Centre for Audio, Acoustics and Vibration, University of Technology Sydney, Sydney, NSW 2007, Australia.
School of Engineering and Information Technology, University of New South Wales, Canberra, ACT 2600, Australia.
Phys Rev Lett. 2022 Oct 21;129(17):174501. doi: 10.1103/PhysRevLett.129.174501.
Acoustic meta-atoms serve as the building blocks of metamaterials, with linear properties designed to achieve functions such as beam steering, cloaking, and focusing. They have also been used to shape the characteristics of incident acoustic fields, which led to the manipulation of acoustic radiation force and torque for development of acoustic tweezers with improved spatial resolution. However, acoustic radiation force and torque also depend on the shape of the object, which strongly affects its scattering properties. We show that by designing linear properties of an object using metamaterial concepts, the nonlinear acoustic effects of radiation force and torque can be controlled. Trapped objects are typically small compared with the wavelength, and are described as particles, inducing monopole and dipole scattering. We extend such models to a polarizability tensor including Willis coupling terms, as a measure of asymmetry, capturing the significance of geometrical features. We apply our model to a three-dimensional, subwavelength meta-atom with maximal Willis coupling, demonstrating that the force and the torque can be reversed relative to an equivalent symmetrical particle. By considering shape asymmetry in the acoustic radiation force and torque, Gorkov's fundamental theory of acoustophoresis is thereby extended. Asymmetrical shapes influence the acoustic fields by shifting the stable trapping location, highlighting a potential for tunable, shape-dependent particle sorting.
声学超原子作为超材料的基本组成部分，其线性特性旨在实现波束转向、隐身和聚焦等功能。它们还被用于塑造入射声场的特性，从而实现了对声镊的声辐射力和力矩的操控，提高了其空间分辨率。然而，声辐射力和力矩也取决于物体的形状，这强烈影响其散射特性。我们表明，通过使用超材料概念设计物体的线性特性，可以控制辐射力和力矩的非线性声学效应。被捕获的物体通常比波长小，被描述为粒子，诱导单极子和偶极子散射。我们将这些模型扩展到包括 Willis 耦合项的极化率张量，作为非对称性的度量，捕捉几何特征的重要性。我们将我们的模型应用于具有最大 Willis 耦合的三维亚波长超原子，证明了力和力矩相对于等效对称粒子可以反转。通过考虑声辐射力和力矩中的形状不对称性，Gorkov 的声泳理论得到了扩展。不对称形状通过改变稳定的捕获位置来影响声场，突出了可调谐、形状依赖的粒子分选的潜力。
