Researchers at EPFL’s MICROBS laboratory have developed small hollow structures that propel themselves using sound. Described in the journal Science Advances, this technique allows miniature boats and microdrones to move forward without a motor, gears, or magnetic components.
- EPFL researchers developed hollow structures that move using sound.
- The principle relies on Helmholtz resonance, without motors, gears, or magnets.
- Tuned cavities convert sound into thrust for miniature boats and microdrones.
- This approach could lead to smaller, lighter, sound-responsive robots.
The principle is inspired by a well-known phenomenon, Helmholtz resonance. When you blow across the neck of a bottle, the air inside oscillates back and forth, and certain frequencies greatly amplify this movement, producing the bottle’s familiar sound. The researchers used this physics to create hollow structures, shaped like a round cavity or a bell.
“Instead of pushing objects with sound waves, we created tuned acoustic resonators to capture sound at precise frequencies in order to generate directional thrust and controlled movement,” says Selman Sakar, head of the laboratory. “Our work demonstrates the feasibility of transforming a simple, cleverly designed mechanical part into robotic matter.”
Boats and Microdrones
While other approaches already used sound to levitate passive objects, the EPFL team designed devices that themselves convert sound into propulsive force. When waves excite the air inside a cavity, it is expelled as a concentrated jet, while the air entering is more diffuse. This imbalance creates thrust capable of moving small vehicles. The cavities can be made from various materials, including 3D printing plastics, rubber-like polymers, and glass.
On a centimeter scale, the researchers built miniature boats equipped with up to three cavities, each tuned to a different audible frequency. By changing the frequency emitted by a loudspeaker, they can selectively activate one cavity or another, steer the boats, make them navigate around obstacles, and even program them for autonomous navigation.
On a much smaller scale, the team used a nanometric 3D printing technique to create “microdrones,” ultralight flying vehicles with three microscopic cavities integrated into their polymer structure. Unlike the boats, these microdrones are powered by ultrasound, inaudible to the human ear. One version weighing only 150 micrograms uses its cavities to produce upward vertical thrust, like a rocket. Another combines the cavities with tiny blades, which spin up to 13,000 revolutions per minute, to generate stable helicopter-like lift.
Towards Sound-Sensitive Robotics
Since these devices rely on hollow cavities rather than motors, they can be made extremely small and light using various 3D printing methods. “Our concept is compatible with even further miniaturization, which will enable advanced designs that push the boundaries of robotics and aeronautics,” says Junsun Hwang, first author of the study and a PhD student at the MICROBS laboratory.
Selman Sakar also envisions, in the future, integrating several sound-sensitive structures into a single flexible device, each reacting to a different sound frequency. “This would allow specific parts of the device to move, bend, or vibrate, potentially leading to aerodynamic robotic devices capable of changing shape in response to sound,” he explains.
