What is the Lotus Effect?

Foster Lyons Building Science: What building science can learn from a self-cleaning leaf

Editor’s note: I recently received several blog posts from building scientist and writer Foster Lyons. These were originally intended to be republished on the Green Building Advisor website, but circumstances prevented that from happening. The pieces are far too interesting to go unread anywhere beyond Foster’s business site, so I’ll be sharing them here on Northern Built from time to time. I find them both educational and fascinating, and I hope you enjoy them as much as I do.

Nelumbo nucifera, also known as Indian lotus, sacred lotus, bean of India, Egyptian bean or simply lotus, is an aquatic plant with beautiful flowers. It is native to tropical Asia and northern Australia. It is the national flower of both India and Vietnam. Generally, it grows in wet, muddy, dirty places.

Culturally, it is sacred to both Hindus and Buddhists. Historically, it has been viewed as an example of divine beauty and since it grows up out of the mud, it is taken to represent spiritual promise.

But, most interesting of all, its leaves don’t get wet and they’re always clean! They so don’t get wet that people have been writing about it for hundreds of years. For example, in the Gita, written back when they still taught cursive in grade school, you can find the following passage: “One who performs his duty without attachment, surrendering the results unto the Supreme Lord, is unaffected by sinful action, as the lotus is untouched by water.”

How cool is that? A leaf that lives in the water but never gets wet and cleans itself! Imagine if we could make roofing material or clothing or paint or bricks like that. Then guys like me could sleep late, eat more bacon and go home early every day. Well, it turns out we can make stuff like that.  More on that another day—like a summer day, in the evening, on a porch with grilled steak and wine.

Why Water Sits on Some Surfaces and Spreads on Others

Here’s what’s going on. The surface of the lotus leaf has a unique chemistry and is not flat or smooth—but only if you look at it at under tremendous magnification. That combination of features is the magic that allows the lotus leaf to never get wet and always stay clean.

Here’s what a drop of water on a typical surface looks like:

It just lays there a little spread out and sort of flat. It takes that shape because water is wet. Duh!  What that means is that the water is a little sticky with the surface and gets spread out as it sticks. There is an attractive force between the water molecules and the countertop surface molecules. The countertop is a little hydrophilic, or water loving.

At the top of the droplet, the water molecules grab onto each other like friends squeezing into a group selfie—everyone pulling tight to stay in the frame. That little huddle creates a stretched, springy skin across the surface of the drop.

When the force of hydraulic pressure caused by gravity matches the opposite force of the surface tension at the outside edge of the drop (which is really a combination of the drops surface tension and the waters adhesion to the countertop) then the drop stops spreading. For most every-day surfaces that means the water drop or spill will be about 1/16-in. tall or deep. Typical hydrostatic pressure of a drop of water on a surface is 1/16-in. of water column. For guys like me that think about building envelope physics and mortar chemistry all day and night this number is a big deal.

What Happens on a Lotus Leaf

Here’s what a drop of water on a lotus leaf looks like:

It’s almost a perfect sphere. Not spread out at all. WTF? The attraction between the leaf surface and the water is never as great as the attraction between the water molecules themselves. The adhesive forces between the water surface and the leaf surface are less than the cohesive forces between the water molecules on the drop’s surface. That’s because the chemistry of the surface is hydrophobic, or water fearing. It’s made up of wax crystal bumps that are tens to hundreds of nanometers in size. Wax and water don’t mix; there is no attractive force between them. Think of surfboard wax or ski wax. And that’s why the lotus leaf doesn’t get wet.

Building scientists and uber-cool architects think of surfaces and water like this:

It’s all about contact angle. When the contact angle is greater than 150 degrees the surface gets a special name: super- or hyper-hydrophobic. Lotus leaves are super-hydrophobic.

How the Lotus Cleans Itself

The “doesn’t get dirty” thing is something else. But first, a little side bar to scale:

• A nanometer is one billionth of a meter.
• Your fingernails grew about one nanometer while you read that sentence.
• A sheet of paper is about 100,000 nanometers thick.
• A raindrop is roughly a million nanometers across.
• A water molecule is about 0.1 nanometers.
• Liquid water can’t squeeze through pores smaller than ~50 nanometers. (And that, of course, is how Tyvek house wrap works—individual water vapor molecules fit through, liquid water does not. Liquids aren’t really individual molecules all close to each other; they are clumps of loose and limber crystals. Sort of. Gases are individual molecules. Usually.)

As Kanye West and Jay Z said “That s#*t crazy”!

Here’s a photo of the surface of a lotus leaf from an electron microscope:

All those bumps at the microscopic level of the leaf’s surface are of a certain size: the waxy bumps are about 1 nanometer; the regular lumps and bumps are about 5,000 to 10,000 nanometers in height and they are set about 10,000 to 15,000 nanometers apart. It turns out this is the perfect arrangement for keeping specs of dirt, dust, pollen, etc. sitting on the tops of all the bumps. All those things are about 10,000 to 1,000,000 nanometers across and too big to fall into the cracks and valleys, they just perch on top of the bumps barely contacting the leaf.

Next thing to know: dirt, dust, pollen, etc. stick well to water. All those things are usually hydrophilic; if they weren’t life probably would not have survived on Earth.

So, dirt falls onto a lotus leaf and just sits there on top of the bumps. Then it rains, and those water droplets roll around on the leaf not sticking to it but sticking excellently to the dirt specs. The dirt gets into the water droplet, and the water droplet rolls off the leaf. That’s the lotus effect.

Of course, if it rained bacon grease it would totally stick to the leaf. The waxy stuff on the leaf would be chemically attracted to the fat in the grease. But that’s a discussion for another day.

Final Thought

The lotus spent millions of years perfecting something we’re just beginning to mimic. Nature already solved water management and dirt resistance at the molecular scale. We just have to pay attention.

And maybe someday, because of lotus-inspired building materials, building scientists really will get to go home early.

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Foster Lyons is an engineer with over thirty years of experience in the Architecture-Engineering-Construction industry. During his career Foster has provided educational training for thousands of architects, engineers and contractors on numerous technical building topics and has been a featured speaker at many regional and national conferences. Foster Lyons Building Science