The nonlinear interaction of a time-harmonic acoustic wave with an anisotropic particle gives rise to the radiation force and torque effects. These phenomena are at the heart of the acoustofluidics technology, where microparticles such as cells and micro-organisms are acoustically manipulated. We present a theoretical model considering a generic acoustic beam interacting with a subwavelength spheroidal particle in a nonviscous fluid. Concise analytical expressions of the radiation force and torque are obtained in the scattering dipole approximation. The radiation force is given in terms of a gradient and scattering force; while the radiation torque has two fundamental contributions, namely, the momentum arm and acoustic spin (spin-torque effect). As a practical example, we use the theory to describe the interaction of two crossed plane waves and a prolate spheroidal particle. The results reveal the particle is transversely trapped in a pressure node and is axially pushed by the radiation force. Also, the momentum arm aligns the particle in the axial direction. At certain specific positions, only the spin-torque occurs. Our findings are remarkably consistent with finite-element simulations. The success of our model enables its use as an investigation tool for the manipulation of anisotropic microparticles in acoustofluidics.

中文翻译：

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声波与球状粒子的非线性相互作用：辐射力和转矩效应

时谐声波与各向异性粒子的非线性相互作用会产生辐射力和扭矩效应。这些现象是声流技术的核心，在声学技术中，诸如细胞和微生物之类的微粒被操纵。我们提出了一种理论模型，考虑了与非粘性流体中的亚波长球形粒子相互作用的通用声束。在散射偶极子近似中获得了辐射力和扭矩的精确解析表达式。辐射力以梯度和散射力的形式给出；而辐射转矩具有两个基本贡献，即动量臂和声学自旋（自旋转矩效应）。举一个实际的例子 我们使用该理论来描述两个交叉平面波和一个长球形颗粒的相互作用。结果表明，粒子被横向捕获在压力节点中，并被辐射力轴向推动。而且，动量臂使粒子在轴向上对齐。在某些特定位置，仅发生自旋扭矩。我们的发现与有限元模拟非常一致。我们模型的成功使它可以用作研究声流体中各向异性微粒的研究工具。我们的发现与有限元模拟非常一致。我们模型的成功使它可以用作研究声流体中各向异性微粒的研究工具。我们的发现与有限元模拟非常一致。我们模型的成功使它可以用作研究声流体中各向异性微粒的研究工具。