Back in the summer we spent a day filming with BBC Studios. This was for a new series called The Edge of Science, hosted by Rick Edwards and others (including Colin Furze and Dianna Cowern). Rick visited the lab and I talked him through ultrasonic levitation. Behind the scenes we had spent the previous month designing and building an acoustic vortex levitator that we had promised the BBC could break a world record. The build team were Sam Elfring, Tatsuki Fushimi and Luke Cox.
For this programme we constructed a slightly larger version of the levitator we used in our recent paper on virtual vortices which had 192 ultrasonic loudspeakers. The new supersized levitator had 800 ultrasonic speakers. The levitator creates an acoustic vortex which in turn generates vertical and horizontal force, but it also causes the levitated object to spin rapidly, which destabilises it. To counter this effect we switch rapidly between vortices rotating clockwise and anticlockwise. The aim being to generate the intensity profile of the vortex that we know is good for levitation, but without the spin.
The levitator started working the day before filming thanks to the huge efforts from this team. The results can be seen in the film below. We levitated an expanded polystyrene sphere of 19.53mm in diameter, which is 2.3 wavelengths in size. The previous best was 16mm (or 1.85 wavelengths). So, we believe that this is the world record for the largest object levitated in an acoustic tractor beam (so far its unofficial). Note that for the world record we used a 6th order vortex, which means the phase cycles 6 times around the array for every wavelength. The higher the order of the vortex, the larger the node in the vortex centre, and hence the larger the object that can be levitated. So, our world record could be beaten, if a higher order vortex of sufficient power could be generated. Maybe next year!
Bruce Drinkwater 