“In the past, we have used different acoustic fields to trap particles: standing waves, twin traps or bottle traps,” Dr. Asier Marzo, from Bristol’s Department of Mechanical Engineering, told Digital Trends. “They are generated by having several speakers emitting with the same amplitude and frequency, but different coordinated phases. However, all these fields have in common that the separation between the low-intensity areas is always around half of the wavelength of the sound used. For instance, operating in air at 40kHz gives a wavelength of 8.6mm. Particles get trapped in the nodes, so particles larger than half-wavelength span across several nodes and cannot be stably trapped.”
The Bristol researchers’ breakthrough, allowing them to trap particles larger than the wavelength of sound, is the result of a special type of acoustic field called vortices. Marzo describes these as being “like tornadoes of sound, with a silent core in the center.” One interesting capability of vortices is that they can increase their aperture. Unfortunately, when you normally try and put a particle in the core of a vortex it begins spinning and is swiftly ejected.
To get around this, the team emitted very short pulses of vortices with opposite directions: Emitting one counter-clockwise vortex for one millisecond, and then a clockwise vortex for another millisecond. This sequence of short-pulsed vortices allowed the team to trap larger particles, without them spinning.
In the future, with more acoustic power, the team’s hope is that it will be possible to hold even larger objects — possibly including humans. This will be achieved without lowering the pitch of the sound, which would make it audible to humans and potentially dangerous as a result. Before we reach that point, however, Marzo has more immediate applications in mind.
“For me, the interesting applications of acoustic trapping are in the smaller scale,” he said. “For instance, to trap and dispose objects that are inside our body, such as kidney stones or eye floaters. The ability of being able to trap wavelength-size particles permits [us] to employ the same frequency to image and trap particles. This could lead to ultrasonic imaging machines that can see inside you, but also manipulate particles that are there.”