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WATCH: “Acoustic tweezers” lift tiny ball off a board with sound


Futuristic it may seem, but the idea of using sound waves to levitate tiny objects is something we’ve seen demonstrated in a number of interesting ways. Scientists in Japan have now added another one to the list that might just bring this experimental technology a little closer to practical use, by demonstrating for the first time how it can lift small polystyrene balls off of a rigid surface.

In recent years, we’ve looked at various types of “sound tweezer” systems that use carefully directed sound waves to levitate water dropletslab-grown cartilage and even leverage “acoustic tractor beams” to lift up relatively large objects. All work by using a transducer to send sound waves down towards a concave reflector, which bounces them back upwards and causes them to meet up in the middle to form a new type of wave with sufficient pressure to hold small objects in place.

Scientists at Tokyo Metropolitan University sought to break new ground in the area by developing an acoustic tweezer system that can pick an object up off a rigid stage that reflects sound, in this case a wooden board. Doing so involved developing a novel hemispherical array of ultrasound transducers, and overcoming the technical hurdles associated with accurately controlling the varying sound waves in real-time in a way that allows levitation of small objects above the platform surface.

To tackle this complex problem, the team divided its hemispherical array of transducers into eight blocks and reversed the polarity of half in each block. The phase and amplitude of the sound waves emanating from each block was individually optimized, enabling the system to generate an acoustic trap at a very specific position, which can be shifted by altering the behavior of the individual blocks.

This was demonstrated through a series of simulations, and then through experiments using the array to lift and levitate a polystyrene ball just three mm (0.11 in) in diameter above the wooden board.

The technique wasn’t perfect, with the ball not lifting up every time and sometimes scattering away, but the scientists hope to improve on this as they work towards practical uses for the technology. In any case, this is the first demonstration of “non-contact pick-up of a particle on a rigid stage with reflection using a multi-channel-controlled hemispherical ultrasonic transducer array,” the researchers write in their paper.

That paper was published in the Japanese Journal of Applied Physics, while the video below shows this impressive system in action.

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