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Discovery
Quenching the world's water and energy crises, one tiny droplet at a time

In pursuit of beetle biomimicry, NSF-funded engineers develop new, textured materials to trap and channel small amounts of liquid

A black beetle with water droplets on its back

A beetle in the Namib Desert of Africa drinks 12 percent of its body weight in fog each day.
Credit and Larger Version

July 24, 2014

In the Namib Desert of Africa, the fog-filled morning wind carries the drinking water for a small black beetle.

The beetle slakes its thirst by tilting its back end up and sipping from tiny droplets that collect on its textured back and slide down to fall into its mouth. Incredibly, the beetle gathers enough water through this method to drink 12 percent of its body weight each day.

Scientists surmise that the beetle's back may be made up of hydrophilic (water-loving) areas, and hydrophobic (water-fearing) surfaces that cause the water to accumulate or disperse.

More than a decade ago, news of this creature's efficient water collection system inspired engineers to try and reproduce these surfaces in the lab.

Advances in small-scale fluid physics, materials engineering and nanoscience since that time have brought them close to succeeding.

These tiny developments, however, have the prospect to lead to macroscale changes. Understanding how liquids interact with different materials can lead to more efficient, inexpensive processes and products, and might even lead to airplane wings impervious to ice and self-cleaning windows.

Emulating fog basking in the lab

Using various methods to create intricately patterned surfaces, engineers can make materials that attempt to closely mimic the beetle's back.

"Ten years ago no one had the ability to pattern surfaces like this at the nanoscale," says Sumanta Acharya, a National Science Foundation (NSF) program director. "We observed naturally hydrophobic surfaces like the lotus leaf for decades. But even if we understood it, what could we do about it?"

What researchers have done is create surfaces that so excel at repelling or attracting water they've added a "super" at the front of their description: superhydrophobic or superhydrophilic.

Many superhydrophobic surfaces created by chemical coatings are already in the marketplace (water-repellant shoes, shirts, iPhones!).

However, many researchers focus on materials with physical elements that make them superhydrophobic.

These materials have micro- or nano-sized pillars, poles or other structures that alter the angles at which water droplets contact their surface. These contact angles determine whether a water droplet beads up like a teeny crystal ball or relaxes a bit and rests on the surface like a spilled milkshake.

By varying the layout of these surfaces, researchers can now trap, direct and repulse small amounts of water for a variety of new purposes.

"We can now do things with fluids we only imagined before," says mechanical engineer Constantine Megaridis of the University of Illinois at Chicago. Megaridis and his team have two NSF grants from the Engineering Directorate's Division of Chemical, Bioengineering, Environmental and Transport Systems.

"The developments have enabled us to create devices--devices with the potential to help humanity--that do things much better than have ever been done before," he says.

Megaridis has used his bio-inspired designs to put precise, textured patterns on inexpensive materials, making microfluidic circuits.

Plastic strips with superhydrophilic centers and superhydrophobic surroundings that combine or separate fluids have the potential to serve as platforms for diagnostic tests (watch "The ride of the water droplets").

"Imagine you want to bring drops of blood or water or any liquid to a certain location," Megaridis explains. "Just like a highway, the road is the strip for the liquid to travel down, and it ends up collecting in a fluid storage tank on the surface." The storage tank could hold a reactive agent. Medical personnel could use the disposable strips to field-test water samples for E. coli, for example.

Devices such as these--created in engineering labs--are now working their way to the marketplace.

Water, water in the air

NBD Nanotechnologies, a Boston-based company funded by NSF's Small Business Technology Transfer program, aims to scale up the durability and functionality of surface coatings for industrial use.

One of the most impactful applications for superhydrophobic or hydrophobic research is improved condensation efficiency. When water vapor condenses to a liquid, it typically forms a film. That film is a barrier between the vapor and the surface, making it more difficult for other droplets to form. If that film can be prevented by whisking away droplets immediately after they condense--say, with a superhydrophobic surface--the rate of condensation increases.

Condensers are everywhere. They're in your refrigerator, car and air conditioner. More efficient condensation would let all this equipment function with less energy. Better efficiency is especially important in places where large-scale cooling is paramount, such as power plants.

"NBD makes more durable coatings that span large surface areas," says NBD Nanotechnologies senior scientist Sara Beaini. "Durability is an important factor, because when you're working on the micro level you depend on having a pristine surface structure. Any mechanical or chemical abrasion that distorts the surface structures can significantly reduce or eliminate the advantageous surface properties quickly."

NBD, which you might have guessed stands for Namib Beetle Design, has partnered with Megaridis and others to improve durability, the main challenge in commercializing superhydrophobic research. Power plant condensers with durable hydrophobic or superhydrophobic coatings could be more efficient. And with water and energy shortages looming, partnerships such as theirs that help to transfer this breakthrough from the lab to the outside world are increasingly valuable.

Other groups have applied hydrophobic patterning methods in clever ways.

Kripa Varanasi, mechanical engineer at the Massachusetts Institute of Technology and NSF Faculty Early Career Development CAREER awardee, has applied superhydrophobic coatings to metal, ceramics and glass, including the insides of ketchup bottles. Julie Crockett and Daniel Maynes at Brigham Young University developed extreme waterproofing by etching microscopic ridges or posts onto CD-sized wafers.

With all these cross-country efforts, many are optimistic for a future where people in dry areas can harvest fresh water from a morning wind, and lower their energy needs dramatically.

"If someone comes up with a really cheap solution, then applications are waiting," said Rajesh Mehta, NSF Small Business Innovation Research/Small Business Technology Transfer program director.

Correction: Aug. 19, 2014. This article has been modified from its original version to reflect ongoing research on the fog-basking beetle.

--  Sarah Bates, (703) 292-7738 sabates@nsf.gov

Investigators
Constantine Megaridis
Sara Beaini
Julie Crockett
Kripa Varanasi
Brent Webb
R Daniel Maynes

Related Institutions/Organizations
University of Illinois at Chicago
Iowa State University
Brigham Young University
NBD Nanotechnologies, Inc.
Massachusetts Institute of Technology

Locations
Ames , Iowa
Provo , Utah
Boston , Massachusetts
Chicago , Illinois

Related Programs
Fluid Dynamics
Thermal Transport Processes

Related Awards
#1235881 Convective Thermal Transport at Superhydrophobic Surfaces
#1066426 Investigation of icephobic behavior of surfaces with tunable properties
#1331817 STTR Phase I: STTR Proposal on Atmospheric Water Capture using Advanced Nanomaterials
#1066356 Turbulent Flow Drag Reduction Using Surfaces Exhibiting Superhydrophobicity and Riblets
#1235867 Collaborative Research: A Micropatterned Wettability Approach for Superior Boiling Heat Transfer Performance
#0952564 CAREER: Fundamental Studies of Condensation Phenomena on Heterogeneous and Hierarchical Nanoengineered Surfaces

Total Grants
$1,790,992

Related Websites
Water capture by desert beetle: http://1.usa.gov/1pftGp8
Ride of the water droplets: https://www.youtube.com/watch?v=UG4IMfH7I3E&feature=youtu.be
Professors' super waterproof surfaces cause water to bounce like a ball: http://news.byu.edu/archive14-may-superhydrophobic.aspx
Droplets break a theoretical time barrier on bouncing: http://newsoffice.mit.edu/2013/droplets-break-a-theoretical-time-barrier-on-bouncing-1120

Water droplets rolls across a water-repellant surface
If researchers perform some beetle biomimicry, that may mean a new source for water in dry areas.
Credit and Larger Version

Red and green colored water flow upward on plastic strips
Researchers can trap, direct and repulse small amounts of water for a variety of new purposes.
Credit and Larger Version

Green lotus leaf with water bouncing off of it.
Engineers look to naturally water-repellant surfaces like the lotus leaf for inspiration.
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Beads of water on a surface
Superhydrophobic coatings speed up the rate at which water vapor can condense on a surface.
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Red colored water droplets in a star pattern
As superhydrophobic materials become cheaper, their potential applications become more numerous.
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Water droplets travel up a tiny plastic strip
At the microscale, who needs a pump?
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Water droplets bouncing on different surfaces.
The less time water spends in contact with a cold surface, the less likely it is to freeze.
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Water droplets bouncing from two angles
Water-repellant materials make for longer-lasting solar panels and roofs
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Water droplets rolls across a water-repellant surface
Ten years ago no one had the ability to pattern surfaces like this at the nanoscale.
Credit and Larger Version

water droplets
View Video
Ride of the water droplets.
Credit and Larger Version



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