Robotic water strider is propelled by surface tension – and alcohol
You may think that water strider insects move across the water's surface simply by wiggling their legs, but they actually utilize what is known as the Maragoni effect. Scientists have now utilized the phenomenon in a tiny, silent, water-striding robot.
The Maragoni effect is defined as "the mass transfer along an interface between two fluids due to a gradient of the surface tension." Water striders harness it by secreting water-insoluble compounds called lipids, creating a surface tension imbalance that pulls them forward. Led by Asst. Prof. Hassan Masoud, a team at Michigan Technological University built a small robot that works in the same fashion.
The device sits on a side-by-side set of pontoon-like floats. Instead of using lipids for propulsion, though, it creates the surface tension imbalance via the controlled release of isopropyl alcohol.
The liquid is contained within the bot's cylindrical body, and is dispensed out of a horizontally swivelling nozzle in the rear. By remotely controlling the direction in which that nozzle is facing, it's possible to steer the robot in real time. Its speed is controlled by varying the alcohol's flow rate.
In its current incarnation, the robot has a top speed of about 100 mm per second, and a fuel efficiency of around 600 mm per milliliter of alcohol. The scientists are working on improving both of those figures, in hopes that the technology could one day find use in applications such as wildlife observation or environmental monitoring in difficult-to-access locations. It's even possible that autonomous "swarms" of the robots could work together on such tasks.
The robot was recently described in a paper published in the journal Bioinspiration & Biomimetics, and can be seen in action in the video below.
It should be noted that researchers at the Harbin Institute of Technology and Harvard University have previously designed water-strider-inspired robots, although they didn't utilize the Maragoni effect in the same fashion as the Michigan Tech bot.