Two-dimensional manipulation of microparticles using phase-controllable ultrasonic standing waves

The ability to trap, and then manipulate, micro-particles in a fluid, is of interest as a research tool in the biosciences. Applications include tissue engineering, particle sorting and improving alignment with bio-sensors. This paper relates to the use of phase-controllable counter-propagating ultrasonic waves to generate a standing wave with pressure nodes whose positions are determined by the relative phases of the component counter-propagating travelling waves. As dense (relative to the fluid) particles are forced to nodes in the pressure field this allows particles to be trapped at particular points and moved to arbitrary positions. Counter-propagating waves are generated using pairs of opposing transducers, matched and backed to minimise reflection. Using one pair of transducers allows particles to be trapped and manipulated in one dimension. Using two pairs of transducers, positioned orthogonally, and adjusting the relative phases appropriately, allows trapping and manipulation in two dimensions. The device is shown experimentally to be capable of trapping and manipulating 10-micron-diameter polystyrene beads in two dimensions.