Scientists levitate objects with sound

La lévitation acoustique peut aussi se faire à la maison, avec des haut-parleurs suffisamment puissants ! © Forance, Fotolia

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[EN VIDÉO] Superconductivity: The Secrets of Quantum Levitation It’s hard not to be fascinated by superconductivity. This quantum property, which, among other things, levitates objects, is today at the center of a large number of cutting-edge research. Here is a video overview of the most beautiful quantum levitations. The first “acoustic clamp” was produced in 2016 by CNRS researchers: a complex system based on sound waves, made up of emitters and reflectors, that manages to trap a particle and lift it up! This device was recreated all over the world and then improved many times. Unlike acoustic levitation, which holds an object in the air previously placed where it should levitate, this technique causes the object to fly off a reflective surface. Although they had already manufactured such a device last year, this time the researchers at the University of Tokyo decided to improve its stability, more precisely that of the particle once it is lifted. The details of his project were presented in a study, published in the Japanese Journal of Applied Physics. Perfection of the device called “acoustic clamp” for the manipulation of objects without contact. © TMU, YouTubeThe acoustic clamp, an evolution of acoustic levitation Their acoustic clamp device is based on the use of ultrasonic transducers, i.e. devices that convert an input signal into ultrasound. For the 180 transducers used, the output frequencies were 40 kHz. At this frequency, the human ear is no longer able to hear the signal. The assembly was placed in the form of a hemispherical network, with the aim of surrounding the target particle. But how can sound waves “trap” a particle? Everything is based on acoustic radiation pressure: sound waves exert pressure on the medium in which they propagate, using either several sources of the same frequency that combine or reflect, or a single one that reflects, it is possible to generate what is called a stationary wave: a wave that contains points called “nodes”, for which the amplitude remains constant over time, and between these nodes a “belly” where, conversely, the amplitude varies over time. At the level of the nodes, the amplitude does not vary. Thus, by adjusting it so that the force exerted by the waves compensates for gravity, it is possible to immobilize a particle at the level of the acoustic pressure nodes. It’s acoustic levitation! For the acoustic clamp, it’s more complicated. The emitters must be tuned so that the particle moves, using a continuous variation of the emission frequency. “A particle levitates at the nodes of a standing wave. Therefore, changing the signal frequency of the transducers allows the transfer of a particle,” describe the researchers. So the nodes move slowly and the levitating particle will follow these nodes. However, the movement takes place unilaterally. Two modes, “in phase” and “out of phase”, to bring the particle to its destination The acoustic clamp developed by the researchers is based on a hemispherical network of ultrasonic transducers. The manipulated particles are of the order of a millimeter, and they move following the acoustic pressure field created by this network. But within that field, the particle oscillated until they upgraded their acoustic tweezers: “A major concern when picking up objects using the acoustic radiation force is the reflection effect caused by scene. Because the sound field is altered by this reflection, it is not possible to keep the object stable,” the researchers explain. A problem the scientists solved by “locking and controlling the phase and amplitude of the excitation of a network of transducers”. Specifically, they use “adaptive switching between in-phase and opposite-phase excitations”. The particle is first captured from the ground by phase excitation, not all transducers are used. The shift to the center is then performed in an out-of-phase (opposed-phase) mode, this time of the entire network, and then maintained using the full network, always in phase opposition. The signal thus created allows the particle to be gently lifted, then moved and held in a central position. This type of device could have applications in several fields, particularly in those where the manipulation of components could be done without contact: electronics, or even chemistry. This experience can also be reproduced more easily in space, where gravity no longer has to be compensated for.Interested in what you just read?
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