Acoustic manipulation of micro-objects

Abstract

Sound has been widely used to manipulate micro-objects by the acoustic radiation force and torque (ARF&T) arising from wave scattering with the attributes of being biocompatible and contact-free. This study is aimed at the theoretical analysis of the ARF&T and experimental realization of ultrasonically driven assembly and controllable rotation of micro-objects.

Gor’kov’s theory is applied to describe the motions of numerous particles homogeneously suspended in the fluid. Typically, the particles cluster at the pressure nodes or anti-nodes for a scaffold pattern. Here, we demonstrate that these particles may move along wave attenuated directions. An attenuation coefficient is derived to quantify the sound attenuation when the wave propagates in an unbounded space. Considering that the scattered energy from the particle-wave interaction is partially absorbed by the matched layers of a bulk acoustic wave (BAW) device, the unbounded attenuation coefficient can be employed to approximate the pressure attenuation inside the bounded device, resulting in a convergent potential field and particle agglomeration. A parametric analysis has been performed to evaluate the transition conditions for particles that cluster into a scaffold pattern or agglomerate together. Statistically, the particles satisfying size parameter ka>1 and concentration > 0.5 wt% can be agglomerated, and the phenomena could be improved by increasing the supplied power on the transducers.

For multi-sphere systems, the partial-wave expansion method is employed to evaluate the ARF&T on the particles over a wide-size range. The interaction effects among particles can be involved in the theoretical framework with the help of the translation addition theorem. The results hint that the interaction effects indeed play an important role in particle manipulation.

Later, the theory is extended to accommodate the axisymmetric irregular geometries. The shape of the object strongly affects its scattering properties, thereby the ARF&T as well as the acoustophoretic process. The conformal transformation approach is used to map a non-spherical object to a sphere to capture the geometric features. In this way, the wave equation subjecting to the spherical boundary conditions can be solved, and thus the ARF&T are asymptotically obtained under the far-field limitation. Based on the proposed theoretical framework, an open-access, GUI-based software, Soundiation, is provided to evaluate the ARF&T while supporting the acoustophoretic prediction.

The theoretical predictions reveal that the geometric features are a potential degree of freedom in manipulating non-spherical objects. Utilizing this property, we experimentally achieve rotational manipulations of the particles under a standing wavefield using a low-cost and simple BAW device. Theoretical and experimental results have confirmed that the particles will be locked in the pressure nodes, and controllably rotated by tuning the standing BAW device. Lyapunov stability analysis reveals more than one stable equilibrium state for stable rotation of the objects.

Overall, this study derives the ARF&T exerted on the objects for various scenarios, including numerous spheres, multiple spheres, and a single non-spherical object. All the theoretical methods perfectly match the numerical simulations, while presenting higher computational efficiency. Experiments have been performed to validate the theoretical analysis.