(with S. Oberti, A. Neild, J. Wang, T. Schwarz, and D. Möller)

Primary acoustic radiation forces are increasingly used for the handling of micron sized particles suspended in a fluid. These forces arise as a nonlinear effect when an acoustic wave interacts with the particles. In addition secondary acoustic forces arise. Typically a resonance (at upper kHz - lower MHz frequencies) is set up in the system consisting of chip, fluid, particles and transducer. Both solid and fluid parts vibrate and are excited e.g. by piezoelectric elements. The pattern of the pressure distribution in the fluid then determines where the particles are located.

The analytical formula by Gorkov predicts the location of spherical elastic particles in the bulk of the fluid. Several fields might be superimposed to produce time independent or time varying patterns of particles in the fluid. Excellent agreement between theory and experiment is found. For particle handling, the acoustic manipulation can be combined with microfluidic flow, microgrippers, wire loops, optical tweezers, DEP, etc. depending on the application.

For more complicated situations numerical solutions have to be found. Recently a code has been developed that can compute forces on rigid particles in viscous fluids in general situations, e.g. for particles near walls or near other particles, as well as for particles of arbitrary shape.  The code is based on the FVM ( Finite Volume Method ), solves the Navier-Stokes equations directly and also yields the acoustic streaming pattern. The viscosity increases the apparent size of the particle by the Stokes layer. Thereby also the force is increased.