mM NaCl, 100 mM NaCl, 10 mM KCl, 10 mM NaBr, 10 mM KNO₃, 10 mM ZnSO₄, deionized water (<1 μS cm⁻¹) and rainwater collected in the Mainz area.

Charge deposited by each drop reduced the contact electrification of subsequent drops. Because the surface required several minutes or longer to return naturally to electrical neutrality, an Aerostat PC ionizing air blower (Simco-Ion) was used to continuously neutralize the tilted surfaces. This allowed each drop to experience comparable contact electrification. In practice, the surfaces were neutralized during the 12-s interval between drops. Ionized-air neutralization required several seconds and had a negligible effect during sliding, which lasted approximately 100 ms, while remaining effective between drops. Surface corrosion was also observed without the ionizing blower (Supplementary Fig. 21).

A side-view high-speed camera (FASTCAM MINI UX100, Photron; 25,000 fps; 1,280 × 200 resolution; 1× SilverTL telecentric lens, Edmund Optics) recorded drop behaviour during impact on Teflon-coated copper. A standard FLIR Blackfly S camera was used to record the in situ development of corrosion patterns from the underside.

Water-Drop Sliding Experiments

Samples used for sliding tests were mounted on a grounded metal plate tilted at 50°. Water drops measuring 35 μl and containing 1 mM NaCl were released onto the upper edge of each tilted sample from a grounded syringe needle. Drops were released from approximately 5 mm at 12-s intervals, with solution continuously supplied from a reservoir by a MINIPULS 3 peristaltic pump (Gilson). The ionizing air blower remained on throughout each experiment. Approximately 3,000 drops were released continuously per test. A bottom-view high-speed camera (Phantom TMX 7510, Photron; 40,000 fps; 1,280 × 800 resolution; 10× UPlanSApo microscope objective, Olympus) recorded drop motion during sliding.

Confocal Laser Scanning Microscopy

Surface morphology and fluorescence were characterized with a confocal microscope (LSM 880, Carl Zeiss) equipped with a Zeiss Plan-Apochromat 10×/0.45 objective. An argon-ion laser (λ = 488 nm), coupled to the microscope by optical fibre, provided excitation. The observation volume was positioned at the sample surface. Images were collected in reflection or fluorescence mode using the appropriate dichroic mirror and spectral detection unit (Quasar, Carl Zeiss), which included a diffraction grating and a 32-channel GaAsP multianode photomultiplier array. Fluorescence emission was detected over 517–696 nm. Emission spectra were recorded in lambda mode across 520–690 nm using 32 separate detection channels. Three-dimensional surface morphology images (Supplementary Figs. 15 and 23) were obtained with a confocal white-light microscope (μsurf, NanoFocus AG) equipped with a 20×/0.46 objective and point-by-point line-scanning laser profilometry.

Water-Drop and Substrate Charge Measurements

Electrical charge was measured using a current amplifier with a response time of 0.8 ms (FEMTO DDPCA-300). Current signals from water drops and metal substrates during impact were recorded with a National Instruments USB-6366 X Series data-acquisition board.

Water-drop measurements: The current amplifier was connected to 0.7-mm-diameter tungsten wires coated with gold to improve conductivity. Current was measured from drops after they had slid 4 cm across the surface (Extended Data Figs. 1 and 5a) and from drops bouncing off the surface (Extended Data Fig. 5c). Contact between a drop and the wire electrode generated a measurable current. Electrode contact was defined as 0 ms (t₀), and discharge continued until t₁. After t₁, a low positive current of approximately 10 nA continued until the rear of the drop separated from the electrode after about 20 ms. This current resulted from continued negative-charge deposition as the drop moved while still in contact with the electrode. For all samples except Tradescantia spathacea leaves, t₁ was typically 2 ms; for leaves, it was approximately 10 ms. The charge generated by contact electrification was calculated by integrating the current from t₀ to t₁. Drop current and charge were also measured for different salt concentrations, release intervals and natural rainwater (Supplementary Figs. 4, 21 and 22).

Metal-substrate measurements: The current amplifier was connected directly to the metal layer. When a water drop struck the surface, the resulting current was recorded (Extended Data Fig. 5b). Drop contact was defined as 0 ms, and discharge continued until t₁, which was approximately 2 ms. The charge transferred from the drop to the metal was calculated by integrating the current between t₀ and t₁.

AFM and AFM-IR Surface Analysis

Atomic force microscopy (AFM; Dimension Icon, Bruker) was used in tapping mode to characterize surface morphology. The prepared surfaces were uniform and smooth (Supplementary Figs. 23 and 24). Commercial copper foil was rougher, although its roughness decreased slightly after Teflon coating (Supplementary Fig. 25). The cantilever had a nominal resonance frequency of 300 kHz and a spring constant of 26 N m⁻¹ (OTESPA, OPUS). Scan areas were 0.5 × 0.5, 20 × 20 or 50 × 50 μm².

Products formed by dielectric breakdown in the PS film were analysed using AFM-based nano-infrared spectroscopy (Vista One, Molecular Vista). Nano-IR signals were acquired in photo-induced force microscopy (PiFM) mode. Surface topography was measured by exciting the cantilever’s second eigenmode and maintaining constant oscillation amplitude with an electronic feedback circuit. The first eigenmode was used to record the nano-IR response. Incident infrared light was modulated at the frequency difference between the first and second eigenmodes, which was tuned to maximize the first-eigenmode response.

SEM-EDS Elemental Analysis

Scanning electron microscopy (SEM) was performed using a HITACHI SU8000 microscope (Hitachi High-Technologies Europe GmbH). The microscope was equipped with an XFlash 5010 X-ray detector for simultaneous energy-dispersive X-ray spectroscopy (EDS) and elemental analysis.

Raman Spectroscopy

Raman spectra were collected with a WITec alpha300 R confocal Raman spectrometer using a 10× objective, a 600/1,200 grooves mm⁻¹ grating and 5 mW laser power. Samples were excited at 532 nm with a 120-s integration time. Pristine copper foil was analysed as a reference (Supplementary Fig. 26a).

X-Ray Diffraction Measurements

X-ray diffraction (XRD) measurements were performed with a Rigaku SmartLab diffractometer equipped with a rotating copper anode (8 keV, λ = 1.5406 Å), Kβ filter and HyPix-3000 detector. Samples were mounted on a rotating stage and measured in θ–θ geometry from 10° < 2θ < 80° at a scan rate of 1° min⁻¹. The X-ray beam width was 1 mm. Pristine copper foil was used as a reference sample (Supplementary Fig. 26b).

Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was performed with a Metrohm Autolab N Series potentiostat (Autolab PGSTAT204) in a three-electrode configuration. Samples were mounted horizontally in a flat cell, with the exposed working area defined by a 6-mm-diameter O-ring. An Ag/AgCl electrode in 3 M KCl served as the reference electrode, and a platinum wire served as the counter electrode. Measurements were conducted in air at ambient temperature using either 10 mM or 600 mM aqueous NaCl.

Impedance spectra were recorded at open-circuit potential from 10⁵ Hz to 0.1 Hz using a 10 mV sinusoidal perturbation. For 5-μm PS films, the amplitude was increased to 20 mV to improve the signal-to-noise ratio of these highly resistive samples. EIS results showed that the developed coatings substantially increased interfacial resistance, confirming their strong barrier properties (Supplementary Information Discussion 4). Coating barrier performance was also assessed with a digital multimeter (Supplementary Fig. 28).