Nonlinear Interaction of Acoustic Waves with a Spheroidal Particle: Radiation Force and Torque Effects

The nonlinear interaction of a time-harmonic acoustic wave with an\nanisotropic particle gives rise to the radiation force and torque effects.\nThese phenomena are at the heart of the acoustofluidics technology, where\nmicroparticles such as cells and microorganisms are acoustically manipulated.\nWe present a theoretical model considering a generic acoustic beam interacting\nwith a subwavelength spheroidal particle in a nonviscous fluid. Concise\nanalytical expressions of the radiation force and torque are obtained in the\nscattering dipole approximation. The radiation force is given in terms of a\ngradient and scattering force; while the radiation torque has two fundamental\ncontributions, namely, the momentum arm and acoustic spin (spin-torque effect).\nAs a practical example, we use the theory to describe the interaction of two\ncrossed plane waves and a prolate spheroidal particle. The results reveal the\nparticle is transversely trapped in a pressure node and is axially pushed by\nthe radiation force. Also, the momentum arm aligns the particle in the axial\ndirection. At certain specific positions, only the spin-torque occurs. Our\nfindings are remarkably consistent with finite-element simulations. The success\nof our model enables its use as an investigation tool for the manipulation of\nanisotropic microparticles in acoustofluidics.\n