The study, carried out by a research team from the University of Tsukuba in Japan and University of Bristol, was published in the journal Physical Review Letters.
Acoustic levitation is a technique that uses soundwaves to suspend objects in mid-air without physical contact – meaning it has the potential to be hugely beneficial for handling fragile materials, contamination-sensitive samples, and hazardous substances.
Conventional acoustic levitation systems rely on soundwaves being generated within an enclosed space, but the new technique is the first time a single-sided design has been able to demonstrate stable acoustic levitation in three dimensions.
Yusuke Koroyasu, lead author and PhD student at the University of Tsukuba, said: "This research prospered through the close collaboration between researchers at Bristol and Tsukuba. My time in Bristol provided invaluable opportunities to discuss ideas and develop the work together, and I am grateful for the support and guidance I received from colleagues at both institutions.'"
Bruce Drinkwater, Professor of Ultrasonics at the University of Bristol, explained: "When you have a conventional acoustic levitator, the soundwaves from opposing directions stabilise the object within the device. Past attempts to develop single-sided levitators have struggled because the force that keeps the object in place gets weaker as the object moves further from the source.
"Eventually the force starts pushing the object away instead of holding it steady, making long-distance levitation impossible. To overcome this we used a special type of ultrasonic beam called a zero-order Bessel beam, which unlike a normal sound beam, stays narrow and maintains a high-intensity central core over a much longer distance."
By exploiting the Bessel beam's unique properties, the researchers successfully achieved stable levitation of a 1.5mm-diameter polystyrene sphere at distances of up to 40cm from the sound source, approximately six times further than previously achieved using single-sided acoustic traps.
The team was able to manipulate the levitated object in three dimensions using ultrasound from only one side, and were able to levitate multiple objects, non-spherical objects, and objects located beyond other physical objects.
Professor Tatsuki Fushimi from the University of Tsukuba said: "Because our technique enables long-range, non-contact manipulation in open environments, we anticipate this method could be used for automated experiments, three-dimensional displays, handling fragile materials and hazardous substances."
Paper
'Midair Single-Sided Acoustic Levitation in High-Pressure Regions of Zero-Order Bessel Beams', by Y. Koroyasu et al. in Physical Review Letters [open access]
