Particle manipulation using acoustic radiation forces in micromachined devices

Acoustic radiation forces are increasingly used for the handling of micron sized particles suspended in a fluid. The primary radiation forces arise as a nonlinear effect when an acoustic wave interacts with a single particle. In addition, secondary acoustic forces arise when several particles are present. Typically a resonance (at upper kHz to 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, for example, by piezoelectric elements. The pattern of the pressure distribution in the fluid then determines where the particles are located. The analytical formula by Gor'kov predicts the location of spherical compressible particles in the bulk of the fluid based on the acoustic field. Several fields might be superimposed to produce time independent or time varying patterns of particles in the fluid, resulting in the formation of lines, clumps or even in particle rotation. Excellent agreement between theory and experiment is found. For further particle handling, the acoustic manipulation can be combined with microfluidic flow, microgrippers, wire loops, optical tweezers, DEP, etc. depending on the application. In more complicated situations numerical solutions have to be found. Recently a code has been developed that can compute forces on fixed 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 due to the Stokes layer, with the effect that the force is also increased.