Acoustophoresis deals with the manipulation of sub-wavelength scatterers in an incident acoustic field. The geometric details of manipulated particles are often neglected by replacing them with equivalent symmetric geometries such as spheres, spheroids, cylinders or disks. It has been demonstrated that geometric asymmetry, represented by Willis coupling terms, can strongly affect the scattering of a small object, hence neglecting these terms may miss important force contributions. In this work, we present a generalized formalism of acoustic radiation force and radiation torque based on the polarizability tensor, where Willis coupling terms are included to account for geometric asymmetry. Following Gorkov's approach, the effects of geometric asymmetry are explicitly formulated as additional terms in the radiation force and torque expressions. By breaking the symmetry of a sphere along one axis using intrusion and protrusion, we characterize the changes in the force and torque in terms of partial components, associated with the direct and Willis Coupling coefficients of the polarizability tensor. We investigate in detail the cases of standing and travelling plane waves, showing how the equilibrium positions and angles are shifted by these additional terms. We show that while the contributions of asymmetry to the force are often negligible for small particles, these terms greatly affect the radiation torque. Our presented theory, providing a way of calculating radiation force and torque directly from polarizability coefficients, shows that in general it is essential to account for shape of objects undergoing acoustophoretic manipulation, and this may have important implications for applications such as the manipulation of biological cells.
Coupled Decorated Membrane Resonators with Large Willis Coupling
我们报告的实验研究声散射体组成的一对耦合装饰膜谐振器（DMR），表现出近极端的反射不对称性和强威利斯耦合系数的振幅为2.5和极化交叉项为1.1 × 10-8 ms 2的对比度。该设计是基于理论分析，使用格林函数公式，我们开发的早期。威利斯系数的实部和虚部的清晰线条形状清楚地表明，即使通过目视检查，Kramers-Kronig关系也是满意的。理论分析与实验结果的主要特征吻合得很好。
Soundiation: A multi-functional GUI-based software in evaluation of the acoustophoresis by the acoustic radiation force and torque on arbitrary axisymmetric objects
Wavefront Shaping of Ultrasound Vortex through the Human Skull Enabled by Binary Acoustic Metasurfaces
School of Engineering Medicine, Beihang University（北京航空航天大学工程医学院）；Beijing Advanced Innovation Center for Biomedical Engineering（北京生物医学工程高精尖创新中心）；Shenzhen Institute of Beihang University（北京航空航天大学深圳研究院）
EPFL（洛桑联邦理工学院）；Vienna University of Technology (TU Wien)（维也纳工业大学）；Univ. Bordeaux, CNRS, Bordeaux INP, I2M（波尔多大学）；Arts et Metiers Institute of Technology（巴黎高科国立高等工艺学院）
