Scientists at the Public University of Navarra and the University of Bristol have developed an intriguing "sound tweezer" device that coordinates the tiny mechanical forces exerted by five hundred miniature speakers to levitate and manipulate dozens of small objects at the same time through nothing but thin air.

Just two months ago, 96-year-old Arthur Ashkin was awarded a Nobel Prize in Physics for his work on optical tweezers, which can harness the radiation pressure of light to manipulate microscopic objects. Since that initial discovery in 1970, scientists have managed to transfer the same principles to the realm of sound.

So far, we've seen sound tweezers used for an array of nifty but hardly practical applications, from home gadgets that levitate small water droplets to sonic tractor beams and even rudimentary levitating displays. The technology, however, could really come into its own when applied to surgical applications. In this area, it holds many crucial advantages over its optical counterpart: most importantly, it can easily operate through opaque materials (such as human organs) and it runs on much lower power, meaning there is little risk of damaging delicate living tissue.

Researcher Asier Marzo Pérez and professor Bruce Drinkwater have now taken an important step in this direction by developing a way to finely manipulate dozens of small, levitating particles independently using sound waves alone.

They did so by creating two arrays of 16 by 16 sound transducers (in essence, small speakers) that generate sound waves at a frequency of 40 kHz, outside the human hearing spectrum, and an algorithm that adjusts the output of each speaker 90 times a second to make the object move as needed. The result is a device that can simultaneously and independently control the movement of up to 25 small objects up to 2.5 cm (1 in) in diameter in a three-dimensional space.

To demonstrate the performance of their system, the scientists attached two small spheres to the ends of a thread and used their acoustic tweezers to "sew" the thread onto a piece of cloth. The team is confident that the technique could soon adapt to the handling of such tasks while submerged in water and, shortly after that, to use on biological tissue with the goal of allowing new means to perform noninvasive surgery.

Aside from medical applications, this advance could also be used to create a new form of highly interactive 3D displays where physical "pixels" move about to form evolving shapes and could be seen from any angle.

"We are accustomed to the two-dimensional pixels in our monitors, but we would like to see a technology where the objects are formed by tangible pixels that float in the middle of the air," said Marzo.

The advance is described in an open-access paper for the scientific journal Proceedings of the National Academy of Science. You can get a better feel for the degree of precision achieved (including a levitating "sewing" demonstration) by taking a look at the video below.

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