A small object can remain levitating nearly 40 cm from a sound source.
This experiment relies on acoustic levitation. Ultrasound, too high-pitched to be heard by the human ear, exerts a force on an object and can counteract its weight. The technique can therefore keep small objects in the air without touching them, which is of particular interest for handling fragile materials or materials sensitive to contamination.
Experimental demonstration and numerical simulation of levitation beyond an obstacle.
(a) Simulated acoustic pressure field, showing the reconstruction of the beam profile beyond the obstacle.
(b) Photograph of a particle levitating above the obstacle.
Usually, acoustic levitation uses two sides: opposing sound sources, or a source facing a reflector. The waves then combine to create areas where the object remains trapped. Single-sided devices also exist, but their range had remained limited to a few centimeters.
The team from the University of Tsukuba, the University of Bristol and Pixie Dust Technologies changed the way the object is held. Instead of placing it in an area of low sound pressure, they keep it at the heart of an area where this pressure is high. This configuration had been predicted by theory, but its three-dimensional stability had not yet been demonstrated experimentally in air.
To achieve this, the researchers use a particular ultrasound beam called a Bessel beam. Its central part remains narrow and intense over a relatively long distance. The device contains 256 small emitters operating at 40 kHz, controlled together to shape the sound waves in space.
With this system, a 1.5 mm-diameter expanded polystyrene bead was held between 141 and 397 mm from the source. The maximum range therefore reaches almost 40 cm. Under the same conditions, a conventional one-sided technique used for comparison reached 66.7 mm, or about six times less.
The bead is not merely suspended at a fixed point. By modifying the beam, the team was able to move it in three dimensions. The researchers also levitated several objects at the same time and tested non-spherical shapes, including pieces of dried tea leaf and aerogel.
Another distinctive feature is that the beam can reform after encountering certain obstacles. The researchers were thus able to hold objects beyond an obstacle placed in its path. This property leaves more free space around the working area than a system that encloses the object with several sources.
The tests currently concern very light, millimeter-sized objects. The method could be used for automated experiments, contactless manipulation or certain three-dimensional displays. Future research will need to clarify its behavior with objects of different types and in less controlled environments.