Strategies for single particle manipulation using acoustic radiation forces and external tools

The use of primary acoustic radiation forces has been shown to be a valid technique for the handling of micron sized suspended particles, such as beads or biological cells. These forces arise as a nonlinear effect when an acoustic wave or vibration, which is set up in the fluid by exciting to resonance the system containing the suspension, interacts with the particles. The typical frequencies (upper kHz-lower MHz range) and the periodicity (in the range of hundreds of micrometers) of the acoustic field make this technique particularly suited for the handling of particles within microfluidic systems. A variety of devices for separation, fractionation, trapping and positioning of beads or biological cells, working both in batch [12] or fluid flow [3-4] mode, have been proposed. With the exception of the ports used to inject or remove the sample or the carrier medium, these systems can be considered as closed systems. Nevertheless, access to the particles with external tools is sometimes needed after acoustic manipulation has been performed. For instance, particles or cells pre-positioned in a sequence along the centerline of a channel using acoustic radiation forces need to be removed from it using a microgripper for further handling. Furthermore, in the field of crystallography research protein crystals have to be placed one by one onto a nylon loop prior to X-ray analysis with synchrotron radiation. This is usually done using the loop to pick up the crystal from the solution where it has been growing with other ones. As this process is sometimes repeated for a large number of crystals there are efforts to automate it. To this purpose it would be advantageous to bring the crystals spatially separated into a known position where they than can be sequentially collected with the loop. Here strategies for single particle manipulation are presented combining the effects of acoustic fields, fluid flow, surface tension and external tools. They are discussed by means of numerical results from FE-simulations of both two and three dimensional models as well as corresponding experiments.