A new breakthrough in sound levitation

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

Sound waves, like invisible 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 research and industry, including the manipulation of delicate materials like biological cells.

Researchers from the University of Technology Sydney (UTS) and the University of New South Wales (UNSW) have recently demonstrated that to precisely control a particle using ultrasonic waves, consideration must be given to both the shape of the particle and how it affects the acoustic field. Their findings were recently published in the journal Physical examination letters.

Sound levitation occurs when sound waves interact and form a standing wave with nodes that can “trap” a particle. Gorkov’s fundamental theory of acoustophoresis, the current mathematical foundation of acoustic levitation, assumes that the trapped particle is a sphere.

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

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

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

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

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

“A better understanding of the specific structural dynamics of these natural objects – how they vibrate or resist forces – could enable the development of new, nature-inspired materials for use in industries such as construction, defense or development. of sensors. ”

Researchers have focused on understanding the mechanical properties of termite sensing organs in order to build and innovate hypersensitive vibration sensors. They recently identified structural details of the sub-sex organ, located in the leg of a termite, which can detect micro-vibrations.

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

“So the far-reaching application of this current theoretical research is to use non-contact analysis to extract new material principles to develop new acoustic materials.”

References:

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

“Low radiodensity μCT scans to reveal detailed morphology of the termite leg and its subgenal organ” by Travers M. Sansom, Sebastian Oberst, Adrian Richter, Joseph CS Lai, Mohammad Saadatfar, Manuela Nowotny, and Theodore A. Evans , July 8, 2022, Structure and development of arthropods.
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.