Acoustic radiation torque on an off-axis elliptical cylinder in Gauss beams

Abstract

As one of the nonlinear effects of acoustic waves, the time-averaged acoustic radiation torque expression is derived from the transfer of angular momentum from the incident beam to the object. In recent years, the acoustic radiation torque has received substantial attention since it is the underlying principle of well-controlled particle rotations and spins, which provides a new degree of freedom in particle manipulation and acousto-fluidic applications in addition to the translational displacement caused by the acoustic radiation force. Cylindrical particles, such as fibers, carbon nanotubes and other surface acoustic wave devices, are commonly encountered in various applications. The acoustic scattering coefficients for an elliptical cylinder arbitrarily located at the field of Gauss beam in two-dimensions are computed based on the partial-wave series expansion method and the Graf’s additional theorem for cylindrical functions to obtain the off-axis beam shape coefficients. It is worth mentioning that both the rigid and non-rigid cylinders are considered in this work, which requires different boundary conditions at the cylinder surface. Moreover, the closed-form expression of the acoustic radiation torque in this case is derived. On this basis, several numerical simulations are performed with particular emphasis on the off-axis distance, the incident angle and the beam waist. The simulated results show that both the positive and negative acoustic radiation torque can exist under certain conditions, which means that 1) the elliptical cylinder can be rotated in either the clockwise or the counterclockwise direction, 2) rigid elliptical cylinders are more likely to experience a strong acoustic radiation torque than non-rigid elliptical cylinders at low frequencies, 3) the incident wave field with a specific frequency can excite a different resonance scattering mode for a non-rigid elliptical cylinder, therefore the acoustic radiation torque peak is more related to the beam frequency than to the elliptical cylinder’s location in the field, and 4) increasing the beam width can enlarge the scattering cross section area, and thus enhancing the acoustic radiation torque on the elliptical cylinder. The results in this study are expected to provide a theoretical guide for the controllable rotation of a particle and the viscosity inversion of fluid by using the acoustic radiation torque. The exact formalism presented here by using the multipole expansion method, which is valid for any frequency range, can be used to validate other approaches by using purely numerical methods.