Researchers demonstrate acoustic levitation of a large sphere

August 12, 2016 by Lisa Zyga, feature
Acoustic levitation of a polystyrene sphere, the first spherical object to be acoustically levitated that is larger than the acoustic wavelength. Credit: Andrade et al. ©2016 AIP Publishing

When placed in an acoustic field, small objects experience a net force that can be used to levitate the objects in air. In a new study, researchers have experimentally demonstrated the acoustic levitation of a 50-mm (2-inch) solid polystyrene sphere using ultrasound—acoustic waves that are above the frequency of human hearing.

The demonstration is one of the first times that an object larger than the wavelength of the acoustic wave has been acoustically levitated. Previously, this has been achieved only for a few specific cases, such as wire-like and planar objects. In the new study, the levitated sphere is 3.6 times larger than the 14-mm acoustic wavelength used here.

The researchers, Marco Andrade and Julio Adamowski at the University of São Paulo in Brazil, along with Anne Bernassau at Heriot-Watt University in Edinburgh, UK, have published a paper on the demonstration in a recent issue of Applied Physics Letters.

"Acoustic levitation of small particles at the acoustic pressure nodes of a standing wave is well-known, but the maximum particle size that can be levitated at the pressure nodes is around one quarter of the acoustic wavelength," Andrade told "This means that, for a transducer operating at the ultrasonic range (frequency above 20 kHz), the maximum particle size that can be levitated is around 4 mm. In our paper, we demonstrate that we can combine multiple to levitate an object significantly larger than the acoustic wavelength. In our experiment, we could increase the maximum object size from one quarter of the wavelength to 50 mm, which is approximately 3.6 times the acoustic wavelength."

Although there are several different ways to acoustically levitate an object, most methods use an ultrasonic transducer, which converts electrical signals into ultrasonic waves. The current setup uses three ultrasonic transducers arranged in a tripod fashion around the sphere.

As the researchers explain, the angle and number of transducers can be changed, and this does not interfere with the setup's ability to levitate a large object. The ability to levitate the large sphere occurs because the three transducers produce a standing wave in the space between the transducers and the sphere. In previous methods, small objects are levitated by being trapped at the pressure nodes of the standing wave, but this is not the case here.

Demonstration of the acoustic levitation of a polystyrene sphere using ultrasound waves. Credit: Andrade et al. ©2016 AIP Publishing
"In a typical acoustic levitation device, a small particle can be levitated at the pressure node of a standing wave formed between the transducer and a reflector," Andrade explained. "Instead of trapping the object at the pressure node, the key to levitating an object larger than the acoustic wavelength is to generate a standing wave between the transducer and the object."

Although this strategy is based on earlier work, the tripod arrangement used here provides greater 3D stability.

"The idea of levitating an object larger than the acoustic wavelength was based on the paper of Zhao and Wallaschek," Andrade said. "In their paper, they showed that a large planar object can be levitated by generating a between the object and the transducer. However, their setup was only capable of providing vertical acoustic forces, and they used a central pin in contact with the object to prevent it from escaping laterally. By using three ultrasonic transducers in a tripod configuration, we obtain vertical and lateral acoustic forces. Consequently, we can levitate an object larger than the acoustic wavelength without any contact with external surfaces."

Using the new method, the researchers demonstrated that the sphere could be levitated to a height of about 7 mm, or approximately half the wavelength of the acoustic waves. They predict that the method can be used to levitate even larger spheres, and can also be extended to levitate objects of different shapes and sizes and at different positions.

"At the moment, we can only levitate the object at a fixed position in space," Andrade said. "In future work, we would like to develop new devices capable of levitating and manipulating large objects in air."

Acoustic levitation has applications for handling and manipulating various materials, such as very hot materials and liquid samples in space. In microgravity, the lower surface tension allows liquid droplets to reach larger sizes than they do on Earth, and acoustic levitation can be used to control and analyze these large liquid samples.

Explore further: Acoustic levitation made simple

More information: Marco A. B. Andrade et al. "Acoustic levitation of a large solid sphere." Applied Physics Letters. DOI: 10.1063/1.4959862

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2.3 / 5 (6) Aug 12, 2016
Interesting. I wonder how much power they are running? How loud would it be if it were in the human hearing range. (Can dogs stand to be in the room?)
5 / 5 (3) Aug 12, 2016
@carbon-unit: the paper referenced at the bottom of the article says "the transducer operated at a frequency of 25230 Hz with a displacement amplitude of 15 μm" and the acoustic pressure just below the sphere ranges from 1 to 8 kPa. A quick back-of-the-envelope (literally!) calculation tells me that this would be over 1000 dB (referenced to 20 μPa), which would not just be unpleasant but deafening. Do not try this at home :)
Those with more experience in acoustics may like to check that and correct me if necessary.
1 / 5 (7) Aug 12, 2016
Well, being ultrasonic it is WAY above the range of hearing where we would get to hear anything, you can see the person in the film setting the styrofoam ball into place with bare hands, So, you can deduce a few things, one, while a lot of energy is going into it, it is way faster than our ears can perceive, it did no damage to his hands nor, apparently, to the ball itself. I think it is a matter of very carefully timed sets of rotation of the vibratory pattern, an offset of each of the three transducers seen, and so it is held in place by the counteracting forces being manipulated much faster than it can respond with any noticeable effect. Very nice, and yes, much better than levitating droplets of liquids or various size and power of magnets.
2 / 5 (4) Aug 12, 2016
180 db is nearing physical dissolution, so it can't be much higher than 170-190db per ultrasonic horn. After all we are talking logarithmic calculations.

Most commercially available ultrasonic devices in the 25kHz range are about 700-1000 watt, so it again, can't be much above 160-180 These devices, the three horns... are going to be fed by standard commercial gear, in my estimation. which means that 700-1000 watts per horn.

The next problem is that they are designed to be terminated, so the ultrasonic power has a sink. Those horns are terminated in air, and we don't see them turned on for very long, probably due to that issue of backemf/resonance overheating.

Interesting.... but I don't see more than about $20k of standard industrial ultrasonic gear being re-applied... into being fun time gear, here.
5 / 5 (2) Aug 13, 2016
Acoustics is powerful stuff.
Aug 13, 2016
This comment has been removed by a moderator.
not rated yet Aug 13, 2016

Most commercially available ultrasonic devices in the 25kHz range are about 700-1000 watt, so it again, can't be much above 160-180 These devices, the three horns... are going to be fed by standard commercial gear, in my estimation. which means that 700-1000 watts per horn.

If it was making that much heat I would expect to see some kind of optical distortions. The roof of your car at most gets 1000 watts per meter square and you can definitely see the air shimmering over it.
Aug 14, 2016
This comment has been removed by a moderator.
1 / 5 (2) Aug 14, 2016
Since the video blinks the information too quickly for most humans.
sphere diameter: 50 mm
sphere mass: 1.46 g
transducer frequency: 25 kHz
transducer diameter: 20 mm
(Sphere is polystyrene - low density styrofoam)
1 / 5 (2) Aug 14, 2016
They could get better "lift" with shaped waveforms too. Like a Parenthesis shape '( ' hitting the sphere repeatedly, but I don't think their transducers have a bored cup at the end as they appear quite flat.

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