Acoustic radiation forces and torques on elastic micro rings in standing waves

Acoustic radiation forces and torques are very effective in manipulating micron-sized objects such as cells, droplets, particles, and organisms. In this work, we present analysis of acoustic radiation forces and torques on ring-shaped slender microstructures under a standing wave in an inviscid fluid using on a three-dimensional finite-element-method (FEM) model. The influence of geometric and physical parameters on the radiation forces and torques is characterized. The radiation force tends to push the rings towards the pressure nodes or anti-nodes depending on the contrast factor and exhibits a volumetric dependence in magnitude on geometric parameters. Moreover, a nonzero net torque develops when the ring is not co-planar with plane waves and varies in magnitude and direction depending on the position, orientation, and material properties of the ring. Large variations are observed only in torque values for specific combinations of geometric and physical parameters as an indicative of resonance. Furthermore, the FEM results are compared with a reduced-order model called chain-of-spheres, which works well in estimating the radiation forces at a fraction of the computational cost but deviates significantly in torque evaluations. Lastly, a segmented ring is used to understand the relative effect of secondary forces due to self-scattering. The findings of the study are applicable to development of acoustic manipulation systems for ring-like elastic microfilaments and slender bodies with arbitrary shapes and orientations. These results also can be used in directional reinforcing of ring-shaped composites.