The question Ni, Berger, Butt, and their colleagues asked was what this charge does to the surface that the drop lands on next. To find out, the team released 35-microliter water drops, about the size of a large raindrop, containing a pinch of salt to mimic rainwater, onto a surface tilted at 50 degrees. The drops slid about four centimeters, picked up a charge, rolled off the edge, and then fell five millimeters onto a copper plate coated with a 60-nanometer film of Teflon, which is one of the most chemically resistant coatings on the market today.
That, btw, is the part that might not model real raindrops accurately--I'll address that in a moment.
The tilted surfaces were mostly chosen to mimic raindrops in the real world. One was a leaf from a Tradescantia spathacea plant growing in one of the researchers’ offices. Another was a PVC foam board from a hardware store. The third one was a sheet of transparent polystyrene sold as window glazing. Only the fourth one, the fluorinated coating on quartz, was more of a lab creation than something people usually see everywhere around them. The charges the drops picked up ranged from 0.2 nanocoulombs off the leaf to two nanocoulombs off the fluorinated quartz. A nanocoulomb in something the size of a raindrop works out to be a few thousand volts.
And yes, electrostatic effects (at raindrop speeds that's a good approximation) distort the raindrop into something pointy that creates a high voltage difference in the fraction of a second before impact, up to the kilovolt scale, which will cause a tiny dielectric breakdown. The researchers find damage to the surface.
The polymer coating is changed by the zap, and the underlying surface begins corroding.
The researchers identified the corrosion products on copper by Raman spectroscopy and X-ray diffraction. They found cuprous oxide and basic cupric chloride, the pale green compound familiar from weathered copper roofs. Elemental mapping of the damaged zones showed oxygen and chlorine flooding in and fluorine and carbon from the Teflon flooding out—exactly what should happen when a coating has been disrupted.
Another study looking at rain on solar cells finds a voltage spike as the drop rolls off.
It seems like a plausible real effect, and suggests that protective coatings (like paint) need to include other factors besides chemistry.
OTOH, the first other study I found (1953) found that raindrops had a little less charge. A nanocoulomb is about 3 esu.
The average measured free charge brought down by positively charged rain was 0.022 esu (.007 nanocoulomb) per drop, and by negatively charged rain was 0.031 esu (.01 nanocoulomb) per drop. The ratio of the negative free charge to the positive free charge brought down by rain was 1.2, while the ratio of the number of negative drops to the number of positive drops was 0.88.
That's quite a bit less charge on the natural raindrops that aren't sliding off a roof or something. Under a tree, the raindrops can pick up an order of magnitude more charge than an unimpeded drop, but still an order of magnitude less than sliding off flourinated quartz. So, under a tree, raindrops could hit a few hundred volts. Unimpeded drops, maybe a few dozen.
Some other studies of raindrops are comparable. This has obviously been a matter of some study--trying to understand what is the relationship of rain and lightning, for example.
And therefore, of course, one can find other, contrary estimates: "At the rain-forming level" 50 esu/gram times .03 gram (.3 for a heavy storm drop) is 1.7 esu (.6 nanocoulomb) (6 for a heavy storm's drop). And another that didn't find much difference in charge with raindrop size. And different proportions of positive and negative depending on the rain type (pre-monsoon, monsoon I, monsoon II).
And "The results show that the magnitude of the electric charges range between 1 and 50 pC (.001 and .05 nanocoulomb) and more than 90% of the charges are mainly carried by raindrops >1 mm, even though most of the raindrops are smaller than 1 mm."
Interesting. My guess from a quick-and-dirty literature review is that the effect in real life is smaller than the lab effect. Nature is always finding ways to erode what you build.
