Scientists at the Institute of Science and Technology Austria (ISTA) have successfully tackled a significant challenge in the field of acoustic levitation, allowing multiple objects to hover simultaneously without clumping together. This advancement could lead to revolutionary applications in areas such as 3D printing, mid-air chemical synthesis, and micro-robotics.
Acoustic levitation utilizes sound waves to lift small particles, typically ranging from tens of microns to millimetres in size, into the air. The process relies on the momentum transferred to these particles as sound waves bounce off them. While effective for individual particles, multiple particles often aggregate due to the attractive forces generated when they scatter sound waves. This phenomenon, known as “acoustic collapse,” has hindered progress in utilizing acoustic levitation for more complex applications.
The research team, led by physicist Scott Waitukaitis, discovered a solution by integrating a hybrid structure that combines attractive acoustic forces with repulsive electrostatic forces. This innovative approach allows for stable levitation of multiple particles while keeping them separated.
To demonstrate this technique, the researchers first levitated a single silver-coated poly(methyl methacrylate) (PMMA) microsphere, measuring between 250 and 300 μm in diameter, above a reflector plate coated with a transparent, conductive layer of indium tin oxide (ITO). They imparted a controlled electrical charge to the microsphere by placing it on the ITO plate while the acoustic field was off, applying a high-voltage direct current (DC) potential. This process created a capacitive charge build-up on the particle, which could be estimated using Maxwell’s equations.
After charging, the team activated the acoustic field and then introduced the electric field within a mere 10 milliseconds. This brief period allowed either field to propel the particle toward the center of the levitation setup. Once the electric field was switched off, the particle remained stably levitated, adhering to the charge calculated from previous approximations.
The method proved equally effective for multiple particles, enabling the researchers to load them into the trap with high efficiency and control the charge as needed, limited only by the breakdown voltage of the surrounding air. Remarkably, they found they could adjust the charge levels to maintain particles in separate positions or combine them into a single dense cluster. They could even create hybrid states, mixing both separated and collapsed particles.
The Dance of Particles
In an exciting development, Sue Shi, a PhD student at ISTA and the lead author of a paper published in PNAS, observed spontaneous rotation among the compact parts of the hybrid structures while the expanded sections remained stationary. Shi described the phenomenon as “a visually mesmerizing dance,” marking the first observation of such acoustically and electrostatically coupled interactions in an acoustically levitated system.
The implications of this research extend beyond the immediate applications in materials science and micro-robotics. Shi notes that the techniques developed could facilitate the study of non-reciprocal effects, which lead to the observed rotations and oscillations. Understanding these complex interactions could pave the way for advancements in various scientific fields.
As physicists continue to explore the potential of acoustic levitation, the work done at ISTA highlights the innovative approaches that may redefine how we manipulate materials in mid-air, opening up new avenues for research and technology.
