声波与球形粒子的非线性相互作用：辐射力和扭矩效应
Nonlinear interaction of acoustic waves with a spheroidal particle: radiation force and torque effects
摘要：
时谐声波与各向异性颗粒的非线性相互作用产生辐射力和力矩效应。这些现象是声流体技术的核心，其中诸如细胞和微生物的微粒被声学操纵。本文提出了一个非粘性流体中声束与亚波长椭球粒子相互作用的理论模型。在散射偶极子近似下，得到了辐射力和辐射力矩的简明解析表达式。辐射力由梯度和散射力给出;而辐射力矩有两个基本贡献，即动量臂和声自旋（自旋力矩效应）。作为一个实际例子，我们用该理论描述了两个交叉平面波与一个长椭球粒子的相互作用。结果表明，粒子被横向捕获在一个压力节点中，并被辐射力轴向推动。此外，动量臂使粒子在轴向方向上对齐。在某些特定的位置，只有自旋力矩发生。我们的研究结果是非常一致的有限元模拟。我们的模型的成功，使其成为一个调查工具，用于操纵各向异性微粒在声流体。
英文摘要：
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 microorganisms 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.
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