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
In a new study, researchers have demonstrated the acoustic levitation of a 50-mm (2″) solid polystyrene sphere using ultrasound—acoustic waves that are above the frequency of human hearing. It 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.
“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 1/4 of the acoustic wavelength,” Andrade said. “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 ultrasonic transducers to levitate an object significantly larger than 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 3 ultrasonic transducers arranged in a tripod fashion around the sphere. 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.
Although this strategy is based on earlier work, the tripod arrangement used here provides greater 3D stability.
“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.” The sphere could be levitated to a height of ~7 mm, or ~1/2 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.
http://phys.org/news/2016-08-acoustic-levitation-large-sphere.htmljCp https://www.youtube.com/watch?v=UNTi2MbeIEM
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