Acoustofluidics 19: ultrasonic microrobotics in cavities: devices and numerical simulation.

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.