Scientists in Japan are exploring the possibilities – and limitations – of suspending objects in midair using sound waves. The team of innovators was able to make a matchstick, drops of water, screws and nuts float and even move around using only ultrasound.
The process, called “acoustic levitation,” has a history that dates back to the 1940s. It is the technique of using ultrasonic speakers to create changes in air pressure, moving some air molecules closer together and others further apart. When an object is placed at a certain point within a sound wave, the force of gravity is counteracted by the force exerted by the sound wave, trapping the object in the sound waves’ valleys and crests.
Researchers in Japan used four speakers, arranged in a square and facing each other, to blast sound waves at small objects at a frequency of 40 kHz – above the uppermost limits of human hearing at 20 kHz. In other words, we can’t hear them.
Previous experiments with acoustic levitation resulted in bees, ants and even fish becoming airborne, according to a 2006 study published in the journal Nature. But this is the first time scientists were able to move levitated objects around in a three-dimensional space rather than simply lift them off the ground.
"It is a real advance, and it opens new possibilities for levitation," acoustic levitation expert Rick Weber of Materials Development Inc. in Arlington Heights, Ill., told National Geographic. "With three-dimensional controls, you could combine more materials in more steps than you could previously without ever using a container.”
The new study, led by Yoichi Ochiai of the University of Tokyo, was submitted to Cornell University’s science research site. The authors describe the “essence of levitation technology” as being the “countervailing of gravity.”
“It is known that an ultrasound standing wave is capable of suspending small particles at its sound pressure nodes and, so far, this method has been used to levitate lightweight particles, small creatures and water droplets,” they wrote. “The acoustic axis of the ultrasound beam in these previous studies was parallel to the gravitational force, and the levitated objects were manipulated along the fixed axis (i.e. one-dimensionally) by controlling the phases or frequencies of bolted Langevin-type transducers.
“In the present study, we considered extended acoustic manipulation whereby millimeter-sized particles were levitated and moved three-dimensionally by localized ultrasonic standing waves, which were generated by ultrasonic phased arrays.”
To put it simply, the Japanese scientists put their predecessors to shame. Check out the video below, first uploaded to YouTube, to see what they can do.
Researchers from the University of Tokyo hope to improve the technique so one day it could be used to manipulate delicate electronic components when assembling hardware.