Abstract
The transfer of orbital angular momentum (OAM) from a vortex wave field to an object is interesting for developing vortex-based acoustic or optical tweezers, particularly for biomedical applications. When the object is not aligned with the vortex's core, it is unclear how OAM transfer relates to energy transfer. The author finds that here the ratio of torque to absorbed power is not given by---and can even be opposite to---the ratio of vortex topological charge to angular frequency. This suggest a negative radiation torque, associated with energy absorption, that spins an off-core axisymmetric object around its center of mass, in the direction opposite to the wave vortex's handedness.
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