"Is there a clear evolutionary advantage for these leaves to be superhydrophobic?"<p>From:
<a href="http://biomimetic.pbworks.com/f/The+dream+of+staying+cleanSolga.pdf" rel="nofollow">http://biomimetic.pbworks.com/f/The+dream+of+staying+cleanSo...</a><p>"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).<p>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).<p>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)."
Hey - I'm the author of this piece. Thanks for voting it up to the front page. I love finding cool science in ordinary/everyday places, and am curious to hear people's responses to the piece. Cheers.
This is something which has been known for thousands of years, unless I am missing something: <a href="http://en.wikipedia.org/wiki/Lotus_effect" rel="nofollow">http://en.wikipedia.org/wiki/Lotus_effect</a><p>Edit: The relevant Bhagavat Gita verse: <a href="http://www.bhagavad-gita.us/bhagavad-gita-5-10/" rel="nofollow">http://www.bhagavad-gita.us/bhagavad-gita-5-10/</a><p>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.
Superomniphobic materials using this property are being made:<p><a href="http://pubs.acs.org/doi/abs/10.1021/ja310517s" rel="nofollow">http://pubs.acs.org/doi/abs/10.1021/ja310517s</a><p>But this is a great article! I love getting into the nitty gritty of why nature is so worth replicating.
If you like this sort of thing, you might enjoy reading "The Gecko's Foot: Bio-inspiration: Engineering New Materials from Nature"
It explains this and other neat tricks of nature.
I'm going to guess it was an adaptation that provided a reproductive advantage. You are supposed to be very careful about getting Tomato plant leaves wet when watering them because getting them wet makes them much more susceptible to diseases. Being resistant to such things would be a huge advantage. I've noticed such water-phobic behavior on other plants in my garden, specifically kale and broccoli.
And don't forget the whole thing self assembles with inputs of primarily light, water, and air at zero cost. Biology makes the rest of our manufacturing technology look ridiculously inefficient.
I was going to say before I read the rest of the article that there's probably an uneven surface on the leaf that's allowing the droplet to retain its superior surface tension.<p>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.<p>Ah, I found the paper, I think [1]: 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.<p>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")<p>Can somebody stick all that into the Wikipedia article? I'm lazy.<p>[1] <a href="http://wildfowl.wwt.org.uk/index.php/wildfowl/article/download/107/107" rel="nofollow">http://wildfowl.wwt.org.uk/index.php/wildfowl/article/downlo...</a>
[2] <a href="http://www.ncbi.nlm.nih.gov/pubmed/20437221" rel="nofollow">http://www.ncbi.nlm.nih.gov/pubmed/20437221</a> (a modern paper confirming the results)
Two reasons for this come to mind, in addition to the "rain on the roots" idea: dry surfaces are less able to support fungal/bacterial growth, and suspended water is less likely to damage the leaf cells with jagged ice crystals in the event of frost. Awesome article! :)
I would love to see a SEM (scanning electron microscope) picture of the exact area where the water comes in contact with these fibers (instead of this non-SEM fuzzy picture: <a href="http://www.wired.com/wp-content/uploads/2014/06/plume-poppy-contact-angle-mod.png" rel="nofollow">http://www.wired.com/wp-content/uploads/2014/06/plume-poppy-...</a>). They have the equipment. They should have done it!
If recent science is correct <a href="http://www.nature.com/srep/2013/130912/srep02617/full/srep02617.html" rel="nofollow">http://www.nature.com/srep/2013/130912/srep02617/full/srep02...</a> then we should be able to actually touch the leaf and "feel" the needles ourselves.
Very nice article, fun and informative.<p>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.
Did want to share that I came across this phenomenon on the underside of Silver Maple leaves the other day, and on the underside of Weeping Willow today. (The tops wet really well) What fascinates me is that the Silver Maple is native to the same areas as other Maples and yet, those other types are not hydrophobic at all - not on the top or the underside. Why would two plant leaves of the same genus, growing in the same area, have such different leaves?
I believe the explanation for why the leaves are so hydrophobic is quite simple. Those leaves are huge. Since they are so huge, they act as a canopy, preventing needed water from reaching the ground... unless the water could just somehow bead up and roll off the leaf. Saying a leaf is hydrophobic is another way of saying that water doesn't stick to them. If water doesn't stick to them, it falls off. Very simple, and ingenious!
It's called the "lotus effect" :<p><a href="http://en.wikipedia.org/wiki/Lotus_effect" rel="nofollow">http://en.wikipedia.org/wiki/Lotus_effect</a><p>And it works even with honey:<p><a href="https://www.youtube.com/watch?v=LJtQ6dvcbOg" rel="nofollow">https://www.youtube.com/watch?v=LJtQ6dvcbOg</a>
I immediately thought of jewelweed which in addition to having water repellent leaves is also medicinal - <a href="http://www.wildmanstevebrill.com/Plants.Folder/Jewelweed.html" rel="nofollow">http://www.wildmanstevebrill.com/Plants.Folder/Jewelweed.htm...</a>
I really liked looking at the vibrations of the drops. Especially the drop at 15s. I think it is the l=2 mode?<p><a href="https://www.youtube.com/watch?v=VGUDapZ7WlQ&t=0m15s" rel="nofollow">https://www.youtube.com/watch?v=VGUDapZ7WlQ&t=0m15s</a>