The Water-Repelling Surface That Lives Outside My Window

http://www.wired.com/2014/06/empzeal-superhydrophobic-leaf/

by aatish • 12 years ago

350 62 12 years ago

wired.com

kamalkr• 12 years ago

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

CapitalistCartr• 12 years ago

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

yellowapple• 12 years ago

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

joezydeco• 12 years ago

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

devindotcom2• 12 years ago

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.

tempestn2• 12 years ago

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

Karellen• 12 years ago

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.

chrisbennet• 12 years ago

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.

aatish7• 12 years ago

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

devindotcom2• 12 years ago

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.

aatish7• 12 years ago

Thanks!

arjn• 12 years ago

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.

aatish7• 12 years ago

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.

aatish7• 12 years ago

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.

giarc2• 12 years ago

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.

hrjet• 12 years ago

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.

maxerickson2• 12 years ago

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

rcthompson• 12 years ago

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

mrb2• 12 years ago

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

aatish7• 12 years ago

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.

amanfredi2• 12 years ago

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.

aatish7• 12 years ago

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

kevinwang• 12 years ago

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

ultimatedelman• 12 years ago

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 :)

pdevr• 12 years ago

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.

mhitza• 12 years ago

What, they had microscopes thousands of years ago? TIL

TeMPOraL2• 12 years ago

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

Panoramix3• 12 years ago

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.

peterwwillis• 12 years ago

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)

mrb2• 12 years ago

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!

imaginenore• 12 years ago

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

lokki2• 12 years ago

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.

TheScythe• 12 years ago

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! :)

latortuga• 12 years ago

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.

suprgeek• 12 years ago

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.

jrkelly2• 12 years ago

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.

JumpCrisscross• 12 years ago

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.

jrkelly2• 12 years ago

zero dollars.

TeMPOraL2• 12 years ago

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.

usaphp• 12 years ago

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

js2• 12 years ago

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

woodchuck64• 12 years ago

"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)."

nhebb• 12 years ago

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

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

nikster2• 12 years ago

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.

spyder• 12 years ago

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

asimpletune• 12 years ago

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!

im3w1l• 12 years ago

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

mplishka• 12 years ago

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?

The Water-Repelling Surface That Lives Outside My Window

Related Stories