Researchers at the Institute of Science and Technology Austria (ISTA) have made a significant breakthrough in acoustic levitation, successfully overcoming the challenge of “acoustic collapse” that previously hindered the levitation of multiple objects simultaneously. This advancement opens new avenues for applications in areas such as 3D printing, chemical synthesis, and micro-robotics.
In acoustic levitation, sound waves can cause small particles, typically ranging from tens of microns to millimetres, to hover in mid-air. While effective for individual objects, the technique often causes multiple particles to cluster together due to attractive forces created when sound waves scatter off their surfaces. The ISTA team, led by physicist Scott Waitukaitis, developed a method to maintain separation between these particles using a combination of acoustic and electrostatic forces.
Innovative Approach to Levitation
The researchers began their experiments by levitating a single microsphere made of silver-coated poly(methyl methacrylate) (PMMA), measuring between 250 and 300 micrometres in diameter, above a reflector plate coated with a transparent conductive layer of indium tin oxide (ITO). By applying a high-voltage direct current to the ITO plate while the acoustic field was turned off, they were able to charge the particle effectively.
This process involved estimating the charge based on Maxwell’s equations, which describe the behavior of electric fields. Once the particle was adequately charged, the team activated the acoustic field and, after just 10 milliseconds, introduced the electric field. This innovative setup allowed the particle to be propelled toward the centre of the levitation apparatus, where it remained stable.
The researchers found that this method not only worked for single particles but also allowed for efficient loading of multiple particles into the levitation trap. By finely tuning the amount of charge on each particle, they could control whether the particles remained separate or merged into a single object.
Dynamic Interactions Observed
One of the standout moments in the research occurred when the team noticed intriguing behavior in the hybrid structures formed by the levitated particles. Sue Shi, a PhD student and lead author of the paper published in the Proceedings of the National Academy of Sciences (PNAS), described the phenomenon as a “visually mesmerizing dance” of the particles. Compact parts of the structure began to rotate while the expanded components oscillated, responding to the rotation.
“This is the first time that such acoustically and electrostatically coupled interactions have been observed in an acoustically levitated system,” Shi stated.
The implications of these findings extend beyond mere visual spectacle. Shi noted that the technique could facilitate the study of non-reciprocal effects, enhancing the understanding of complex interactions among particles in various systems. This research represents a significant step toward unraveling the complexities of many-body interactions and could influence future advances in multiple scientific fields.
As the ISTA team continues to explore these dynamics, the potential applications of their work in materials science and micro-robotics remain promising. The ability to manipulate particles with precision using sound and electric fields could lead to novel approaches in technology and manufacturing.
The breakthrough achieved by the ISTA researchers not only highlights the innovative capabilities of acoustic levitation but also sets the stage for future exploration and application in diverse scientific disciplines.
