If you blow across the top of an empty bottle, you hear a clear, steady note. Engineers have now harnessed that same physical phenomenon—known as Helmholtz resonance—to power microscopic robots capable of movement without any onboard motors.
These devices operate entirely on sound energy. When exposed to the correct frequency, precisely shaped hollow chambers within their structure resonate, expelling focused jets of air that generate enough thrust to propel small boats or lift miniature aerial robots.
The breakthrough, detailed in the journal Science Advances on August 12, builds upon principles first investigated by German physicist Hermann von Helmholtz in 1856. Originally studying how sound could aid in tuning musical instruments, Helmholtz discovered that resonating air within enclosed cavities produces faint directional airflow. While barely noticeable in acoustic instruments due to low pressure, modern engineering allows this effect to be scaled up significantly.
“It isn’t very powerful when you do it with a musical instrument, because pressure is low,” explained study co-author Selman Sakar, an associate professor of mechanical engineering at EPFL. “But if you could increase the pressure, that jet becomes significant—and can be harnessed for functional machinery.”
By miniaturizing the resonators, researchers shifted the required activation frequencies into the ultrasonic range—beyond human hearing and easier to direct with precision. Traditional larger-scale systems would require dangerously loud audible sound levels to achieve comparable force output.
Employing advanced two-photon 3D printing techniques, the team fabricated hollow structures following Helmholtz’ original mathematical models. Laboratory testing and computational simulations confirmed these micro-devices produced thrust exactly as predicted.
The researchers engineered small boats measuring approximately 2 inches (5 centimeters) in length, each equipped with multiple resonators tuned to different frequencies and oriented in various directions. By adjusting the pitch emitted from nearby speakers, they successfully steered the vessels left, right, or forward.
At even smaller scales—just 0.04 inches (1 millimeter) across—the team developed “microfliers” that generate lift through two distinct mechanisms: some expel air downward like rockets, while others spin tiny attached blades similar to helicopter rotors, both powered exclusively by sound waves.
This approach offers a crucial advantage over conventional actuation methods: scalability. Traditional motors face fundamental miniaturization limits due to essential components like magnets, coils, and shafts. In contrast, these acoustic resonators consist solely of precisely engineered hollow cavities, presenting no inherent size constraints.
Looking ahead, the researchers envision applications extending beyond robotics, including contactless manipulation of small objects suspended in midair, and adaptive soft materials capable of bending and morphing when activated by specific frequencies—potentially revolutionizing biomedical devices such as responsive heart stents.
For now, this research establishes foundational design principles for future development. “This paper is significant in that we’ve outlined core design strategies,” noted Sakar, suggesting subsequent efforts might focus on refined applications, control systems, and navigational improvements.
Hwang, J., Angéloz, Q., Murugan, A. S., Lissek, H., & Sakar, M. S. (2026). Acoustic resonators as wireless actuators in air for small-scale robots. Science Advances, 12(33), eaef5620.
