Physicists at the Institute of Science and Technology Austria (ISTA), led by Assistant Professor Scott Waitukaitis, have demonstrated a practical way to overcome one of the best-known limits in acoustic levitation. The technique normally works well for a single floating particle but breaks down once several particles are involved, as they quickly pull together into a tight cluster. By introducing a controllable source of electrostatic charge, the team has shown that it is possible to keep multiple particles separated in mid-air, opening a path to experiments and applications that were previously out of reach.
Shi, S., Hübl, M. C., Grosjean, G., Goodrich, C. P., & Waitukaitis, S. (2025). Electrostatics overcome acoustic collapse to assemble, adapt, and activate levitated matter. Proceedings of the National Academy of Sciences, 122(50). https://doi.org/10.1073/pnas.2516865122
Acoustic levitation uses focused sound waves to suspend small objects without physical contact. It has been around for decades and is used for studying droplets, creating acoustic holograms, assembling materials, and exploring non-contact manipulation at small scales. But when more than one particle is introduced into the levitation field, the sound waves reflected from each particle create attractive interactions. These forces cause the group to collapse inward, a behavior sometimes referred to as acoustic collapse. For researchers attempting to study particle interactions or assemble stable structures in mid-air, this collapse has been a persistent obstacle.
Assistant Professor Scott Waitukaitis from Institute of Science and Technology Austria (ISTA) stated,
“At first, it was frustrating to see these hybrid configurations and weird rotations and dynamics, they were preventing me from getting the clean, stable crystalline structures I was aiming for”.
Waitukaitis’s group began working with levitation over a decade ago with the goal of using sound as a tool for studying how small objects interact. Once the lab at ISTA was established, the team started building experiments to explore how levitated particles might form organized arrangements. The initial idea was to create crystalline patterns, but the particles always drew together faster than they could be positioned. According to several reports, the researchers were not primarily trying to solve the collapse problem at first. The breakthrough came once they began considering whether another physical force could counteract the sound-driven attraction.
The solution turned out to be electrostatic repulsion. By giving the particles a controlled charge, the team created an additional force that could push them apart strongly enough to balance the acoustic attraction. The work was led experimentally by Ph.D. researcher Sue Shi, who developed a method for charging the particles and the reflective plate beneath them. With the right charge levels, the researchers could produce fully separated particle arrangements, completely collapsed ones, and intermediate configurations containing both clustered and isolated components.
The group also found that they could manipulate the system dynamically. By briefly bouncing the particles off the charged reflector plate, they could switch the arrangement from one configuration to another. In collaboration with ISTA’s Carl Goodrich and Ph.D. student Maximilian Hübl, the researchers built simulations showing that all of these arrangements arise from the balance between sound scattering and electrostatic repulsion.
Once the collapse problem was controlled, the team began to observe behaviors that had been predicted in previous theoretical studies but were not measurable experimentally. Several particle arrangements began to rotate on their own, and in some cases, pairs of particles appeared to chase each other. These effects highlight what physicists refer to as non-reciprocal interactions, in which the force that particle A exerts on particle B is not the mirror of the force that B exerts on A. Newton’s third law is not violated in the conventional sense because momentum is exchanged with the sound field, but the effective interaction between particles no longer behaves like a standard reciprocal system.
These observations had been anticipated in the literature, but experimental verification was not possible as long as levitated particles collapsed into a single clump. With the introduction of electrostatic stabilization, researchers now have a platform where these subtle effects can be directly examined. The ISTA team has already begun using the method to study these interactions in greater detail. What originally appeared as an obstacle—irregular hybrid structures, drifting rotations and unexpected transitions between states—has become a research direction in its own right.
The ability to maintain well-separated particles in mid-air could influence work in materials science, micro-robotics, contact-free assembly, and studies of dynamic systems where conventional mechanical supports interfere with measurements. Acoustic levitation is valued for allowing objects to be manipulated without containers or probes, and this development expands the range of structures that can be built and controlled in that environment.
Like many experimental advances, the result emerged gradually rather than through a single insight. As Shi has noted in several discussions of the work, the most interesting behaviors were the ones that appeared to undermine the original goal of forming clean crystalline arrangements. Over time, however, these anomalies revealed themselves as features of a system that had never before been accessible. With electrostatic control now providing a way to keep levitated objects apart, the field gains a new tool for exploring how small particles behave when forces, sound fields, and motion interact in non-standard ways.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
