A sonic screwdriver: acoustic angular momentum transfer for ultrasonic manipulation

Dexterous manipulation of objects in travelling wave beams can be achieved by modulating the phase profile of a coherent source to generate reconfigurable and complex interference patterns that can impart momentum on objects in the field. In this paper we use a commercially available matrix array operating at 550 kHz to generate helical ultrasonic beams to demonstrate the complexity of beam shapes that can be achieved from a system with approximately 1000 elements. Helical ultrasonic beams carry orbital angular momentum, due to the inclination of the wavefront, that increases with vorticity, or number of intertwined helices. The beams are used to both levitate and rotate a 100 mm diameter disk of acoustic absorber material in a water-filled chamber above the matrix array. The observed levitation of the absorber decreases with increasing vorticity because of the inclination of the wavefront away from the beam axis. The angular momentum transferred to the absorber increases with both the vorticity and the power of the ultrasonic beam. There was no evidence of aliasing of phase pattern applied to the array even with vorticity greater than 10, demonstrating the capability of a matrix array system for dexterous manipulation of objects with ultrasonic beams.