A simple acoustic system consisting of a pair of parallel singe layered piezoelectric transducers submerged in a fluid used to form standing waves by a superposition of two counter-propagating waves is reported. The nodal positions of the standing wave are controlled by applying a variable phase difference to the transducers. This system was used to manipulate polystyrene micro-beads trapped at the nodal positions of the standing wave. The demonstrated good manipulation capability of the system is based on a lowering of the reflection coefficient in a narrow frequency band near the through-thickness resonance of the transducer plates.
Grinenko, A., Ong, C. K., Courtney, C. R. P., Wilcox, P. D., & Drinkwater, B. W. (2012). Efficient counter-propagating wave acoustic micro-particle manipulation. Applied Physics Letters, 101(23), . https://doi.org/10.1063/1.4769092