Can you make a ball float in the air by just shouting? It's not magic, it's the power of sound

No one touches the ball, but it stays in the air. Is this magic? No, it's the magic of ultrasonic levitation .

Arthur C. Clark, a famous science fiction writer, once said: "Any sufficiently advanced technology is indistinguishable from magic." Acoustic levitation technology is such a magical existence that anyone who has seen it will be amazed by its magic.

Ultrasonic levitated water droplets

(Image source: Argonne National Laboratory)

Part 1

It is not only magnetism that can make objects levitate, but also sound.

When it comes to suspension, you may think of the maglev train, which uses electromagnetic force to achieve non-contact suspension and guidance between the train and the track, thus significantly improving the speed. However, many people may not know that in addition to magnetic suspension, there is also an acoustic suspension technology.

In 1866, German physicist August Kundt discovered in a sound speed measurement experiment that dust particles in a reaction tube could happily float and dance under sound waves. This is the acoustic levitation phenomenon.

The acoustic radiation force generated by high-intensity sound waves acts on objects and balances the gravity of the objects, allowing the objects to hover in space for a long time without contacting the walls of the container. This is the acoustic levitation phenomenon , which avoids problems such as wear and pollution.

Compared with other suspension technologies, the main advantages of acoustic levitation are: few restrictions on the target suspended material; simple equipment foundation and low difficulty in implementation; strong suspension ability, etc.

A simple ultrasonic levitation device (left) and the distribution of the ultrasonic field in the levitation state

(Image source: Wikipedia)

Part 2

Unstable at high temperatures? One system solves it

In 2022, researchers from the Institute of Acoustics, Chinese Academy of Sciences developed an ultrasonic suspension system for measuring the physical properties of high-temperature objects.

To ensure that this system can suspend an object while performing laser heating on it, that is, to ensure that the object remains stable after being heated and undergoing a phase change (a common phase change is the change from solid to liquid to gas), it is necessary to comprehensively consider the effects of laser heating, sample phase change, and gas turbulence temperature field on the acoustic field. By combining gas lift and laser heating, the consistency of the transducer is achieved by designing the surface shape of the transducer and improving the transducer bandwidth. By optimizing the coupling parameters of the temperature field and acoustic field, the effects of high temperature, airflow velocity and size on the acoustic field are resolved, allowing the system to stably suspend high-temperature objects and self-adjust related parameters.

Schematic diagram of a stable acoustic suspension system with high temperature field and acoustic field coupling

(Image source: Institute of Acoustics, Chinese Academy of Sciences)

The core device of the system consists of three groups of six acoustic transducers facing each other, with a controller and control software to stably suspend metal balls. The system uses a focused laser to heat the balls in the ultrasonic suspension field and turn them into a molten state. After fully considering the impact of high temperature and airflow on the acoustic field, by adjusting the system's resonant distance, power and other parameters, high-temperature molten objects above 1800°C can be kept in a stable suspension state. At the same time, a low-frequency signal (less than 200Hz) is used to excite the droplets to measure their density, viscosity coefficient and other parameters, solving the problem of failure of conventional acoustic suspension measurement methods due to high temperature.

High temperature temperature field and acoustic field coupling to stabilize acoustic suspension system

(Image source: Institute of Acoustics, Chinese Academy of Sciences)

Ultrasonic levitation is a hot topic in the research of levitation technology. How to improve the levitation stability of the system, especially the stability of the system when high-temperature objects exist, is the focus and difficulty of the research. In the past, researchers have achieved the levitation of small particles, droplets and even small insects, but rarely levitated and oriented high-temperature objects.

Small creatures suspended by ultrasound

(Image source: Reference [1])

The ultrasonic suspension system developed by the Institute of Acoustics of the Chinese Academy of Sciences for measuring the physical properties of high-temperature objects combines the research on the optimal distribution and control of the acoustic and temperature fields, and uses laser heating, gas lift and other means to achieve non-contact stable suspension of objects with a density below 10,000 kg/m^3 (such as droplets, steel balls, copper balls, oxide metal balls) in a space of more than ten centimeters.

The system has received widespread attention in the fields of semiconductor manufacturing, micro-electromechanical systems, mechanical assembly, biochemistry, drug preparation, solid-state physics, ground-based and space experimental research, and has broad application prospects.

Suspended droplets

(Image source: Institute of Acoustics, Chinese Academy of Sciences)

Thanks to the efforts of scientists, the "magic" of acoustic levitation has demonstrated increasingly powerful power. I believe that acoustic levitation technology will exert greater "magic" in various fields in the future.

References

[1] Xie, WJ, et al. Acoustic method for levitation of small living animals[J]. Applied Physics Letters 89.21 (2006): 214102.

Produced by: Science Popularization China

Author: Wang Xiaozhen, Xu Delong (Ultrasonics Laboratory, Institute of Acoustics, Chinese Academy of Sciences)

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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