Marzo, A., Caleap, M., & Drinkwater, B. W. (2018). Acoustic Virtual Vortices with Tunable Orbital Angular Momentum for Trapping of Mie Particles. Physical Review Letters, 120(4), 44301.
An acoustic vortex is a sound beam with spiralling wave fronts. Imagine a wave travelling like a rotating corkscrew. Before this paper, these vortices had been shown to be candidates for acoustic tractor beams (a beam of sound that can apply both pushing and pulling forces). The acoustic radiation forces present in acoustic vortices can trap objects against gravity. However, they also cause the trapped object to spin. As the power of the acoustic wave is increased to make the trap stronger, this rate of spinning increases, and eventually the object is ejected from the trap. This spinning effect is worse for larger objects, limiting acoustic vortices to the trapping of small objects. As ever in waves, small means significantly less than the wavelength. So, for the commonly used 40 kHz, where the wavelength of sound is 8mm, these vortices were only good for objects up to a maximum of 1mm. In this paper we came up with a nice simple idea to extend this size range. We rapidly switch the handedness of the vortex, one moment it’s right-handed, next moment it’s left-handed. Or if you prefer, clockwise and anti-clockwise. If we do this switching at the right speed, say 1000 times a second, then we find that the spin and trapping force of these components just sums. This means that the spin, which is in opposite directions cancels, whereas the force, which does not change sign doubles. What you are left with is a vortex without any spin, what we call a virtual vortex. It’s virtual because it never exists. At any moment in time the sound field is always a vortex with a particular handedness. But if the switching is sufficiently fast then the particle feels the combined effect of both vortices, so it’s as if the virtual vortices really did exist. This is useful as it makes these beams more stable as the spin is removed) and allows us to trap much larger objects. In fact we even managed to trap objects larger than the wavelength. The biggest being 2cm in diameter. That’s what the reference of “Mie” in the title means, a Mie particle being one that is of wavelength scale.
The image below show the virtual vortex experiment in action. We use many small emitters to create the vortices.