
The Water-Repelling Surface That Lives Outside My Window - aatish
http://www.wired.com/2014/06/empzeal-superhydrophobic-leaf/
======
woodchuck64
"Is there a clear evolutionary advantage for these leaves to be
superhydrophobic?"

From:
[http://biomimetic.pbworks.com/f/The+dream+of+staying+cleanSo...](http://biomimetic.pbworks.com/f/The+dream+of+staying+cleanSolga.pdf)

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

[https://www.asme.org/engineering-
topics/articles/energy/self...](https://www.asme.org/engineering-
topics/articles/energy/self-cleaning-solar-panels-maximize-efficiency)

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aatish
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.

~~~
giarc
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
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.

~~~
giarc
The wax could also refract sunlight better.

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pdevr
This is something which has been known for thousands of years, unless I am
missing something:
[http://en.wikipedia.org/wiki/Lotus_effect](http://en.wikipedia.org/wiki/Lotus_effect)

Edit: The relevant Bhagavat Gita verse: [http://www.bhagavad-gita.us/bhagavad-
gita-5-10/](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.

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

~~~
Panoramix
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](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.hydrobead.com/)
[http://www.ultraeverdrystore.com/](http://www.ultraeverdrystore.com/)

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

~~~
nikster
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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devindotcom
Superomniphobic materials using this property are being made:

[http://pubs.acs.org/doi/abs/10.1021/ja310517s](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.

~~~
aatish
Thanks!

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chrisbennet
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.

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

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latortuga
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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jrkelly
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
> _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.

~~~
jrkelly
zero dollars.

~~~
dj-wonk
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...](http://www.sciencedirect.com/science/article/pii/S0921800902000915)

~~~
Panoramix
Elsevier wants me to pay $40 for this publicly funded work. Here's a direct
link:
[http://yosemite.epa.gov/sab/sabcvpess.nsf/0/1c7c986c372fa8d4...](http://yosemite.epa.gov/sab/sabcvpess.nsf/0/1c7c986c372fa8d485256e29004c7084/$file/howarth%20and%20farber.pdf)

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).

~~~
jp555
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.

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

~~~
lokki
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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peterwwillis
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...](http://wildfowl.wwt.org.uk/index.php/wildfowl/article/download/107/107)
[2]
[http://www.ncbi.nlm.nih.gov/pubmed/20437221](http://www.ncbi.nlm.nih.gov/pubmed/20437221)
(a modern paper confirming the results)

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

~~~
ultimatedelman
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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mrb
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-...](http://www.wired.com/wp-content/uploads/2014/06/plume-poppy-
contact-angle-mod.png)). They have the equipment. They should have done it!

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

~~~
lokki
_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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suprgeek
If recent science is correct
[http://www.nature.com/srep/2013/130912/srep02617/full/srep02...](http://www.nature.com/srep/2013/130912/srep02617/full/srep02617.html)
then we should be able to actually touch the leaf and "feel" the needles
ourselves.

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

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

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

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

------
arjn
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.

~~~
aatish
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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mplishka
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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usaphp
I would love to see more of these kind of articles here on HN

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asimpletune
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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spyder
It's called the "lotus effect" :

[http://en.wikipedia.org/wiki/Lotus_effect](http://en.wikipedia.org/wiki/Lotus_effect)

And it works even with honey:

[https://www.youtube.com/watch?v=LJtQ6dvcbOg](https://www.youtube.com/watch?v=LJtQ6dvcbOg)

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js2
I immediately thought of jewelweed which in addition to having water repellent
leaves is also medicinal -
[http://www.wildmanstevebrill.com/Plants.Folder/Jewelweed.htm...](http://www.wildmanstevebrill.com/Plants.Folder/Jewelweed.html)

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im3w1l
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](https://www.youtube.com/watch?v=VGUDapZ7WlQ&t=0m15s)

