We've already seen sound waves being used to levitate small numbers of foam balls, drops of liquid and even bits of lab-grown cartilage. Scientists at the Universities of Sussex and Bristol, however, have recently taken things a step further. They've created a low-res "display" in which the pixels are actually physical, tangible spheres, that float and can be individually rotated in mid-air.

The technology is known as JOLED, and it utilizes an array of expanded polystyrene spheres/pixels that are coated in titanium dioxide to give them an electrostatic charge.

A series of acoustic transducers (basically miniature speakers) above and below that array create opposing ultrasound waves that are inaudible to humans, but that are forceful enough to hold the lightweight pixels in place in the air. By varying the intensity of those sound waves from speaker to speaker, it's possible to move either the whole array of pixels around, or to move individual spheres.

Rotation of the pixels is accomplished by manipulating an electric force field that surrounds the display – this is where the titanium dioxide coating comes in. By painting the sides of the pixels different colors, they can appear to change color as they rotate, making it possible to create constantly-changing dot-matrix-style patterns within the array.

Down the road, it is hoped that JOLED displays with much higher resolution could be used in places such as theme parks or art galleries.

"Traditionally, we think of pixels as tiny colour-changing squares that are embedded into our screens," says research associate Asier Marzo, of the University of Bristol. "JOLED breaks that preconception by showing physical pixels that float in mid-air. In the future we would like to see complex three-dimensional shapes made of touchable pixels that levitate in front of you."

There's more information on how JOLED works, in the following video.