"The evolutionary benefits of sophisticated superhydrophobic surfaces are diverse. With plants, a water film affects the gas exchange which is crucial for many physiological processes (Brewer and Smith 1994 ,Brewer 1996). This is true particularly for the underside of the leaf where the stomata are usually located. Consequences of a disturbed gas exchange are inhibition of photosynthesis and suppression of plant growth; the latter can even become chronic (Ishibashi and Terashima 1995). This might explain why in numerous plant species the underside of the leaf is less wettable than the upper surface (Smith and McClean 1989). Moreover, a water film significantly increases leaching of nutrients (Tukey 1970). The prevention of a water film has an important side effect: the period during which dissolved air pollutants can damage the plant is distinctly shortened (Haines et al 1985).
Clearly, the ability of a plant to clean itself is an additional benefit. Naturally and artificially emitted dust that is deposited on photosynthetic plant organs causes shading, enhanced reflection, increased leaf temperature, decreased gaseous diffusion and increased transpiration through stomata and cuticle (Thompson et al 1984 , Eveling 1986 , Hirano et al 1995 , Sharifi et al 1997). As a result, the photosynthetic rate is reduced and the plant gets under stress sometimes to the point of damage to its surface (Eveling 1986).
Another important function of the self-cleaning mechanism is its role in the protection against pathogen attacks. Spores of pathogenic fungi are completely washed off surfaces of certain crops with well-developed epicuticular waxes, provided that the surface microstructure is intact (Neinhuis et al 1992). Moreover, a dense layer of wax crystals makes it more difficult for fungi to penetrate a plant surface (Schwab et al 1995). The almost permanent dryness of superhydrophobic self-cleaning surfaces is an obstacle particularly to pathogens producing spores which require free water for germination (Juniper 1991)."
Picture a leaf in the Amazon, do you picture a big, waxy leaf? I do. Rainforest = more rain = more wax.
Alternatively, heavy rain and wind can flip leaves upside down, blow plants over, and splash dirt and water up onto the underside of the leaves, so maybe it is an advantage for the whole leaf to shed water.
I agree with your thoughts that this could be a consequence of coating the top surface in the waxy needles. However, I think it's actually somewhat unusual for a leaf to be superhydrophobic on the top and bottom, so I was wondering if there was some specific advantage to this leaf. Also, the needles on the under-side seemed far more dense than those on top, which was interesting.
One, a superhydrophobic surface reduces the surface area of the water that lands on the plant, allowing less of it to evaporate and more of it to fall to the ground.
Two, it may help prevent plant diseases by allowing less water to attach to surfaces that are prone to becoming diseased.
1. More surface area = better ability to capture carbon dioxide from the air.
2. More surface area = heatsink effect = reduced heating under the sun (photosynthesis is less efficient under too much heat).
The fact that these "needles" also create a superhydrophobic effect would just be a side effect.
Edit: The relevant Bhagavat Gita verse: http://www.bhagavad-gita.us/bhagavad-gita-5-10/
Edit 2: @OP: Thank you for the article, it is well-written. From the comments so far, it is obvious that this is something new to many here.
Nowadays there are hundreds of companies that sell materials or sprays taking advantage of this (or related) effects. For example
Still it's always cool to see high fps cameras in action.
And third, the message is to go through life with open eyes, and to wonder about things.
But this is a great article! I love getting into the nitty gritty of why nature is so worth replicating.
The self-assembly of these intricate mechanisms is fascinating. But this doesn't happen for "free". It takes the exploitation of massive thermodynamic imbalances produced by a billion-year thermonuclear detonation no more than 8-and-a-third light minutes away.
I am not an economist, so I would argue that the whole premise is flawed: you can't measure the value of an ecosystem in dollars. You can't eat gold. Money is some sort of abstract concoction, whereas plants and animals are very real. No amount of money would make it alright to destroy them (the paper argues a different point, but still thinks of them in terms of dollars).
Dollars are exactly the same - assigning value lets us communicate and relate otherwise abstract concepts in a meaningful way. While we can't eat gold, we can imagine how much food gold can buy, with all the caveats about exchanges and locations in place. The important bit to take away is providing a framework for common understanding.
Conversely, the waterproofing nature of bird feathers is, apparently, not well understood. It was originally posited that the hydrophilic oils from the Uropygial gland spread during preening added to the waterproofing nature of the feather, but (according to a Wikipedia edit which has no citation that I can find no further evidence of on the Internet) there's some theory about an electrostatic state due to the mechanical process of preening keeping the feathers free from water sticking to them. It's also claimed that powder-down birds use their feather residue as waterproofing, but that seemingly hasn't been proved either.
Ah, I found the paper, I think : Here's a paper from the 1950s which did an intense study of ducks and various states which might affect their water-repellent nature. It turns out that removing the Uropygial gland from newly hatched ducklings resulted in their feathers being just as water-repellent as ducks that still had the gland. Interestingly, the diet of the birds seemed to effect the water-repellent nature more than anything else, but also completely dependent on where they were fed.
The end of page 6 and the rest of 7 point out the theory of the barbules in the feathers being responsible for the waterproof nature. The idea is that when they are properly aligned, air between the barbs (when at a constant distance from each other) keeps a narrow enough distance that the surface tension of the water is maintained, similar to this leaf. So this is probably where the idea of an "electrostatic" force came from (or the paper by Madsen that's cited: "Madsen, H., 1941: Hvad gor Fuglenes Fjer-Dragt vandskyende ?. Dansk Ornithologisk Forenings Tidsskrift, 35: 49-59")
Can somebody stick all that into the Wikipedia article? I'm lazy.
 http://www.ncbi.nlm.nih.gov/pubmed/20437221 (a modern paper confirming the results)
Basic SEM work requires a conductive surface (carbon coatings also work), and in some cases you can skip that if you have a good carrier gas. The real limitation here is that most SEMs require you to pump down to vacuum, which would eliminate the water. Even variable-pressure SEMs that operate close to atmosphere require a purge, first.
So the limit is the vacuum, not the metal coating.
What about glasses/goggles, which are too small for wipers?
What about artificial heart parts, which have problems with blood clotting, presumably because it sticks to the surfaces and stops moving. If we could coat the parts we implant with this stuff, would it prevent blood clots?
Pipes/drains in hard water areas - if the water doesn't stick around, it won't evaporate to leave limescale behind.
There are several products that can coat materials to make them hydrophobic. "Neverwet" and "Ultra-Ever Dry" are two. Check out their videos on youtube.
And it works even with honey: