The RoboBee is a microrobot, smaller than a paperclip, that flies and hovers like an insect, flapping its tiny, nearly invisible wings 120 times per second.

In order to make the transition from air to water, the researchers first had to solve the problem of surface tension.

The robot is so small and lightweight that it cannot break the surface tension of the water.

To overcome this hurdle, the RoboBee hovers over the water at an angle, momentarily switches off its wings, and crashes unceremoniously into the water in order to sink.

Next the team had to account for water’s increased density. “Water is almost 1,000 times denser than air and would snap the wing off the RoboBee if we didn’t adjust its flapping speed,” said team member Farrell Helbling, a graduate student at the Harvard John A. Paulson School of Engineering and Applied Science.

The scientists lowered the wing speed from 120 flaps per second to nine but kept the flapping mechanisms and hinge design the same.

A swimming RoboBee changes its direction by adjusting the stroke angle of the wings, the same way it does in air. Like a flying version, it is still tethered to a power source.

The researchers prevented the robot from shorting by using deionized water and coating the electrical connections with glue.

While this robot can move seamlessly from air to water, it cannot yet transition from water to air because it can’t generate enough lift without snapping one of its wings.

“Solving that design challenge is the next phase of the research,” said team member Farrell Helbling, also of the Harvard John A. Paulson School of Engineering and Applied Science.

“What is really exciting about this research is that our analysis of flapping-wing locomotion is not limited to insect-scaled vehicles.”

“From millimeter-scaled insects to meter-scaled fishes and birds, flapping locomotion spans a range of sizes. This strategy has the potential to be adapted to larger aerial-aquatic robotic designs.”

“Bioinspired robots, such as the RoboBee, are invaluable tools for a host of interesting experiments – in this case on the fluid mechanics of flapping foils in different fluids,” said Prof. Robert J. Wood of the Harvard John A. Paulson School of Engineering and Applied Science.

“This is all enabled by the ability to construct complex devices that faithfully recreate some of the features of organisms of interest.”