For some time now, science fiction has been predicting the tractor beam, the strange column of energy that can transport everything from living beings to inanimate objects through space. Now this long-envisioned chunk of technology from science fiction is en-route to becoming science fact. At the Public University of Navarre in Spain, scientists are successfully manipulating tiny objects in midair. They are doing this with what they claim are acoustic holograms.
Ordinarily, holograms are three-dimensional optical structures, that is, they are made from light. More specifically, they are made by photons diffracting through interference patterns on a holographic plate. Although not popularly known, sound waves can do this as well. Sound waves, when interfering constructively and destructively with ultrasonic waves, can generate 3-D structures. That allows the sound waves to exert force on objects and behave in the same way a tractor beam does.
For instance, scientists have earlier demonstrated acoustic levitation techniques. These can suspend particles within the standing ultrasonic wave created by a single array aimed at a reflector, or between a pair of ultrasound emitter arrays. By varying the phase of the ultrasound, the nodes can move, thereby transporting the particles along a single axis. However, the acoustic levitation technique is fundamentally limited because the design relies on a fixed enclosure. Acoustic holograms are a step forward since they accomplish the same with only a single acoustic emitter and do not require any special enclosure.
These 3-D structures, made of sound or acoustic holograms, are actually bridging the gap between optical and acoustical trapping – shaped in the form of bottles, twisters, or tweezers. Scientists produce them typically from 400 numbers of 10 mm ultrasonic transducers arranged in an array of 20 x 20. Each transducer generates 40 kHz waves of ultrasonic sound using programmable relative phase modulation.
Within each transducer, there are two elements – one to determine the shape of the ensuing structure, and another, a holographic lens generated when the sound waves emitted phase-coincide at the structure’s nodal point. When the rapidly oscillating sound waves of the transducer array combine with the holographic lens, they are able to suspend small particles of about 3 mm in diameter in midair. In addition, they also grant control over the position and orientation of the particles.
Brüel and Kjær have done further research on the subject of STSF, Spatial Transformation of Sound Fields or acoustic holography. They have combined acoustic holography with transitory calculations. This allows defining any sound field signifier such as particle velocity, sound intensity, or sound pressure as a function of position and time. They have demonstrated through animated maps the control over a specific property change as a function of time.
Scientists at the Public University of Navarre in Spain have published their findings in Nature Communications. They have successfully levitated, rotated, and otherwise manipulated a tiny ball in midair using a grid of Ultrasonic transducers that send out high intensity sound to create a kind of force field around the object. They moved the tiny ball around the grid as though human fingers were doing the work.
