Decades ago, researchers described an intriguing natural phenomenon, which they charmingly called the “lotus effect.” Certain surfaces, like lotus leaves, which look smooth to our eyes but exceedingly rough under a microscope, repel liquid almost completely. Drip water onto the leaf, and it beads up and shimmies off, leaving the surface dry.
This property, known to engineers as “superhydrophobia,” is useful for myriad applications, from roof tiles to waterproof clothes, and James Bird and Samira Shiri wanted to understand it better. Bird, a Boston University College of Engineering (ENG) assistant professor of mechanical engineering, and Shiri (ENG’18), a PhD candidate in mechanical engineering, were curious about a specific property of superhydrophobic materials: heat exchange. Since liquid bounces off these surfaces so quickly, do droplets have time to leave any heat or cold behind? This matters because even a tiny amount of heat, or cold, can quickly add up and affect a material’s properties. Their findings, published online in the June 19, 2017, issue of Proceedings of the National Academy of Sciences (PNAS), found, surprisingly, that smaller drops transfer a larger fraction of their potential heat than larger drops, and that the material properties of the subsurface are important. The results held when they tested a naturally occurring material—bird feathers—opening the door to wider discussions on both ecology and bio-inspired design.
“This is fundamental work, where we’re scratching the surface in an area that we think is interesting,” says Bird, corresponding author on the PNAS paper. “When caught in the rain, people often seek shelter or put on a raincoat. Birds don’t have these options—especially when migrating—and can die of hypothermia. We’re trying to advance some new ways of thinking about heat transfer in these types of systems.”
The results have important engineering implications: spray coolants are useful for chilling electronics during manufacturing, for instance, but cold rain on airplane wings and power lines can turn to dangerous ice. The engineers’ work may lead to novel superhydrophobic materials that better control heat exchange, or even bio-inspired design that could yield warmer and more breathable outdoor wear.
“It’s most interesting for us to relate our findings to the real world,” says Shiri, lead author on the PNAS paper. “I think it’s the hardest part, and it’s the more interesting part. It’s always our goal.”
Shiri started the research using glass slides covered with a thin layer of soot, which provides a rough, water-repellant surface. She dripped single drops of water onto the slides, taking high-speed photos as the drops bounced off—10,000 frames per second (fps) with a normal camera and 200 fps with a thermal-imaging device. The photos, and many months of painstaking drips, allowed her to quantify the heat transfer of drops large and small, cooler and warmer. The amount of heat transferred, as expected, was small: on the order of millijoules. (A joule is the heat required to raise the temperature of one gram of water to .24°C; a millijoule is one-thousandth of that, or, in layman’s terms, a smidgen.)
These numbers were all well and good, but they raised more questions: How exactly did this smidgen of heat transfer through the soot to the glass? Was it something about the soot itself, or was the soot simply acting as a wick, transferring heat to the glass below? They decided to find out. The soot-as-wick model predicts that the bouncing waterdrop would transfer less heat if engineers replaced the glass substrate with an insulating material like rubber or wood. When they tried this experiment with rubber and wood, the hypothesis was confirmed—even under a layer of soot, material makes a difference.
They also found something odd: smaller drops transfer a larger fraction of their potential heat than larger drops, even though they touch the surface for a shorter time. The engineers found that drops transfer the most heat when they first hit a surface, then incrementally less and less over time.
Initial results in hand, the engineers decided to run tests on a naturally superhydrophobic surface—duck feathers—with streaming waterdrops to mimic a downpour. “If you’re caught in the rain, it’s not usually one drop. We’re not just talking about a millijoule of energy here. We’re talking about an aggregate,” says Bird. “This starts adding up.”
“Not all animals are superhydrophobic. Birds are one of a few that have this property,” adds Bird, who notes a “developing consensus” among scientists that animals may have evolved feathers not for flight, at first, but to prevent heat loss. To see if this idea held any water, so to speak, Shiri set up a waterdrop experiment using gray duck feathers ordered on Amazon. “In order to have a larger temperature difference between our feather and our drops, we decided to do the experiment on a cold day outside of the lab,” says Shiri, who set up her apparatus in the parking lot next to her lab with the temperature hovering around 4°C (39°F). To add more realism, Shiri heated the underside of the feather to mimic the duck’s body temperature, then dribbled the icy water onto it, simulating a miserable New England downpour. She found that the superhydrophobic feathers performed the same as the soot-covered glass. “We wanted to see if our finding about smaller drops was really true,” says Shiri. “And we observed that where we had the smaller drop impact, we had more temperature reduction in comparison with the larger one. That was kind of cool.”
Bird is quick to point out that neither he nor Shiri are ecologists—“I don’t know how useful our findings will be to ecology,” he says with a laugh. But he hopes that, at the very least, it may lead to some cross-disciplinary conversations and bio-inspired design.
“A lot of really interesting materials are bio-inspired,” says Bird. “Birds have had a long time to optimize feathers for a variety of uses, including thermal regulation. And if we want our tents or raincoats or outdoor gear to be more breathable while also warmer, then thinking about a natural case like a feather can help us guide those types of designs.”