Object levitation enables new types of display where the content is created from physical particles in air instead of pixels on a surface. Several technologies have been developed to enable levitation, including magnetic levitation and acoustic levitation. The Levitate project focuses on acoustic levitation, but my Pervasive Displays 2018 paper  considers all types of levitation and the novel display capabilities they allow. This synopsis outlines some of those capabilities.
Interacting with Levitating Particle Displays
Voxels in an invisible volume
Levitating objects are held in mid-air, e.g. using sound waves or magnetic forces. This means that unlike most shape-changing displays, the display elements exist within an invisible volume. An invisible volume can allow new interactions. Multiple users around a levitating particle display can view the content but also see each other, potentially improving collaborative interactions. Users can also see surfaces behind and beneath the levitating objects, allowing levitation to augment existing interactive displays.
Reaching into the display
Because of the invisible display volume, it is often possible for users to reach inside the display. With acoustic levitation, this may disrupt the sound waves, but users can still reach in to a certain extent. Other objects may also be placed within a levitating particle display, so long as they are transparent to the levitation forces. With acoustic levitation, this means the objects must allow sound waves to pass through them. Being able to reach into the display and manipulate the display elements, and being able to place objects into the display, could enable new interactions and applications that would not be possible with traditional screens.
Whilst levitating objects are typically quite simple (e.g., small polystyrene beads in acoustic levitation systems), they can be manipulated in order to allow greater expressivity. For example, in Point-and-Shake , we used expressive object movement as a simple but effective means of feedback about selection gestures. In the Pervasive Displays paper  we discuss more about the expressive potential of levitating particles.
Colocated non-visual feedback
Acoustic levitation is based on similar techniques to ultrasound haptics and parametric audio. These techniques could potentially be combined to allow colocated non-visual feedback. Levitating particle displays have the potential to be the first display type where audio and haptic feedback can be presented directly within the display volume.
 Levitating Particle Displays with Interactive Voxels
E. Freeman, J. Williamson, P. Kourtelos, and S. Brewster.
In Proceedings of the 7th ACM International Symposium on Pervasive Displays – PerDis ’18, Article 15. 2018.
 Point-and-Shake: Selecting from Levitating Object Displays
E. Freeman, J. Williamson, S. Subramanian, and S. Brewster.
In Proceedings of the 36th Annual ACM Conference on Human Factors in Computing Systems – CHI ’18, Paper 18. 2018.