Ever dreamed of defying gravity and making objects float in mid-air? Well, scientists at the Institute of Science and Technology Austria (ISTA) have done just that, but with a fascinating twist! Their groundbreaking work has overcome a major hurdle in acoustic levitation, opening doors to exciting new possibilities.
Initially, using sound waves to levitate tiny particles seemed promising, but there was a significant problem: the particles would inevitably clump together, like tiny magnets. This 'acoustic collapse' was a major roadblock. But now, ISTA physicists have found a clever solution: charge.
Back in 2013, Professor Scott Waitukaitis, now at ISTA, began exploring acoustic levitation. He recognized its potential beyond just applications in things like acoustic holograms. He envisioned using it for more fundamental research. His team started experimenting with controlling matter using sound waves.
The challenge? The 'acoustic collapse'. When multiple particles are levitated, the sound waves scattering off them create attractive forces, causing them to stick together. This is where the ISTA team's brilliance shines. They realized they could counteract this attraction with electrostatic repulsion. By charging the particles, they could keep them apart.
Sue Shi, a PhD student and lead author of the study, explained that the key was adding another force to counteract the collapse: electric charge and electrostatic repulsion. The team developed a method to charge the particles, allowing them to arrange them in various configurations. They could achieve completely separated systems, fully collapsed ones, and even 'hybrids' with both. They could also control the particles' arrangements by bouncing them off a charged reflector plate.
Collaborating with Carl Goodrich and Maximilian Hübl, the team developed simulations to understand these configurations, based on the balance between sound scattering and electrostatic forces.
But here's where it gets controversial... The team observed some unexpected and intriguing behaviors, hinting at 'non-reciprocal' interactions, which, in a way, seem to 'violate' Newton's third law. This law states that for every action, there's an equal and opposite reaction. However, in these experiments, particles exhibited spontaneous rotation and chased each other, seemingly defying this principle. Though, in reality, the extra momentum gained by the particles is lost to the sound.
Waitukaitis explains that by introducing electrostatic repulsion, they could maintain stable structures, finally providing a platform to investigate these subtle non-reciprocal effects.
From frustration to triumph! The team's method opens up new avenues in materials science, micro-robotics, and other fields that need dynamic structures from small building blocks.
Shi initially found the hybrid configurations and weird rotations frustrating. They prevented her from getting the clean crystalline structures she initially aimed for. However, presenting her results at conferences and seeing other scientists' excitement helped her appreciate them as a fascinating phenomenon in their own right.
So, what do you think? Are you amazed by this innovative approach to manipulating matter? Do you find the 'violation' of Newton's third law intriguing, or do you have a different interpretation? Share your thoughts in the comments below!
In a notable recognition of his work, Scott Waitukaitis was awarded the Early Career Award for Soft Matter Research by the American Physical Society in November 2026. This award recognized his work in resolving the core mystery of contact electrification and consistently bringing clarity and rigor to complex problems in soft matter through elegant and thoughtful experiments.
