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The Water-Repelling Surface That Lives Outside My Window (wired.com)
350 points by aatish 309 days ago | 62 comments



"Is there a clear evolutionary advantage for these leaves to be superhydrophobic?"

From: http://biomimetic.pbworks.com/f/The+dream+of+staying+cleanSo...

"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)."

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This is the same principle behind self-cleaning solar panels:

https://www.asme.org/engineering-topics/articles/energy/self...

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Common Sense Summary: The same reason humans would not do well if we remained wet at all times. Plants can't towel themselves dry so they are superhydrophobic instead.

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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.

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Could it be that long ago leaves existed without the wax, and when it rained the leaves were weighed down by all the excess water they were absorbing? Leaves with mutations producing wax were able to recover from a heavy rainstorm better than leaves with no wax.

Picture a leaf in the Amazon, do you picture a big, waxy leaf? I do. Rainforest = more rain = more wax.

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That's my first thought too. Moreover, letting the water roll off the leaves and down near the stem would help moistening the soil near the roots, which would ultimately benefit the plant.

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The wax could also refract sunlight better.

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Hi Aatish! I enjoyed it quite a bit. The fibers on the bottom of the leaf may not be significant -- perhaps they are vestigial and only the upper ones provide an advantage.

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.

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Hey, I know an A. Manfredi. Are you Anthony from Swat?

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Yes indeed :)

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Sweet. Hi! This is an odd place to bump into you. :)

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.

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The waxy layer is believed to have developed as plants moved out of the water onto land, it helps to retain water:

http://en.wikipedia.org/wiki/Plant_cuticle

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That explains the need for a waxy layer, but it doesn't explain the need for the superhydrophobic bed-of-needles surface.

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Two possibilities:

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.

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That was my first thought. It's not a good idea to water your plants at night, because without the sunlight and heat of the day the water doesn't evaporate as quickly. Water sitting on the leaves all night can cause fungus and such things to grow, which is bad for the plant. Since plants can't avoid being rained on at night, it seems logical that they would evolve a mechanism for staying as dry as possible.

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My first guess would be that it requires less energy to produce and maintain than other similarly effective arrangements of the wax.

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Leaves blow around in the wind. If water was not being repelled from the underneath of the leaf, it would gain added weight, and have the potential of anchoring the leaf in the wrong direction. Repelling water from the underneath of the leaf keeps the leaf light, which allows the rest of the plant structure to support the leaf in the proper orientation.

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Maybe leaves evolved these microscopic "needles" in order to increase their surface area, because this would help photosynthesis for 2 reasons:

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[1]).

The fact that these "needles" also create a superhydrophobic effect would just be a side effect.

[1] http://www.ncbi.nlm.nih.gov/pubmed/21029116

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Hi. The needles are wax structures that protrude from the leaf, they aren't part of the leaf. So the added surface area doesn't absorb any air, and I doubt it helps in evaporative cooling either.

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This is something which has been known for thousands of years, unless I am missing something: http://en.wikipedia.org/wiki/Lotus_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.

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That it happens was known for thousands of years; why and how it happens is a new piece of knowledge.

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I guess the message is that what the author is sharing is common knowledge since the 70's. With the invention of the electron microscope people started looking at everything, including leaves, discovering what the blogger at hand shares here. http://en.wikipedia.org/wiki/Lotus_effect

Nowadays there are hundreds of companies that sell materials or sprays taking advantage of this (or related) effects. For example http://www.hydrobead.com/ http://www.ultraeverdrystore.com/

Still it's always cool to see high fps cameras in action.

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First, this was news to me. Second it is cool.

And third, the message is to go through life with open eyes, and to wonder about things.

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What, they had microscopes thousands of years ago? TIL

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Superomniphobic materials using this property are being made:

http://pubs.acs.org/doi/abs/10.1021/ja310517s

But this is a great article! I love getting into the nitty gritty of why nature is so worth replicating.

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Thanks!

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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.

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Have been meaning to read that, and just borrowed it from the library on your suggestion. Thanks!

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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.

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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.

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> at zero cost

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.

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While you're right that it's not exactly "free" in energy use terms, it still outperforms our manufacturing technologies by few orders of magnitude. So cheers for the amazing piece of biomolecular nanotechnology; let us learn from it and improve on it.

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zero dollars.

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While we're on the topic of economic cost, I wanted to mention economic value. For example, check out this article, "Accounting for the value of ecosystem services": http://www.sciencedirect.com/science/article/pii/S0921800902...

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Elsevier wants me to pay $40 for this publicly funded work. Here's a direct link: http://yosemite.epa.gov/sab/sabcvpess.nsf/0/1c7c986c372fa8d4...

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).

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Saying you can't measure value in dollars is kind of like saying you can't measure a distance with inches. Inches are also a kind of abstract concoction.

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Not at all. Inches have a direct relationship to a physical quantity.

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Inches are a defined construct, not an absolute. They are convenient chunks so we dumb humans and communicate with some level of accuracy.

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.

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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! :)

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Perhaps they're superhydrophobic to allow water droplets to roll off and be absorbed by the roots?

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If your main source of energy is from light, and water diffracts light, potentially making it harder or less efficient to use, does it not make sense that the better you're able to keep your energy harvesters clear, the more energy you can create? Evolution at its finest :)

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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: http://www.wired.com/wp-content/uploads/2014/06/plume-poppy-...). They have the equipment. They should have done it!

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You can't. SEM requires coating the scanned things in a thin layer of metal.

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nit-pick alert

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.

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If recent science is correct http://www.nature.com/srep/2013/130912/srep02617/full/srep02... then we should be able to actually touch the leaf and "feel" the needles ourselves.

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How hard is it to make artificial hydrophobic material? Wouldn't they make great windshields?

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Pah. We've already solved the windshield problem, if crudely. Think bigger, and also smaller.

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?

Boat hulls.

Skis/snowboards.

Pipes/drains in hard water areas - if the water doesn't stick around, it won't evaporate to leave limescale behind.

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They're making it (I linked above) but my guess is that they're still pretty fragile. The microstructures and nanostructures that minimize contact surface area can be flattened or broken super easily and then your perfect phobic material becomes partially porous. So I would say it's not hard to make them, but it's hard to make them robust.

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And a windshield would be a very difficult place for them, since it's getting continuously blasted by air and grit as you drive.

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You can buy Rain-X and the like at auto parts stores.

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Plus there's that NeverWet stuff Rustoleum sells now; according to their marketing materials, it actually qualifies as superhydrophobic (i.e. in the same ballpark as the article's leaf): http://www.neverwet.com/product-characteristics.php

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There's stronger stuff, like Diamon-Fusion, but they all wear away with use.

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that seems as if it'd be true for any surface that makes use of the three dimensional shape/properties of the surface.

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Very nice article, fun and informative.

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.

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Yeah, I linked to Neverwet in the post. The video demos are amazing although in practice I hear that the coating fades pretty quickly. But it's cool to see commercial applications of superhydrophobic surfaces.

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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?

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I would love to see more of these kind of articles here on HN

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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!

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It's called the "lotus effect" :

http://en.wikipedia.org/wiki/Lotus_effect

And it works even with honey:

https://www.youtube.com/watch?v=LJtQ6dvcbOg

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I immediately thought of jewelweed which in addition to having water repellent leaves is also medicinal - http://www.wildmanstevebrill.com/Plants.Folder/Jewelweed.htm...

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I really liked looking at the vibrations of the drops. Especially the drop at 15s. I think it is the l=2 mode?

https://www.youtube.com/watch?v=VGUDapZ7WlQ&t=0m15s

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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.

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 [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.

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.

[1] http://wildfowl.wwt.org.uk/index.php/wildfowl/article/downlo... [2] http://www.ncbi.nlm.nih.gov/pubmed/20437221 (a modern paper confirming the results)

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