In order to move expanded polystyrene particles of 0.6 mm and 2 mm in diameter, the Japanese scientists at the University of Tokyo and the Nagoya Institute of Technology had to place the objects inside a complex set-up of four arrays of speakers.
“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,” the study stated.
The experiment machine is comprised of audio speakers capable generating inaudible high frequencies sound waves that intersect inside a restrained space. The waves then generate a “moveable ultrasonic focal point,” frequency noise greater than 20kHz, where crossover creates standing waves. Some waves are kept in constant position, serving as a suspending force, while other waves are used to support a floating object jammed in the standing waves.
“Our manipulation system has two original features. One is the direction of the ultrasound beam, which is arbitrary because the force acting toward its center is also utilized. The other is the manipulation principle by which a localized standing wave is generated at an arbitrary position and moved three-dimensionally by opposed and ultrasonic phased arrays,” the study said.
The practice of moving tiny objects acoustically along the fixed axes is not new, but it was previously applied to 2D, starting from 1975.
“The essence of levitation technology is the countervailing of gravity. It is known that an ultrasound standing wave is capable of suspending small particles at its sound pressure nodes,” Yoichi Ochiai from University of Tokyo said.
Currently, acoustic levitators are used mostly in industry and for researchers of anti-gravity effects such as at NASA.
