Acoustic radiation forces are increasingly used for the handling of micron sized particles (cells, functionalized beads, etc.) suspended in a fluid in the chamber of a manipulation device. The primary radiation forces arise as a nonlinear effect when an acoustic wave interacts with a particle. For specific robotic applications, precise control of the acoustic field in the cavity is important, which is excited, for example, by piezoelectric transducers attached to the device. Based on Gor'kov's potential the relevant forces on spherical particles can be computed. The field can be controlled by varying the excitation parameters: chamber and electrode configuration, as well as frequency, amplitude and phase of the excitation and their modulation. In the first part of the present tutorial, a number of examples are described: displacement and rotation of particles in micro machined chambers and macroscopic transport of particles in a larger chamber. In the second part, numerical tools (Finite Volume Method, COMSOL) are used to model the interaction of the acoustic field with a particle beyond a Gor'kov potential: viscosity, effects of walls near particles and acoustic radiation torque to rotate the particle. Excellent agreement between the various methods has been found.

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The numerical prediction of acoustofluidic particle motion is of great help for the design, the analysis, and the physical understanding of acoustofluidic devices as it allows for a simple and direct comparison with experimental observations. However, such a numerical setup requires detailed modeling of the acoustofluidic device with all its components and thorough understanding of the acoustofluidic forces inducing the particle motion. In this work, we present a 3D trajectory simulation setup that covers the full spectrum, comprising a time-harmonic device model, an acoustic streaming model of the fluid cavity, a radiation force simulation, and the calculation of the hydrodynamic drag. In order to make quantitatively accurate predictions of the device vibration and the acoustic field, we include the viscous boundary layer damping. Using a semi-analytical method based on Nyborg's calculations, the boundary-driven acoustic streaming is derived directly from the device simulation and takes into account cavity wall vibrations which have often been neglected in the literature. The acoustic radiation forces and the hydrodynamic drag are calculated numerically to handle particles of arbitrary shape, structure, and size. In this way, complex 3D particle translation and rotation inside experimental microdevices can be predicted. We simulate the rotation of a microfiber in an amplitude-modulated 2D field and analyze the results with respect to experimental observations. For a quantitative verification, the motion of an alumina microdisk is compared to a simple experiment. Demonstrating the potential of the simulation setup, we compute the trajectory of a red blood cell inside a realistic microdevice under the simultaneous effects of acoustic streaming and radiation forces.

This study is aimed at the theoretical analysis of the acoustic radiation torque and the experimental realization of a controlled rotation of non-spherical particles by ultrasound. A finite element model has been developed and validated to calculate the acoustic radiation torque on a microfiber. The influence of different parameters such as the frequency, fiber size and position in the acoustic field are evaluated. The rotational motion of a non-spherical particle and the resulting drag torque are analyzed as well. This allows for the calculation of the angular velocity of a fiber. Various rotation methods for non-spherical particles with the acoustic radiation torque have been developed, tested experimentally with a microdevice at frequencies in the MHz range and compared to each other. The first method relies on successive change of the wave propagation direction in discrete steps. Three additional rotation methods have been developed which allow for a continuous rotation and alignment at defined orientations. The methods are characterized by the modulation of one single parameter (amplitude, phase or frequency) over time.