Science1 distinct publisher3 min readPublished
Corrosion has long been taught as abrasion plus acid chemistry. A German group has now damaged a Teflon-coated copper surface using only the electric charge that water picks up while sliding down a leaf or a plastic board first.
The Scientist · Science desk
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Strip the experiment to its comparison and it is one variable wide. Copper carrying a 60 nm Teflon film took water drops placed straight down and showed no corrosion [7]. The same target, struck by drops that had first run down a plant leaf, a PVC construction board or a PFOTS hydrophobic film of the type applied to window glass, showed damage in the coating and in the metal beneath after roughly 3000 impacts, imaged by atomic force and confocal microscopy [8][9]. The slide is the only thing added, and removing it removes the damage.
Two supporting measurements make the electrical route hard to argue away. Charge tracking in the copper showed negative charge flowing toward drops as they dripped off the insulating slope [11], and high-speed video caught the dripping drop stretched into a cone with positive charge concentrated at the tip, where the directly deposited drop stayed spherical right up to impact [12]. The group's reading is that the landing potential difference can exceed 1 kV, beyond the dielectric breakdown strength of the film, so the drop discharges through it and leaves the metal open to further oxidation [10]. A kilovolt across 60 nm is a nominal average field near 17 volts per nanometre [18], which is why a film that thin loses this contest.
Thickness is also why the result stays in the lab for now. Hans-Jürgen Butt of the Max Planck Institute for Polymer Research in Mainz puts technical coatings on cars and ships at 100 microns and thicker, about 1700 times the film his group perforated, and says the process is probably not of direct relevance there [3][13][17]. He points instead at monuments and other outdoor historical objects [14], and is candid about the gaps: surfaces do change when charge is slid across them, but the real consequences are unknown [15], and the charge separation itself should be energetically unfavourable and yet happens [16]. The older reason nobody looked here is that triboelectric charging was thought not to work in liquids, since solid-on-solid charging depends on atomic-scale roughness and a fluid cannot be rough [4].
The thing this does not tell you is a rate. The reported work gives about 3000 impacts with no elapsed time and no corrosion rate, so there is nothing yet to weigh against the abrasion and acid-chemistry pathways that conventional accounts credit [19][2].
My read, with its conditions: this belongs in corrosion testing before it belongs in corrosion budgets. A weathering rig that drips water onto a coupon from a nozzle is reproducing the null condition of this experiment rather than the damaging one [7]. Whether that matters depends on whether anything in the field carries a barrier thin enough to break down at a kilovolt, and the man raising that doubt is the paper's own senior author [13].
Ranked by verification strength, evidence, and original report placement.
Researchers in Germany conclude that water droplets sliding across solid surfaces can acquire electric potentials of thousands of volts and corrode non-conductive surfaces on metals when they discharge.
Conventional wisdom holds that most corrosion is caused by physical abrasion from droplet motion combined with chemical degradation from natural acids and anthropogenic pollutants in the water; the idea that electrochemistry plays a significant role had not been seriously considered, according to Hans-Jurgen Butt.
Hans-Jurgen Butt is a physical chemist at the Max Planck Institute for Polymer Research in Mainz.
Until relatively recently the triboelectric effect was thought not to work in liquids, because solid-on-solid charging relies on atomic-scale roughness to concentrate charge at points where bonds break, and fluids cannot by definition be rough.
Butt says the mechanical force from surface tension on a sliding droplet is much too low to generate enough local energy to move an electron or an ion onto the surface.
Over the past 10 years it has become increasingly clear that sliding droplets do become highly charged.
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Specific bench work, one relay
The measurements are concrete and well matched to the claim: 60 nm of Teflon on copper, about 3000 impacts, atomic force and confocal microscopy for the damage, in-copper charge flow and high-speed footage of the drop pinching into a charged cone for the mechanism. Better still, there is a null condition — drops placed directly on the surface did nothing — which is what turns a correlation into a finding. Against that, we are reading a magazine account of a Nature paper rather than the paper, there is no timescale attached to those impacts, and no second lab has tried it.
Nothing yet in the world
There is no uptake to measure. This is a mechanism demonstrated on a laboratory stack, and the one real-world surface named — the PFOTS coating used on window glass — appears as a charging prop in the experiment, not as a deployment. No conservator reports this damage on a monument, no manufacturer has changed a coating spec, and the researchers describe themselves as chasing fundamental physics.
Deflation supplied by the authors
Unusually, the caution is inside the reporting rather than missing from it. Butt spends his applied quotes shrinking the claim — 100-micron coatings out, consequences unknown, effect in some ways trivial — and Physics World lets him. The 'nothing but pre-charged water' framing is literal. If anything the piece undersells its own quietest line: a plant leaf, a construction board and an ordinary window coating all worked equally well as chargers, which is a much more everyday setup than a headline about Teflon and copper suggests.
Interested voices, plainly signposted
Two stakes, both visible. Butt is describing his own paper, yet his quotes narrow its reach rather than widen it, which is not how promotion reads. The outside comment comes from Zhong Lin Wang, who was among the first to find the triboelectric effect in liquids and helped build the nanogenerator used to measure it, and who frames the result as vindication for contact-electro-catalysis, a field he proposed. Expert, yes; disinterested, no. Physics World's own incentive is the ordinary one of a physics title with a Nature paper in hand.
Solid observation, single channel
We are fairly sure of what happened on the bench and much less sure of what it means anywhere else. The core is peer-reviewed and internally coherent, but it reaches us through one outlet, every figure comes from one group, and that group says outright that the charge separation should not occur and it cannot explain why it does. Confidence sits where the microscopes were pointed: at a 60-nanometre film.