A Rhagovelia water strider showing off its fans. Credit: Victor Ortega-Jimenez/UC Berkeley
Rhagovelia, also known as ripple bugs, are a semiaquatic genus of water strider roughly the size of a grain of rice. They make the highly dynamic, wavy and turbulent waters of natural streams their home, and have evolved an ingenious way of navigating them.
“It’s as if Rhagovelia have tiny wings attached to their legs, like the Greek god Hermes,” says Dr Victor Ortega-Jimenez, an integrative biologist at the University of California, Berkeley in the US.
Ortega-Jiminez is co-lead author of a new study, published in the journal Science, which found that the wing-like fans on Rhagovelia feet propels them across fast-moving streams without relying on energy produced by muscles.
The research shows that this motion happens passively due to surface tension and elastic forces.
The insect’s feather-like feet automatically fan out when they enter water, creating thrust, and collapse when pulled out, minimising drag.
“Observing for the first time an isolated fan passively expanding almost instantaneously upon contact with a water droplet was entirely unexpected,” says Ortega-Jimenez.
The motion works like an oar. It allows Rhagovelia to skitter across the surface of the water at speeds of up to 120 body lengths per second and execute full body turns in just 50 milliseconds.
“They literally row day and night throughout their lifespan, only pausing to moult, mate or feed,” says Ortega-Jimenez, who knows this because he monitored the bugs in the lab for 24-hours straight.
Rhagovelia in Mexico. Credit: © Leonardo Hernández Escudero, some rights reserved (CC-BY-NC https://creativecommons.org/licenses/by-nc/4.0/)
The team mimicked the flat, ribbon-shaped fans to develop an insect-scale microrobot which can turn, accelerate and brake on the surface of water while consuming minimal energy.
“We initially designed various types of cylindrical-shaped fans,” says Dongjin Kim, a postdoctoral researcher at Ajou University in South Korea and co-lead author of the study.
“However, the functional duality of the fan – rigidity for thrust generation and flexible for collapsibility – could not be achieved with cylindrical structures. After numerous attempts, we overcame this challenge by designing a flat-ribbon shaped fan.
“We strongly suspected that biological fans might share a similar morphology [shape] and eventually discovered that the Rhagovelia fan indeed possess a flat-ribbon micro architecture, which had not been previously reported. This discovery further validated the design principle behind our artificial flat-ribbon fan.”
The 1mg elastocapillary fan consists of 21 flat, ribbon-shaped barbs that mimic the natural fan’s structure. It was integrated into an insect-sized robot (0.23g) to allow it to turn, accelerate and brake while consuming minimal energy.
“The Rhagobot … can travel quickly along a flowing stream thanks to its intelligent fan structure, which is powered by surface tension and the drag forces from the water surface,” says Professor Je-sung Koh, a senior author of the study, from Ajou University.
Professor Je-sung Koh, also a senior author of the study from Ajou University, adds: “It is a form of mechanical embedded intelligence refined by nature through millions of years of evolution. In small-scale robotics, these kinds of efficient and unique mechanisms would be a key enabling technology for overcoming limits in miniaturisation of conventional robots.”
The findings have wide-ranging implications for the design of bioinspired robots capable of navigating challenging environments like rivers, wetlands, or flooded urban areas for applications such as environmental monitoring and search-and-rescue.