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Why We Have Eyelashes

When his daughter was born, fluid dynamicist David Hu was astonished at the length of her eyelashes. The fact that the newborn was otherwise hairless prompted him to question why we have these hairs at the edge of our eyelids to begin with. No one had an answer.

So Hu and his colleagues at Georgia Institute of Technology in Atlanta set out to measure the eyelashes of 22 mammals. What they found was that the length of eyelashes is roughly one-third the width of the mammalian eye.

They then devised mockups of the eye by building a wind tunnel that could blow air past a cup of water with eyelashes on its rim. That length of one-third the eye’s width reduced evaporation of the cup’s water by a factor of two—essentially helping keep the eyes moist by reducing the airflow. Shorter lashes failed at properly blocking the air and longer ones actually directed more airflow to the eye.

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Similar circumstances—an incident where his infant son urinated on him during a diaper change—led Hu to discover that all animals weighing more than 3 kg (6.6 pounds) urinate for approximately 21 seconds, no matter their body size. That finding, published in the Proceedings of the National Academy of Sciences earned him a 2015 Ig Nobel prize, recognizing “research that makes people laugh and think.”

David Hu studies animals to learn how they have evolved to meet physical challenges. Here, he examines how mosquito wings react to dew. Image: Candler Hobbs

Get past the chuckle and the implications of the paper are profound: Nature uses gravity in a way to optimize a necessary task without wasting energy, a discovery that could advance water systems like fire hoses, water tanks, and water-filled backpacks.

Hu runs a biolocomotion laboratory at the Georgia Tech, where he is an associate professor of mechanical engineering and biology. For him, animals are the key to discovering new, physical ways of dealing with the world. They teach us how to accomplish difficult tasks that many life forms undertake very efficiently, like moving around, eating, drinking, storing and releasing waste, and keeping things clean.

“We try to identify the few real champions, animals that are really typifying optimal ways to do these processes,” Hu said.

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The idea falls in line with the principle popularized in 1929 by physiologist August Krogh. It states that for many tasks there will be one or more animals in nature that could be used as models.

“Evolutionary history provides a phenomenal and very long-duration testing ground for all kinds of environments and uses for systems,” said Sheila Patek, associate professor of biology at Duke University. “We see that revealed through the spectacular diversity of systems around us.”


Hu has been studying the intersection between engineering and natural systems for most of his career. His undergraduate advisor at Massachusetts Institute of Technology was Lakshminarayanan Mahadevan, a mechanical engineer who used math to describe natural processes. It was a time of renewed and growing interest in that field. In class, Hu studied termite mounds and plants, and professors demonstrated mechanical devices that were designed to move like fish.

But it wasn’t until after working for an oil company and returning to school for graduate studies in the early 2000s that Hu really began to focus on this field. It started as a homework assignment on how water striders walk on water for John W.M. Bush, professor of applied mathematics at MIT and an expert in surface tension.

Bush remembers Hu as playful and fun to work with.

“I also appreciated his resourcefulness and tenacity,” Bush said. “When I suggested the water strider problem to him, the first thing he did was to go hunting for water striders at a nearby pond.”

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Researchers believed that water striders were able to walk on water by making waves that help push them forward. That led to a mystery first identified by Stanford biologist Mark Denny: Young water striders couldn’t move their legs fast enough to create the waves necessary to walk on water. But they could, so how did they do it?

Bush, Hu, and mechanical engineering graduate student Brian Chan studied water striders in the lab using high-speed photography coupled with flow-visualization technology. They saw that water strider legs never actually break the water surface. Instead, they row across it.

“Once we had them [water striders] in the lab, we were able to quickly validate our view that they were shedding vortices with each leg stroke, thereby resolving a paradox in the biolocomotion literature,” Bush said.

Chan, with the help of Bush and Hu, then designed a robotic water strider, Robostrider, to mimic what they learned.

“I believe that it was the first non-buoyant water-walking device,” Bush said. “It has certainly spawned an entire generation of more sophisticated devices developed by engineers along the same lines, the utility of which remains to be seen.”

The three detailed the findings in a paper for Nature, and Hu turned it into his doctoral thesis.

At Georgia Tech, Hu is mostly focused on biomechanics of animal locomotion. A lot of his work is centered on everyday wonders that many might not stop to consider. Researchers use it to aid them in developing robots and other useful devices.

Hu has tried to popularize some of these findings in his book, How to Walk on Water and Climb up Walls: Animal Movements and the Robotics of the Future.

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But the down-to-earth focus of his research has drawn not just gentle ribbing, like the Ig Noble Prize, but outright scorn. Former United States Senator Jeff Flake, for instance, included three of his projects as part of a top 20 list of wasteful federally funded science undertakings.

Many scientists are likely to take issue with such criticism.

“There is also a long history of extraordinary discoveries from everyday phenomena and our surrounding biological world,” Patek said. “We wouldn’t be flying around in airplanes without that kind of initial fascination and inspiration. I’ve found that folks who make that argument often just aren’t aware of the necessary and integral interplay of basic and applied research.”


Hu responded directly to Flake via an Emory University TEDx talk. But his work and its forthcoming applications are perhaps the best response to this type of criticism.

Cats’ tongues are covered with bristles. Hu’s team used this insight to create a special hairbrush. Image: Georgia Tech

For instance, Hu, researcher Alexis Noel, and their colleagues recently looked at the structure of cat tongues, which are known for being sandpapery rough. The research team discovered grooves in the millimeter-tall spikes on the top of the tongues that enable them to pick up saliva. As a cat grooms, the saliva is spread through the fur to the skin, essentially washing both.

“People have looked at cat tongues before and no one observed they have this particular shape,” Hu said. He credits the use of 3D scanners and printers to spot the structures and verify their purpose.

Hu and his colleagues are now working on a patent for a cat-inspired hairbrush that would help pet-owners with allergies for whom there are currently few solutions.

“This hairbrush, designed based on a cat’s tongue, can remove some of the allergens on cat fur,” Hu said. “There’s no way we could’ve designed that unless we looked at animals.”

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A large personal care company has already expressed interest in the product.

Another research project involved studying the feeding habits of maggots in containers.

A startup founded by Georgia Tech students raises black soldier fly larvae, Hermetia illucens, to quickly consume food waste. The hope is that the larvae could consume some of the 1.3 billion tons of food waste produced worldwide each year, and that the well-fed larvae could then become high-protein feed for fish, chicken, and other livestock.

“It’s a really novel way to deal with the waste problem,” Hu said.

The problem is figuring out the optimal way to get the wasted food to the larvae. The larvae eat in five-minute bursts, and while they are feeding other hungry maggots are pushing to get a bite, much the way pigs jostle at the trough.

Just drop food in a bin of larvae and some will be well-fed while others will struggle. By observing their behavior and creating models that simulate the larvae eating habits, Hu hopes to better understand how to feed and mix them, turning maggot farm operation into a conveyor belt of insects so there won’t be any blockages, or traffic jams.

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His team is also studying the star-nosed mole, an almost completely blind mammal that has evolved a way to hunt underwater by sense of smell. The mole will breathe out a bubble—a bit like a four-year-old with a cold—and then inhale it back before it pinches off and floats away. The bubble captures chemical traces of the surrounding water and alerts the mole to any nearby prey.

Figuring out how the mole accomplishes this task could help technologists develop new types of underwater sensors, which are currently prone to biofilm formation, a surface growth of algae or bacteria that is especially common in ocean water.

“We believe that this underwater sniffing technique could circumvent the problem entirely,” said Alexander Bo Lee, a doctoral student in quantitative biosciences at Georgia Tech who works with Hu. “It would employ a gas sensor that never touches the water. Instead, a bubble would sniff in and out to probe the water for chemicals and transfer them to the sensor. Without any contact with the water, bacteria and algae can’t form biofilms on the gas sensor.”

The team suspected that the star shape of the mole’s nose was a key factor in the stability of created bubbles. They watched videos of the mole and noticed that it also tilts its body side to side when blowing bubbles. So they built and tested devices that look like a mole’s nose.

“The experiment involved mimicking the star-shaped geometry by laser-cutting plastic stars,” said Lee, who was the first author of a paper on the research that was published in Physical Review Fluids. “We then formed a bubble against the stars. We found that the fins of the star prevent the bubble from rising, but the gaps in between the fins play a role in preventing the bubble from sliding off the star when the entire system is tilted. Bubbles rising through the gaps of the star act as counterweights, keeping the bubble centered on the nozzle.”

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While that first set of experiments focused on static bubble stability, the team is now looking at the dynamics of oscillating bubbles using an “artificial nose” olfactory sensor with the star glued to the end. The hope is to learn what happens as the bubble is exhaled and inhaled at different rates and in bursts.

One of the big factors in how animals sense their environment through odor is that they don’t use continuing motion. “When you have dirty gym socks you don’t smell continuously, you exhale and inhale, in short bursts,” Hu said.

The goal of the star-and-nose experiment is to understand the fluid mechanical consequences of having this type of airflow, one that the mole is able to achieve under water.

For Hu, the world is one large catalogue of wonders, with inspiration at every corner. All one has to do is look, find an exemplary organism, mimic, and build upon its mechanisms.

“It is a quintessential human activity to see what other organisms can do and then wonder whether we, as humans, could build something new or achieve some new capability,” Patek said. “Biological systems have been around for much longer than humans and they reflect a massive ‘database,’ if you will, of inspiration.”

Sara Goudarzi is a Brooklyn-based writer.

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There is a long history of extraordinary discoveries from everyday phenomena and our surrounding biological world. We wouldn’t be flying around in airplanes without that kind of initial fascination and inspiration.Sheila Patek, Duke University

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