You are watching: Why does water roll off the surface of a leaf
This is a picture of a leaf from my backyard, blown up 4,270 times. This bed of ultra-microscopic wax needles, invisible to the naked eye, is the secret to its amazing water-repelling powers.Janine Nunes & Aatish Bhatia
I was heading out from home to get lunch, when I caught a glint of light out of the corner of my eye. I saw what looked like tiny drops of mercury, sitting on the leaves of a plant in my backyard.
Aatish BhatiaHuh. Those balls of mercury were really just very reflective drops of water. But something about this plant mesmerized me, and I stopped to take a closer look. The plant, by the way, is a plume poppy (Macleaya cordata). It's got these lovely fractalesque, large green leaves and is native to China, Japan, and Southeast Asia.
Those water drops are just so.. round. They're like tiny, glass marbles, gently sitting in place. Give the leaf the lightest flick, and they'll roll away.
Aatish BhatiaThat's not how water usually behaves. Water wets things. It clings to the surface and flattens out like a pancake. It doesn't roll around like a glass bead. This leaf must have some kind of natural water-repelling surface that prevents it from getting wet.
A couple of days later, I snapped off a leaf and brought it to my friend Janine Nunes. Janine is a postdoctoral researcher at Princeton University. She's a super-skilled researcher, and she also has access to some of the coolest toys in existence. Among these impressive devices is this Phantom ultra-high speed camera.
So how does this leaf repel water?
To understand this, __we first need to know what it means to get wet. __Since water molecules attract each other, a blob of water wants to shrink inwards. That's why a water blob floating in space is round, like a sphere (it's the most 'shrunken-in' shape). But down here on Earth, water isn't floating in mid-air. It's sitting on some surface, like your table, your bathtub, or a leaf. This surface pulls down on the water, and squishes the sphere into a pancake. So it looks more like this.
Aatish BhatiaIn fact, you can measure just how 'wettable' a surface is by calculating its contact angle.
Aatish BhatiaThe more a surface attracts the water, the more it squishes the ball into a pancake, and the wetter the surface.
Hydrophilic (water-loving) surfaces squish the water ball into a pancake.Aatish Bhatia
On the other hand, hydrophobic (water-hating) surfaces attract the water less, so the drop is more round.
And then you have superhydrophobic surfaces, like Never Wet, which barely attract the water at all. On these surfaces, water drops are almost spherical. It's nearly impossible to get these surfaces wet - the water just rolls off them.
Aatish BhatiaTo find out what's going on with our leaf, we need to measure its contact angle. Janine put a tiny drop of water on the leaf.
Janine Nunes & Aatish BhatiaBOOM. The contact angle works out to about 175 degrees. The leaf is extremely water repellent - it's superhydrophobic.
But how does a leaf become superhydrophobic? The trick to this, Janine explained, is that the water isn't really sitting on the surface. A superhydrophobic surface is a little like a bed of nails. The nails touch the water, but there are gaps in between them. So there's fewer points of contact, which means the surface can't tug on the water as much, and so the drop stays round.
To get superhydrophobic, a drop must lie on a surface that looks like a bed of nails.ZanderZ / Wikimedia Commons (Public Domain)
If this explanation is correct, then the surface of the plume poppy's leaf must be coated in tiny needles. To find out, we stuck one of these leaves inside an electron microscope (didn't I tell you she has access to the coolest toys? I wasn't lying.)
And, just as we expected, we saw this field of tiny wax needles, each needle just a few microns in length!
Janine Nunes & Aatish BhatiaHere's another look at these tiny spikes. You can see the ripples on the leaf behind it.
Janine Nunes & Aatish Bhatia
Zooming in further...
Janine Nunes & Aatish BhatiaThe water drops are suspended on these ultra-microscopic wax needles, and that keeps it from wetting the surface.
Next, we looked at the underside of this leaf with the microscope. We'd noticed earlier that the underside of the leaf was also superhydrophobic, and you could see it was covered with tiny hair-like filaments. But we were blown away with what we saw through the microscope.
Tiny hair-like filaments grow out from the veins under the leaf.Janine Nunes & Aatish Bhatia
Here's a closer look at those fibers.
Janine Nunes & Aatish Bhatia
At this scale, they look like claws reaching out from the veins. To give you a sense of scale, each of these fibers is about as thick as a regular human hair. Let's land on one of them.
Janine Nunes & Aatish BhatiaOnce again, you see a fine mesh of tiny, ultra-microscopic wax needles coating each of these fibers, each needle being only a few microns in length. These needles are way smaller than your eye can see. This ability to touch without really touching, by resting the water on a bed of nails, is the secret to the incredible water-repelling powers of this leaf.
There's one last thing I wanted to know. Why did this plant, and so many others, evolve this incredible ability to keep water at bay? One common explanation is that this allows the leaves to clean themselves. You see, as water rolls around on a superhydrophobic surface, it scoops up dirt and sand with it. Here's Janine demonstrating this neat self-cleaning property of the leaf with genuine Jersey Shore (TM) sand.
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However, I'm not sure that I buy this explanation. Why would a plant evolve a method that cleans the under-side of its leaves? Maybe it produces the wax for some other reason, and as an accidental benefit, this wax just happens to keep the leaves clean? Is there a clear evolutionary advantage for these leaves to be superhydrophobic? I don't know the answer, but I'd love to find out. If you have any leads, drop me a note in the comments.
Oh, and when something interesting catches the corner of your eye, don't forget to stop and check it out.
Update: There's a good discussion of this post brewing at Hacker News, with some thoughtful points about the benefits of being superhydrophobic.
A big thanks to Janine Nunes and to Howard Stone's lab at Princeton U. for indulging me with their time and sharing their equipment. This post wouldn't have been possible without their extensive resources and immense help.
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And a shoutout to my colleague Jaclyn and to Ed Moran for identifying the plant in my pictures.
(I used an ImageJ plugin called DropSnake to calculate the contact angle. It's freely available, and you can learn how to use it here.)