How does this beetle move along water?
When Manu Prakash was a graduate student, he would often search for his thoughts during hikes through the woods in western Massachusetts. On one of these excursions, he stopped by a pond to watch water lilies blossom, and noticed a series of small ripples flash across the water.
It was a perfectly still day; no wind, no rain. A few minutes of investigation revealed the culprit: a tiny insect, called the water lily beetle, which flits from lily pad to lily pad. Prakash scooped up a few of the beetles and took them home, and began studying them as they skittered across water-filled plates on his kitchen table.
Years later, Prakash's curiosity has paid off, as he and his students have discovered the remarkable physics of this beetle's flight.
"The surface tension forces are so large compared to this little thing, but it has the capacity to fly at half a meter per second on the surface of water without ever detaching from the water's surface," said Prakash, now an assistant professor of bioengineering at Stanford. "It's one of the fastest-known locomotion strategies on the surface of water."
Maintaining contact with the water is central to the beetle's ability to travel from lily pad to lily pad. But the unique strategy the beetle employs to essentially fly in two dimensions didn't become apparent until Prakash arrived at Stanford and began formalizing his kitchen experiments.
Prakash, along with a couple of summer interns, Thibaut Bardon and Dong Hyun Kim, painstakingly videoed the beetles as they darted here and there in small aquariums. The video clips caught the eager eye of Haripriya Mukundarajan, a mechanical engineering graduate student in Prakash's lab.
"Every time they presented their work, I just thought it was the coolest project in existence," she said. "The beetles look so beautiful on film, and this project is so rich with opportunity to do really fascinating research in so many fields, including fluid mechanics, biomechanics and non-linear dynamics, and synthesize them into an explanation of this biological phenomenon."
The videos revealed the beetle's unique technique. Once the bug settles on the water, it lifts each leg, one at a time, and carefully places them back so that only the tip comes into contact with the water's surface.
Next, it raises its two middle legs high above its body, unfurls its wings, and flaps in a figure-eight motion. As the wing-beat action settles into a stable frequency – 115 cycles per second – the insect rocks slightly, picks up steam and shoots across the water without ever losing contact with the surface.
But it required careful analysis of the video and the creation of mathematical models that incorporate all aspects of the insect's flight to unravel the complicated physics that make this possible. The researchers found that the preflight leg routine was crucial for minimizing friction with the water and for establishing stability. The research team's calculations also explained the invisible source of the ripples that first caught Prakash's interest.
The claws on the beetle's feet grip the water just enough so that when it beats its wings downward, the beetle actually pulls the surface of the water upward. Conversely, on the upstroke, the water pulls the beetle back down. Amazingly, the beetle is able to beat its wings at a consistent frequency even through all this jostling, and the repetitive up-and-down action is maintained as the beetle travels, creating an unexpected effect.
"This makes the insect feel as though it is on a pogo stick on the water. It makes for a fairly bumpy ride from the insect's perspective, but without this it will either fly off or sink," Mukundarajan said. "You don't usually associate bumpiness with water, but at that scale surface tension is so dominant in the system that it completely turns human intuition on its head. This has provided some fascinating insights into fluid mechanics."
This discovery, Prakash said, serves as a reminder that simply altering one's frame of mind can lead to startling findings. For instance, imagine shrinking yourself down to the millimeter scale. You haven't just shrunk down, the entire world around you is suddenly changed, and forces that you never think about all of the sudden matter, often in ways you wouldn't expect. A drop of water to a human is nothing, but the surface forces of that same drop could feel like superglue to a small creature.
"When you put yourself in the framework of animals that live in this planet, you suddenly realize the way they solve problems is fundamentally different from the way you and I would think about solving an engineering problem," he said. "That's just fascinating, because you suddenly stumble upon completely new and creative solutions."
This research is published in the Journal of Experimental Biology and was funded in part by a Howard Hughes Medical Institute International Student Research Fellowship, an NSF Career Award and a Pew Foundation Fellowship.