Reversals of Orbital Angular Momentum Transfer and Radiation Torque

Our examination of vortex fields of scalar waves illuminating an off-axis-located axisymmetric object shows that the ratio of orbital angular momentum (OAM) transfer to energy transfer is not quantized by the ratio of topological charge l (integer) to wave angular frequency omega. We find that the transfer ratio can even be opposite to the ratio l/omega. The findings reveal how a radiation torque associated with energy absorption can spin an axisymmetric object around its center of mass in a direction reversed with respect to the wave vortex's handedness. We understand the reversal by OAM superposition. The absorption can be either in the absorbing object or in the viscous surrounding fluid. Features of reversed spinning motion of a small particle in a slightly viscous fluid are revealed by use of Bessel-function vortex fields. The results are of interest for the development of acoustic and optical vortex-based tweezers as well as for the field of acoustofluidics.