Radiation torque on a sphere centered on an acoustic helicoidal (vortex) Bessel beam.

Abstract

Circularly polarized electromagnetic waves carry axial angular momentum and analysis shows that the axial radiation torque on an illuminated sphere is proportional to the power absorbed by the sphere [P. L. Marston and J. H. Crichton, Phys. Rev. A. 30, 2508–2516 (1984)]. Hefner and Marston [J. Acoust. Soc. Am. 106, 3313–3316 (1999)] proposed that a helicoidal acoustic beam (an acoustic vortex) also carries axial angular momentum and noted that absorption of such a beam should also produce an axial radiation torque. In the present work the acoustic radiation torque on a sphere centered on an acoustic helicoidal Bessel beam is analyzed and the torque is predicted to be proportional to the ratio of the power absorbed as a result of the interaction of sound with the sphere to the acoustic frequency. The torque is also proportional to the topological charge of the beam. Depending on the beam helicity, the torque is parallel or anti-parallel to the beam axis. The analysis uses a relation between the scattering and the partial wave coefficients for a sphere in a helicoidal Bessel beam [P. L. Marston, J. Acoust. Soc. Am. 124, 2905–2910 (2008)]. [This research was supported in part by ONR and NASA.]