Acoustofluidic Particle Focusing in PMMA Chips Using Low Frequency Lateral Resonances of The Piezoelectric Plate Transducer

In this work we use low frequency lateral resonances of a piezo-electric plate transducer for acoustofluidic particle focusing in a polymeric microfluidic chip. In our experiments we achieved superb particle focusing (1st harmonic) and were able to excite higher modes where particles are split into two (2nd harmonic) and three (3rd harmonic) line. Yet, certain aspects of these results were surprising. On the one hand the observed resonances were found at much lower frequencies than expected concerning the channel geometry and concerning the main thickness resonance frequency of the transducer. On the other hand, we were confronted with a paradox: if one assumes horizontally standing plane pressure waves in the channel being the reason behind the observed particle movement, a laterally symmetric excitation (as employed by us) should not be able to excite odd (1st and 3rd harmonics) standing wave modes. Utilizing computer simulations, we can explain the results of our experiments. With the established model we numerically optimize the chip geometry in order to improve focusing capability of our system. Based on our investigations, we propose that the transducer itself plays a major role in the design of an acoustofluidic device and should not be replaced by simple approximations.