Acoustic Levitation Display
Also referred to as an Acoustophoretic Volumetric Display
This area of research uses ultrasonic soundwaves and the principle of standing waves to "levitate" thin and light particles of material and (in some cases) move them at a high enough speed to make coherent images and shapes. The applications of this technology are somewhat limited to small scale particles and small display areas, but it is worth seeing this as another unusual area of display research.
Another example from the University of Tokyo that levitates small bits of foam with different shapes:
For the above: Yoichi Ochiai / 落合陽一(The University of Tokyo / 東京大学) Takayuki Hoshi / 星貴之(Nagoya Institute of Technology / 名古屋工業大学) Jun Rekimoto / 暦本純一 (The University of Tokyo / Sony CSL) White paper: https://arxiv.org/abs/1312.4006