Topological-charge dependence of radiation torque for an acoustic-vortex spanner
Based on the analyses of the wave front and the wave vector of an acoustic-vortex (AV) spanner generated by a circular array of point source, the principle of object rotation is investigated through the calculation of the orbital angular momentum (OAM) and the radiation torque for AVs with various topological charges. It is demonstrated theoretically that the rotation of the axisymmetric disk centered on an AV spanner is mainly driven by the exerted radiation torque without the contribution of the OAM transfer. The radiation torque on a small-radius object is inversely associated with the topological charge in the center of the AV spanner, and it is enhanced significantly for a larger AV with a higher topological charge. The special case of the radiation torque proportional to the topological charge might be realized when the disk radius is much larger than the wavelength, in which case the acoustic power of the AV spanner can be absorbed as much as possible. With the established sixteen-source experimental setup, the radial pressure distributions of AVs with different topological charges measured at the frequency of 1.3 kHz in air agree well with the simulations. The topological-charge dependence of the radiation torque for AVs is also verified by the quantitative laser-displacement (angle) measurements for disks with different radii. The favorable results demonstrate that, for the object covering the vortex center of an AV spanner, the total OAM transfer might be 0 and the object rotation is contributed by the acting moments. Thus, the radiation torque of an AV spanner is more applicable than the OAM for describing the driving capability of object rotation, and it can be used as an effective tool in clinical applications to noninvasively manipulate objects with a feature size at the wavelength-scale (e.g. kidney stone in lithotripsy) inside body.