A New Sound Levitation Breakthrough

The theory of acoustic levitation is extended by new research, which also highlights potential uses.

Sound waves, like an invisible pair of tweezers, can be used to levitate small objects in the air. Although DIY acoustic levitation kits are readily available online, the technology has important applications in both research and industry, including the manipulation of delicate materials like biological cells.

Researchers at the University of Technology Sydney (UTS) and the University of New South Wales (UNSW) have recently demonstrated that in order to precisely control a particle using ultrasonic waves, it is necessary to take into account both the shape of the particle and how this affects the acoustic field. Their findings were recently published in the journal Physical Review Letters. 

Sound levitation happens when sound waves interact and form a standing wave with nodes that can ‘trap’ a particle. Gorkov’s core theory of acoustophoresis, the current mathematical foundation for acoustic levitation, makes the assumption that the particle being trapped is a sphere.

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“Previous theoretical models have only considered symmetrical particles. We have extended the theory to account for asymmetrical particles, which is more applicable to real-world experience,” said lead author Dr. Shahrokh Sepehrirahnama from the Biogenic Dynamics Lab at the UTS Centre for Audio, Acoustics, and Vibration.

“Using a property called Willis coupling, we show that asymmetry changes the force and torque exerted on an object during levitation, and shifts the ‘trapping’ location. This knowledge can be used to precisely control or sort objects that are smaller than an ultrasound wavelength,” he said.

“In a broader sense, our proposed model based on shape and geometry will bring the two trending fields of non-contact ultrasonic manipulation and meta-materials (materials engineered to have a property not found in nature) closer together,” he added.

Associate Professor Sebastian Oberst, head of the Biogenic Dynamics Lab, said that the ability to precisely control tiny objects without touching them might enable researchers to explore the dynamic material properties of sensitive biological objects such as insect appendages, insect wings or ants, and termite legs.

“We know that insects have fascinating abilities – termites are extremely sensitive to vibrations and can communicate through this sense, ants can carry many times their body weight and resist significant forces, and the filigree structure of honey bee wings combines strength and flexibility.

“A better understanding of the specific structural dynamics of these natural objects – how they vibrate or resist forces – could allow for the development of new materials, based on inspiration from nature, for use in industries such as construction, defense, or sensor development.”

The researchers have been focused on trying to understand the mechanical properties of termite sensing organs in order to then build and innovate hyper-sensitive vibration sensors. They recently identified structural details of the subgenual organ, located in a termite’s leg, which can sense micro-vibrations.

“It is currently very difficult to assess the dynamic properties of these biological materials. We don’t even have the tools needed to hold them. Touching them can disrupt measurements and using non-contact lasers can cause damage,” Associate Professor Oberst said.

“So the far-reaching application of this current theoretical research is in using non-contact analysis to extract new material principles for developing novel acoustic materials.”

References: “Willis Coupling-Induced Acoustic Radiation Force and Torque Reversal” by Shahrokh Sepehrirahnama, Sebastian Oberst, Yan Kei Chiang and David A. Powell, 17 October 2022, Physical Review Letters.
DOI: 10.1103/PhysRevLett.129.174501

“Low radiodensity μCT scans to reveal detailed morphology of the termite leg and its subgenual organ” by Travers M. Sansom, Sebastian Oberst, Adrian Richter, Joseph C.S. Lai, Mohammad Saadatfar, Manuela Nowotny and Theodore A. Evans, 8 July 2022, Arthropod Structure & Development.
DOI: 10.1016/j.asd.2022.101191

The study was funded by the Australian Research Council. 

Other researchers who contributed to this study,  include Dr. David Powell from UNSW and Dr. Yan Kei Chiang from UNSW Canberra.