Researchers at the Institute of Science and Technology Austria (ISTA) have made significant advancements in the field of acoustic levitation by successfully levitating multiple objects simultaneously without them clumping together. This breakthrough addresses a longstanding challenge in the application of acoustic levitation, which typically sees small objects aggregate due to attractive forces generated during the levitation process.
Acoustic levitation utilizes sound waves to lift particles ranging from tens of microns to several millimeters in size. The key mechanism is the momentum transferred to these particles as sound waves scatter off their surfaces, generating an acoustic force that holds them aloft. However, when multiple particles are involved, the acoustic forces can inadvertently create attractive interactions, leading to what researchers term “acoustic collapse.”
The team, led by physicist Scott Waitukaitis, devised a solution by incorporating a repulsive electrostatic force that counteracts the attractive acoustic forces. This innovative approach enables the stable levitation of multiple particles while keeping them separate, which could have transformative implications for various fields, including 3D printing, micro-robotics, and chemical synthesis in mid-air.
To demonstrate this technique, the researchers first levitated a single silver-coated poly(methyl methacrylate) (PMMA) microsphere, measuring between 250 and 300 micrometers in diameter. They positioned the particle above a reflector plate covered with a transparent conductive layer of indium tin oxide (ITO). By applying a high-voltage direct current (DC) potential while the acoustic field was turned off, they charged the particle. This capacitive charge was calculated based on Maxwell’s solutions for conductive spheres, allowing the researchers to estimate the charge effectively.
After charging, the acoustic field was activated, followed by the introduction of the electric field within just 10 milliseconds. This combination enabled the particle to be launched towards the center of the levitation setup. Once the electric field was deactivated, the particle remained stably levitated within the trap.
The technique proved to be equally effective for multiple particles. The researchers were able to control the charge of the particles, allowing for high-efficiency loading into the trap and the ability to manipulate their interactions. This capability enabled them to either keep the particles separated or bring them together into a single dense object, even creating hybrid states that combined both configurations.
A particularly exciting moment for the team occurred when they observed the compact parts of the hybrid structures rotating while the expanded sections oscillated in response. Sue Shi, a PhD student at ISTA and lead author of a paper detailing the research published in the Proceedings of the National Academy of Sciences (PNAS), described this phenomenon as “a visually mesmerizing dance.” She noted that this was the first recorded instance of such acoustically and electrostatically coupled interactions in an acoustically levitated system.
The implications of this research extend beyond mere observation. According to Shi, the developed technique has the potential to facilitate the study of non-reciprocal effects that lead to particle rotation or oscillation. Such insights could significantly enhance the understanding of complex non-reciprocal forces and many-body interactions, influencing the behavior of various systems.
As physicists continue to explore the possibilities of acoustic levitation, this breakthrough marks a notable step forward, opening doors to new applications in materials science and micro-robotics. The ability to control and manipulate multiple particles in mid-air could lead to innovative advancements that reshape how we approach both scientific research and practical applications in various technology sectors.
