Acoustic radiation force and spin torque on a viscoelastic cylinder in a quasi-Gaussian cylindrically-focused beam with arbitrary incidence in a non-viscous fluid

Abstract

The axial and transverse acoustic radiation force components for a viscoelastic phenolic polymer circular cylinder placed arbitrarily in the field of a zeroth-order quasi-Gaussian focused beam in 2D are computed, based on the partial-wave series expansion (PWSE) method and Graf’s addition theorem for the cylindrical wave functions to compute the off-axial beam-shape coefficients. Moreover, the analysis is extended to derive the expression for the acoustic spin radiation torque experienced by the cylinder when the beam is shifted off-axially with respect to the axis of wave propagation. The emergence of the acoustic spin radiation torque experienced by the circular cylinder is a consequence of acoustic attenuation inside its absorptive material. Depending on the direction of the shift, the spin torque is negative or positive, causing the rotation of the cylinder in the polar plane in either the clockwise or the anticlockwise direction, respectively. Moreover, for a fixed position of the cylinder in the off-axial configuration, a spin torque reversal is predicted due to the variation of absorption of both the longitudinal and shear waves. The effect of increasing the surrounding fluid’s density is also investigated, which has a substantial effect on the radiation force and spin torque plots. Numerical results are performed with particular emphasis on the properties of the incident beam, the amount of the shift from the center of the cylinder as well as the properties of the surrounding fluid. The analytical formalism allows evaluating the axial and transverse radiation force components and radiation torque for any 2D beam that is generally an exact solution of the Helmholtz equation. The sought applications are in controlled rotation of a particle and its manipulation using “acoustical sheets” (i.e., finite beams in 2D).